Fireside slagging, fouling, and high-temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels

Pr,,,g z~-,w~,~ (‘o,,,h,,ti. 3, Vol. 72. pp ?Y IX. 1996 1996 Elsebw Sc~rncc Ltd Copyright Prmtrd ,n Great Brltaln. All rl@hta rewvrd 0360 ,lXS 96 S39 011

c

Pergamon

0360-1285(95)00012-7

FIRESIDE SLAGGING, FOULING, AND HIGH-TEMPERATURE CORROSION OF HEAT-TRANSFER SURFACE DUE TO IMPURITIES IN STEAM-RAISING FUELS Richard

W. Bryers

Abstract-The process of steam raising as a source of heat or means of generating electricity using combustible fuels began with the turn of the century. From the very beginning. impurities in the fuels were responsible for added maintenance, a reduction in rate of heat transfer and corrosion due to fireside deposits of sintered or molten ash. The nature and severity of deposit formation. i.e. slagging and fouling, changed as the fuels and their impurities changed, the steam raising process evolved and the steam generators increased in size and efficiency. With the introduction of computer science. the empirical art of ash deposition from impurities in combustion gases is rapidly being transformed into the science of mineral transformation and ash deposition. This manuscript presents in chronological order an overview of the art of ash deposition while firing coal, the mechanistic approach to the problem, the recent introduction of sophisticated analytical procedures, and modeling of mineral transformations. and ash deposition underway. Adaptation of fuels such as ash oil, petroleum coke, municipal waste, wood and biomass to the steam raising process are presented individually in the order in which they were introduced. Empirical indices presently used to characterize the slagging or fouling potentials of impurities in fuels are present. Fundamental data are provided where necessary to illustrate mechanisms for ash deposition. An extensive list of key references is offered for those wishing to investigate details of any particular aspect of fireside slagging, fouling or corrosion. Copyright ‘(? 1996 Elsevier Science Ltd. CONTENTS I. Introduction 2. Fireside Problems with Coal 2.1. Introduction 3. Background for Development of an Empirical Approach 3. I. Ash Fusibility 3.2. Viscosity 3.3. High-Temperature Corrosion Due to Sulfur 3.4. High-Temperature Corrosion Due to Chlorine 3.5. Fouling of Convective Heat Recovery Surface 3.6. Sintering Tests 3.7. Pilot Plant 3.8. Summary of Empirical Approach 4. Mechanistic Approach 4.1. Mineral Matter in Coal 4.2. Mineral Analysis 4.3. Mineral Matter Analysis 4.4. Mineral Distribution 4.5. Thermal Behavior of Minerals 4.5. I. Quartz 4.5.2. Kaolinite 4.5.3. Clays and Shale 4.5.4. Pyrite 4.5.5. Siderite/ankerite 4.5.6. Calcite 4.6. Thermal Behavior of Mineral Matter 4.7. Fate of Mineral Matter During Combustion 4.8. Slagging 4.9. Fouling 4.9. I. Alkali-bonded deposits 4.9.2. Calcium-sulfate-bonded deposits 4. IO. Modeling the Mechanistic Approach 5. Fireside Problems with Heavy Oil 5.1. Introduction and Background Occurrence of Minerals in Oil 5.2. 5.3. Combustion of Heavy Oil 5.4. Deposition on Heat-Transfer Surfaces 5.5. Vanadium Corrosion 29

30 30 30 32 32 37 42 45 49 55 58 62 62 63 66 6X 6X 71 73 73 73 74 75 76 76 77 80 83 84 85 87 90 90 90 90 92 93

R. W. Bryers

30

94 96 97 99 101 101 105 107 107 107 109 110 112 113 114

5.6. Additives 5.7. Low Excess Air 6. Fireside Problems with Petroleum Coke 6.1. Fireside Behavior of Mineral Matter in Petroleum Cokes 7. Fireside Behavior of Inorganic Impurities in Municipal Solid Waste 7.1. Technical Background 8. Combustion of Wood 9. Combustion of Biomass 9.1. Introduction 9.2. Fuel Characterization 9.3. Mineral Matter 9.4. Fireside Behavior of Minerals 9.5. Steam Generator Design and Field Experience 10. Summary References 1. INTRODUCTION

The impurities found in steam-raising fuels have had a major impact on steam generator size and its availability ever since coal was introduced as a means of generating electricity at the turn of the century. Although there have been major changes in combustion system design (i.e., fixed bed, cyclone, suspension firing, and fluid bed combustors with the corresponding changes in combustion kinetics, flame temperatures, and residence times), the fireside behavior of impurities in coal has managed to be a continuous source of slagging, fouling, or corrosion in one form or another. Despite these major changes in combustion system design, the means for characterizing the slagging and fouling potential of ash has changed very little. Changes in design to accommodate increase in capacity, new fuel sources, and new combustion systems have evolved by empirical extrapolation of past experience, regardless of changes in the characteristics of the included minerals. Alternative fuels including oil, petroleum coke, refuse, wood, and biomass containing impurities of slightly different composition and proportion have also been the

source of troublesome fireside problems occurring in slightly different ways depending upon variations in mineral composition. This paper presents an overview of slagging, fouling and fireside corrosion associated with all the steam raising fuels. Each fuel is treated separately and presented in the order of its present economic importance. The presentation of the fireside behavior of impurities in coal has been subdivided into empirical and mechanistic approaches. The former is developed as it historically evolved to gain perspective on the limitations of the relationships presently needed to assess slagging, fouling and corrosion and the parameters they employ as applied to present combustion systems. This approach also serves to point out two approaches presently employed to diagnose and predict slagging and fouling potential. The empirical approach is used primarily by operating and design engineers, whereas a more mechanistic approach has been taken by the fuel technologist. Some repetition will appear in discussing specific types of slagging and fouling in order to maintain continuity in developing the approach.

Table 1. Major elements found in fuel impurities contributing to slagging, fouling, and corrosion Element

Bituminous Si Al Fe

Subbituminous

S S S

Lignite

Oil and Pet

S S S

--------------------------- Contribute

Refuse

to Fouling

Corrosive elements

Lz 2 VI

Zn Ni V

_

_ _ _

_

F C, F

C, F C, F C, F

C Cl C, F C F S C, F S, F C, F C = corrosion-by Cl or S of metal surfaces in excess of 600°F under reducing conditions and 830°F under oxidizing; conditions, by V above 950°F. S = slagging-by partial or fully formed melts due to fluxing of quartz by heavy metals at flue gas temperature >1065”F. F = fouling-fused or sintered ash due to condensation of volatile species, solid-gas reactions with SOs or Cl, solid-state reaction at flue gas temperatures from 648-1037°C between sulfates and oxides, and molten sulfates.

31

Steam-raising fuels

Alkali Trisulfate and Cl Corrosion by Molten Salts

Fused Deposits-Sodium. Calcium Silicates fMrllilites~ Formed From Na,SO.. CaSO,, and Quartz or clays, M.T. -1TBToC

QQQ

Q’

CaSO,-Bonded Deposits High Calcium/Low Sodium Fluxing of Aluminosilicates by FeO. Fe203 Deposition

of Sticky FeS

Calcium Silicates Initiated by CaSO, or Na,SO, Fluxing of Silicates by Lime Cl-Enhanced CO Corrosion Under Reducing Conditions

Fig. 1. Typical slagging, fouling, and corrosion found at various locations in the steam generators The paper is an overview. Although it is lengthy, it does not include details of the technology, which is reported in over 4000 individual publications. Key references are cited for those wishing to explore the technology in greater depth. These include thirteen major international conferences, two literature surveys, and an index of abstracts which include a large portion of the fundamental information. 2. FIRESIDE PROBLEMS WITH COAL

2.1. Introduction Although the impurities found in steam-raising fuels include most of the known elements, only 14 occur in significant concentrations to contribute to fireside problems. Table 1 shows how these elements are distributed in various steam-raising fuels and summarizes their contribution to slagging, fouling, and corrosion. Slagging is defined as deposition of fly ash on heat-transfer surface and refractory in the furnace volume primarily subjected to radiant heat transfer, illustrated in Fig. 1. Although the name ‘slag’ suggests a fused or semi-fused ash, the term ‘slagging’ may also apply to sintered deposits and dry

ash formed in liberally sized, low-pressure steam generator furnaces, or in furnaces fired with coals containing high moisture and alkaline earth ash. Fouling is defined as deposition in the heat-recovery section of the steam generator subject to convective heat exchange by fly ash quenched to a temperature below its predicted melting point, condensation by volatiles, or sulfidation by SOs, as shown in Fig. 1. These deposits may vary from light sintering to complete fusion. The latter is due to the formation of lower melting sulfates. Fouling and slagging are very complex phenomena depending upon the transformation of the inorganic components found in the impurities in fuels upon heating and cooling; the juxtaposition of individual inorganic or carbonaceous species during communition, combustion, and quenching while recovering heat; the chemical reactions between gas, liquid, and solid phases in motion and at rest; the kinetics of transformation of minerals and fly ash; the existence of non-equilibrium conditions frequently associated with supercooling; and, the attachment of impurities to surfaces and the detachment or reentrainment of deposited liquids and solids.’ The complexity of the problem is compounded by

32

R. W. Bryers

the fact that fireside problems cannot be simply represented by a single rate of deposition on a target by ash characterized by a single elemental analysis. The inorganic components (i.e., minerals and organically-associated carbons) in coal and the fly ash they generate are heterogeneous with regard to size and composition. Therefore, individual species behave differently during combustion and their subsequent flight through the steam generator. The degree of fouling and slagging varies throughout the steam generator depending on the impact of the local gas temperatures, tube temperatures, temperature differentials, gas velocities, tube orientation, and local heat flux on each particle. Furthermore, fireside problems manifest themselves in a variety of ways, some of which include: 0 Plugging of slag taps. ?? Formation of agglomerates and clinkers. ?? Loss in furnace heat absorption due to excessive accumulation-be it molten or sintered deposit, or due to a change in the thermal properties of the slag. . Damage due to excessive slag fall from a specific location or type of heat exchange surface and its orientation. ?? Undersized furnace hopper due to a weak slag-totube bond causing excessive sloughing of sintered ash. ?? Freezing of slag on hopper slopes. ?? Slagging of the heat recovery zone due to unpredictable low-melting phases in the ash. ?? Slag accumulations about the periphery of burners. . Fouling of convective heat recovery surface via condensation of volatile fumes or the sulfidation of submicron vulnerable species at the tube surface. ?? Tube wastage due to flame impingement. ?? Erosion/corrosion by Ily ash or soot blowers. ?? Tube wastage due to excessive concentrations of corroding elements. ?? Surface oxidation and exfoliation of inside tube surfaces due to high gas temperatures created by excessive fouling. ?? Slagging due to imbalanced air distribution. . Slagging and/or fouling due to size distribution of the fuel. ?? Slagging due to poor burner adjustments and coal size. ’ The wide variation in type, form, and location in which a deposit may form suggest that slagging and fouling are very much dependent upon design and operating parameters. Boilers of identical design apparently firing identical fuels have often been reported to be encountering quite different slagging and fouling problems. In an attempt to maximize profits by minimizing capital and maintenance costs, through optimized design, it has made steam generators sensitive to fluctuations in operating conditions and fuel ash chemistry. Consequently, slagging, fouling and corrosion are also dependent upon design and operating parameters which include:

Design

Furnace exit temperature. Furnace absorption. ?? Furnace configuration. ?? Burner arrangement. ?? Burner size. ?? Tube size spacing, orientation, ?? Air distribution. ?? Steam conditions. ?? ??

and temperature.

Operation 0 Coal size.

Air distribution between burners. Burner operation. ?? Excess air O2 level. ?? Flame impingement. ?? Soot blower operation. ?? Boiler load. Historically, steam generators were designed to operate on a specific fuel. Availability of vast quantities of coal provided the luxury of selecting fuels with low slagging, fouling, or corrosion potential. Troublesome fuels were simply left in reserve, thereby minimizing the impact of design and operation on fireside problems. The problems most frequently encountered were due to opening of new reserves of untried coals or the use of a new rank of coal. Empirical indices developed for the new fuels became the standard for the acceptability of new coals. Corrective measures such as soot blowing minimized the attention that must be given to operation and design. Unfortunately, reserves are dwindling and changes in economy dictate that the coals previously considered difficult to burn be used. Increased fuel trading activities in the world market require increased fuel flexibility. Tighter restrictions on ash disposal and emissions to meet environmental concerns have resulted in coal blending or switching to coals with entirely different mineral composition and slagging or fouling propensity. The option of selecting an alternate fuel as a means for solving a fireside problem is rapidly disappearing. All fuels will ultimately be used interchangeably and the combustion engineer must adapt his design and operation to deal with variable fuel and ash properties. The philosophy with which one characterizes fuels must change from one of predicting its slagging and fouling potential based on the fluid temperature of a laboratory-generated ash alone, to one which includes a selection of a matrix of engineering parameters which will allow proper location and orientation of the heat-transfer surface in the combustion chamber and heat recovery zone for a broad base of fuel types. Consequently, a more mechanistic approach must be sought. Presently, the cost of fireside problems may be as low as several hundred thousand dollars per installation for yearly cleaning or as high as millions of dollars due to a mismatch of design or operation with the impurities in fuel. It has been estimated ?? ??

Steam-raising

Table 2. Summary Nicholls,

Selvig, Rickets,

Shaefer,

Gauger,

1937

Fe0 + 0.6(CaO

+ MgO + NazO + KzO)

CaO = 100% - [SiOz + A1203 + Fe*Os] Determine S.T.nEM from ternary [SiOz/Al?03/FezO~](~,~,CaO=Conai,

+

1.5K,O

+ 3.57CaO

+ 5.OMgO + 3.22(KzO

+ NazO)]

Deposit weight = x where: xi = 0.03[Ti02] + 0.09[Fez0s] + O.O6l[CaO] + 0.264[MgO] + 0.423[Naz0] xz = 0.044[Ti02]0 094. [FezOslo 499 . [CaOj” 221 . [MgO]O 226 . [Na20]0,7Z* and boiler availability =f(x, ,x2) S,T, =f

1965

- 10.6

[Fe201 + CaO + MgO + Na,O + K,O] [Si02 + AlZ03]

Rhodes,

1974

F.T., ST., I.D. = f (5 I independent Fe203, CaO, MgO, NazO, K20) L,R. = [l.74(Naz0

Sondreal,

1975

Kovitskii, 1975

Karagodina.

Bryers, Taylor,

SiOZ + AlzOs z

&’ = [2.5Fez0,

1964

Winegartner,

+ CaO + MgO + NazO + KzO)

[3.3Si02 + l.96A1203]

dar, 1955

Garner.

33

correlations’

S = SiOz + TiOz + PZOs A = AlzO, T = CaO + 0.7MgO + 2.25Naz0 Determine % S, A, T Obtain S.T. from S, A, T ternary

1951

Majum

of fusibility

(SiOZ + A1203)/(Fez0s Rls=$$.

1933

Estep, Seltz, Osborn,

Duzy,

1932

fuels

variables

+ K20) + 0.73Si02

including

various

groups of AlzOj. SiOz.

+ 0.39(FezOs)]

[I .40(CaO) + MgO]

1975

Marynova.

Kr, = (Si02 + A1203)/(FeZ03 S.T. = 1094 + 42.5Kr, F.T. = 1139 + 48.6Kr,

+ CaO + MgO)

Western S.T.,,d = 2863 - 37.X+0.151X2, S.T.,,d = 2992 - 27.1X+0.27X2, Eastern S.T.,,d = 31 I4 - 38.68 + 0.37X’

Si02/A20s Si02/A120s

> I z 1

where: X = [Fe20, + CaO + MgO + NazO + K20] that fireside problems collectively cost the electrical generating industry $4 billion per year.*

3. BACKGROUND FOR DEVELOPMENT AN EMPIRICAL APPROACH

OF

3.1. Ash Fusibility The fireside behavior of minerals in coal was first investigated at the turn of the century. Initially, fireside problems began as soot from coal-fired, refractory-lined, stoker-fired furnaces.3 Shortly after burning of high-sulfur coals rich in pyrites (FeS2) on grates, it was determined that pyrite was responsible for the formation of clinkers-the first problem to be associated with a specific mineral species4 It was recognized that iron occurring in high-sulfur coals acted as a fluxing agent, lowering the melting temperature of quartz and clays found in coal. These early problems were associated with slagging as the boilers were small and cycle efficiency was low. As steam temperatures and pressures increased, the furnace cavity was assigned to recovering heat with

evaporating waterwall surface. The sensible heat in the non-radiant flue gases was recovered first by convective heat exchange surface used to generate steam and later on, convective surface-to-superheat steam. The furnace exit temperature was selected to optimize the heat exchanged by radiation and convection. An adjustment was made to ensure that entrained ash leaving the furnace was completely dry before entering the tightly-spaced convective heat exchange banks. Furnace configuration was sized to accommodate grates at first and burners later, minimizing ash carryover and providing an essential heat-transfer surface. The configurations were modified to minimize deposition by empirically extrapolating the best available data from early operating boilers. Although knowledge of the fireside behavior of minerals in coal evolved after 1900, the fusibility of the constituents comprising the ash had been studied quite extensively prior to 1900. Prost introduced the concept of fusibility in 1885 and recognized a relationship between the melting temperature of slag and the proportional distribution between basic and acidic constituents.4 A. C. Fieldner indicated at least

R. W. Bryers

34

1. Liwtes

and Wvommg Subbituminous, SiO,lAI,O,>> Subbituminous. SiO,/AI,O,>> 1 3. Wvomine Subbitumtnous. SiO,lAI,O, = 1 4. Eastern Bituminous

I

2. WVOmlng

2900

I

2000 0

10 Percent

Basic

I

I

20

30

[% Z lFe,O,

I

1600 I

I

I

I

I

40

50

60

70

+ CaO + MgO

+ Na,O

+ K,Ol

Fig. 2. Influence of percentage of basic constituents in ash on ash softening temperatures under reducing conditions for different ranks of North American coal.

182 other investigators had examined the fusibility of ash based on eight fundamental oxides most frequently found in coal ash (i.e., SiO*, A1203, TiOz, FezOs, CaO, MgO, NazO, and K20) prior to 1981.’ Since that time at least 17 additional correlations have been developed between ash chemistry and the ASTM ash fusion temperatures in order to improve the reliability of predicting the melting temperature of coal ash.3 The 11 most frequently used are reported in Table 2.3%6 Figure 2 illustrates the ash fusibility expressed in terms of the ash chemistry. The procedure for the ASTM ash fusion temperatures used in the fusibility correlations was accepted as early as 1924.6 The procedure simply requires generating ash from a composite sample of coal in the laboratory, forming cones and monitoring various stages of deformation of these cones as they are heated to a temperature at which they completely melt. Since then, eight different international procedures have been adopted. The diversity among the terms and practices is substantial, making it impossible to interchange results. Details of the terms, procedures, and accuracies have been described in a recent ASME publication.’ Table 3 compares the criteria for determining the various stages of melting used in the various international procedures. In addition to clinkers, operators of stoker-fired

boilers also reported loss of metal from the lower-side waterwalls due to CO and FeS attack. Under reducing conditions in the presence of carbon within the deposit, the pyrite (Fe&) is reduced to pyrrhotite and free sulfur. The latter reacts with tube metal iron to reduce additional FeS. Experiments with FeS have shown that pyrrhotite by itself contributes little to the attack. At the time no mention was made of the role of Cl as a means of enhancing CO attack. The impact of Cl on CO attack was not discovered until much later. The detection of Cl in tube wasted areas is difficult and thus sometimes overlooked. The plugging of convective passes by deposits initiated by a white layer of alkali sulfates was common, suggesting Cl may have been present. The source of the sodium in English coals responsible for fouling was believed to be sodium chloride.’ This initial attack may have been aggravated by Cl. Pulverized coal was introduced in 1920. With the emergence of full furnace-wall cooling and increase in volumetric heat release, fireside corrosion due to flame impingement of partially combusted pyrites on heat-transfer surface worsened. Two types of corrosion occurred: wall wastage due primarily to FeS and/ or CO where the scale was enriched with carbon and FeS. In those instances where a highly water-soluble, bluish-white, glassy enamel-type deposit formed, the

NOfe: Preparation,

support

title, ash binder,

particle

ASMT

= l/2

rate. and reducing

Height = cl.6 mm (1/16in.)

Height base

Spherical mass where height = base

First sign of founding of apex

Pyramid

size, heating

Height = l/3 base at HT

Height = l/3 base at HT

Flow/fluid temperature

(FT)

Height = l/2 base

= I/2

_

cube,

First sign of rounding of top or edges

Pyramid, cylinder

BS

Height base

(ST)

Spherical temperature

cube,

First sign of rounding of tip or edges

Pyramid, cylinder

IS0

Hemispherical temperature (HT)

(ST)

molds

Softening temperature

Initial deformation temperature (IDT/DT)

Specimen

Temperature

Table 3. Characteristic

= l/2

cylinder

to another.

Height = l/3 height at HT; or (microscope onlv)

Height = l/2 base or pyramid tip bends to touch base (tb)

First sign of founding of tip or edges (L)

Pyramid,

gas vary from one procedure

Height = l/3 original height

Height base

First sign of rounding of edges

standards’

USSR

for all ash fusibility

Cube, cylinder

DIN

temperatures

= l/2

= base

Height = l/3 height at HT

Height base

Height

_

First sign of rounding of top or edges

Australia

Height = < mm (T3)

CT21

1.5

Cone bends so that (a) tip touches support; (b) height 5 base

First sign of rounding curving i oftip(T )

Pyramid

PRC

l/2

Height = l/3 height at HT

Height: base

First sign of rounding of tip

Pyramid

South Africa

36

R. W. Bryers

930 920 910 900 890 880 670 060 8” g

650 040

E $ E

630 * 82Q

F

810 800

425

790 760 770 760 400

750 740 730 100

20

1000 SO, Concentration,

Fig. 3. Melting Table 4. A comparison

Coal ash SiOz A1Z03

TiOz Fez03 CaO MgO Na20 K2O so3 p205

Ash fusion temp. Reducing I.D. ST. S.T. F.T. Oxidizing I.D. S.T. S.T. F.T.

points in systems Na2S04-SO3

of slag from a coal with homogeneous-dispersed minerals

Puentes

47.9 30.6 1.0 8.3 3.4 1.5 0.2 1.9 3.5
IO,000 ppm

heterogeneously-dispersed 1.60 Gr. fraction

minerals

with a coal of heterogeneously-dispersed

Coal ash

6.9 8.9 0.2 79.2 1.3 1.0 1.8 0.2 _

0.2

minerals

Kriel homogeneously-dispersed

Furnace slag

5.5 3.6 0.3 90.2 0.6 0.4 _

and KzS04-S03.8~9

48.4 25.2 1.5 1.8 14.2 2.1 3.2 0.5 4.6 0.7

1.80 Sink fraction 48.2 25.3 1.6 6.9 5.9 1.4 2.3 0.4 4.8 _

1237 1282 1437 1537

1248 1260 1271 1293

1315 1404 1537 1537

1282 1310 1337 1360

1243 1293 1304 1360

1104 1537 1537 1537

1326 1526 1537 1537

1471 1537 1537 1537

1293 1343 1371 1393

1271 1304 1315 1393

corrosion was due to alkali pyrosulfates of sodium and potassium. The pyrosulfates form under reducing conditions. Corrosion depends on a molten phase.

minerals Furnace slag 46.4 27.8 1.7 4.1 12.2 2.4 1.9 0.5
Coast et al. showed that KzSz07 and Na&O, was molten above 404°C and 393”C, respectively, at levels of SO3 exceeding 200 and 2000ppm, respectively.

37

Steam-raising fuels Their stability depends upon the SO3 level and generally they decompose at temperatures above SlO”C, as shown in Fig. 3.8.9 The importance of fuel-bearing iron on furnace wall wastage, slag, and clinkers prompted Moody and Langan,‘” as early as 1933, to examine the occurrence of iron in coal and they discovered that fusion characteristics of ash varied from one fraction to the other according to the distribution of mineral species and their juxtaposition with each other and the carbonaceous portion of coal. Gould and Brunjes” established the ‘free fusible material in coal (extraneous pyrites) as an ash index of clinker and slag formation’ in 1944. In 1963, Littlejohn and Watt” examined the distribution of mineral matter in pulverized coal. It was not until 1968 that Watt, Reid, Borio, and Bryers13-” seriously focused their attention on the individual minerals and their juxtaposition in the ‘coal as a cause for fireside slagging and fouling. Borio and Narcisco13 identified a correlation of slagging with respect to the highest gravity fraction, which is usually iron enriched. Bryers and TaylorIs developed a regression analysis for the ash fusion temperatures vs ash chemistry on size and gravity fractionated coal, indicating the variation in homogeneity of the distribution of mineral species in coal had an impact on ash melting temperatures and hence, slagging potential. Watt” provided the industry with the most comprehensive document on the physicochemical behavior of individual minerals in a combustion environment to-date. As a result of the works cited above, it became quite apparent that substantial partitioning of mineral species could be taking place during combustion, resulting in deposited ash with decidedly different compositions from the coal. Table 4 illustrates the difference in slag composition formed by minerals homogeneously and heterogeneously dispersed throughout the coal. Despite the evidence indicating the juxtaposition of pyrites in coal with regard to carbon and other mineral species has a strong influence on the slagging potential of coal, only a few have attempted to include mineral fractionation in their slagging indices.

Temperature, 1100 40,000

1200

1300

1

I

I

OC 1400

I

I 50,

1500

I

I

I

56.9

-

AI,O, 18.8 Equ’valent

10,000 ENlOO

Fe,O, Co0

12.9 8 9

MgO

12

I

I

I

2400

2500

2600

6WO

Alk

_ _

_

I 3

- ??Vlscoslty

measured after cool’ng cycle

+

Vlscoslty meosured after heatmg Cycle \’

2ooO

I

I

I00

2’00

I

2200

2300

2700

Fig. 4. ViscosityPtemperature plot of a typical glassy coal! ash slag.*,19

3.2. Viscosity Interest in cyclone and wet-bottom furnaces in the late 1930s prompted numerous investigators to abandon ASTM ash fusion temperatures as a measure of the high-temperature fusibility of ash and focused their attention on viscosity. Nichols, Reid, Cohen, Watt, and others18-2’ developed viscometers and performed viscosity measurements on coal ash and slags at temperatures as high as 1621°C in oxidizing and neutral atmospheres. They found that completely liquid slags behaved as Newtonian fluids and that the rate of change in viscosity with a change in temperature at a given viscosity was essentially constant. Consequently, the functional relation of viscosity vs temperature in the

20

30

40

x) S’O* ,

I

I

I

60

70

80

90

Penent

Fig. 5. Viscosity of coal/ash slags at 1426°C in air (plotted against SiOZ content on the basis of omitting Al203 and making SiOZ + equivalent FezOX + CaO + MgO = 1OO).8 Newtonian range is the same for all coal slags. By making an appropriate parametric correction for composition, the viscosity curves for all slags could be represented by a single curve.” Figure 4 illustrates a typical viscosity--temperature relationship for coal ash. As early as 1925, Fulcher proposed the following relationship for viscosity in terms of temperature for

R. W. Bryers Temperature, OC 1300

1400

1000

8 ._ Lf f 8 s ._ > 100

I

1c

-‘.d

I

2200

I

2600

2400 Temperature,

,\,

2800

OF

Fig. 6. Viscosity of Illinois no. 6 coal for various gravity fractions using the Watt-Fereday and Urbain correlation. glasses: 77

=

A,eEjRT

and

loglo~=A*+~ 0 where A is a constant, and B is the temperature coefficient (‘Energy of Activation’), and To is a temperature correction required to fit the data in a straight line plot of log of the viscosity against the inverse of the temperature difference.8,22’23 Corey and Reid reported the following relationship between viscosity and temperature developed from North American bituminous coal ash data: n-o.1614= (0,000452)(t) - B

(3)

They found that within the Newtonian fluid range,

the compositional parameter, E, affecting viscosity was not influenced by the aluminum level but was governed by the major basic constituents (i.e., Fe203 + CaO + MgO) when treated as a sum.‘.*’ Hence, they developed the silica ratio defined as the percent silica in an ash normalized to include only Si02, Fe203, CaO, and Mg0.8S24The impact of silica ratio on viscosity is illustrated in Fig. 5 at a constant temperature.’ Investigations performed at BCURA,8.‘4 applying the same silica ratio to a wider range of coal ash compositions, produced the following relationship:

Watt and Fereday, also using BCURA data, derived an expression for viscosity in terms of temperature from the Arrhenius equation, identical

Steam-raising fuels

‘ooon--

i

2 f; je

1100

1200

IXCJ

IO

1500

1400

Temperature.

‘C

Fig. 7. Flow characteristics of three typical coal ash slags.” to the relationship

proposed

by Fulcher.“3

Arrhenius

IO’WI ~LNq = ct _ 150)? + c where m and composition:

C

m = 0.00835Si02

+ O.OO6OlAl203 = 0.109

c = 0.0415Si02

are

constants

Eq. (5)

dependent

on

(6)

+ 0.0192A1203 + 0.0276Eq. FezOs

+ 0.0160CaO

- 3 92.

(7)

These three original expressions for viscosity of silicate melts have been recently modified to extend their range of applicability. Whereas Watt and Fereday used the Arrhenius relationship (Eqs (1) and (4)) Riboud and Urbain have found the Frankel equation (Eq. (8)) to give the best fit, and McCauley and Apelian propose the Clausius Clapeyron equation (Eq. (9)).25-27 The latter has been used to successfully relate pressure with temperature, enthalpy, and volume. McCauley and Apehan” indicate ‘many properties including viscosity can be related to an energy barrier, free volume, and temperature’. n =

ATpBIRT

Frenkel

39

terms of temperature, knowing the five major constituents of coal ash, all claiming to be accurate over a given range of slag composition. None of these expressions include modest amounts of the alkalis believed to be influencing the surface fusibility of fly ash and certainly furnace wall slag formed by highsodium-bearing lignites. A plot of viscosity using the Urbain and Watt-Fereday equations on gravityfractionated coal in Fig. 6 indicates substantial disagreement between correlations as iron concentration increases. This suggests that correlations developed to reduce time and cost of determining viscosity should be verified for slags whose composition fall out of the range used to derive the correlations. Figure 6 indicates heterogeneouslydispersed mineral matter produces fly ash with a wide range of viscosities. Important differences between slags develop as they are cooled below the crystallization point. Some slags rich in silica remain glassy over their entire viscous range.s.‘4 No divitrification occurs and viscosity depends only on temperature, not the previous thermal history of the slag.x,‘4 In a second larger group of slags, a solid phase precipitates out abruptly at some given temperature, forming a pseudoplastic fluid. Upon reheating, the solid phase is gradually redissolved but at a temperature level much higher than that at which the crystals were formed. The temperature difference between crystal formation and crystal melt depends on cooling rate. Reid defines a third group in which the temperature at which crystals precipitate upon cooling is the same as the temperature at which the crystals dissolve upon heating.‘K Figure 7 illustrates the three types, along with their chemistry.z8 Freezing is not always as abrupt, as illustrated by Reid.s Some slags reveal a substantial difference in temperature level between the temperature at which crystallization begins and solidification is complete. The crystallizing temperature is important to those designing gasifiers, wet-bottom furnaces, and slag taps, as well as to those modeling slag flow in dry-bottom furnaces. and has become commonly known as the critical viscosity temperature. r,,. The relationship between the beginning of crystallization (or the critical viscosity temperature) and composition is much more complex than predicting the fluidity of the slag. Crystal formation depends on phase equilibria within a complex multi-component system, the presence of catalyst for formation of crystals, and cooling rates of the slag systems.” Despite the complexity of the system, researchers at BCURA have developed the following empirical expression for T,., in terms of the composition alone. based on measured values of 63 coal ash slags.x,‘8

Eq. (8) T,,=2990-

LNq = Cl + C,/T

Presently,

f C3LnT

Clausius

there are five expressions

1470(203)

+360(20,)?

Clapeyron Eq. (9)

- 14.7(FezO,

+ CaO + MgO)

for viscosity in

+ 0.15(Fez03

+ CaO + MgO)‘.

(10)

40

R. W. Bryers Temperature,

OC

1100

1200

1300

1400

1500

I

I

I

I

I

1

I

0

-1500

I

In air

. Under reducing conditions

- 1400

.

-1300

- 1200

.llOO 2ooo’ 2CCO

2200 Temperature

2400

2600

28Ccl

of Critlcal ViscosHy, T,,, F

Fig. 8. Relationship between cone-softening temperature and temperature of critical viscosity.sX’9

Ta.F

X Fa IN GLASS 1_1

1

.(GT w-= 0,.

CRATT

RED -13EI

0,.

nvm.r

13s6 .lsm

*,.

ILL I

117s

0,.

m-f.,

,,M

----

OXIDIZING

-

REDUCING

OX

WE3

TEM?. F

Fig. 9. Schematic illustration of the approximate abundances of the mineral-derived phases in an eastern ash and iron in glass as a function of quenching temperature for a reducing atmosphere.34

41

Steam-raising fuels There appear to be some parallels between ASTM ash fusion temperatures and viscous properties of slag as the initial deformation temperature is an indication of the formation of a melt and the softening temperature, the complete dissolution of the solid phase. The fluid temperature which exceeds the softening temperature by 27.7”C or more, is believed to be a viscous effect impeding the flow of molten slag.” To minimize the cost and timely process of determining the r,, by means of a viscometer, Sage and McIlroy3’ proposed calculating the T,, by simply adding 117°C to the hemispherical temperature. They reported that for approximately 70 ash compositions studied, the T,,was within 111°C of their measured value, except one. This method of determination is reportedly applicable to slags in which 20% of the iron present is in the ferric state.6 The conceptual relationship between the ASTM softening temperature and the critical viscosity has led others to modify earlier regression analyses of ash softening temperature vs basicity of the coal ash by simply substituting critical viscosity for ash softening temperature.30.3’ Corey” showed the relationship between the softening temperature and critical viscosity over a limited range of coals in Fig. 8. As the slag is cooled below the crystallization temperature, the fluxing agents such as calcium and iron begin to precipitate in crystalline form. Crystals of mullite, anorthite. crystobalite, gehlenite, hercynite, moscovite, and fayalite have been identified.32,‘3 Kalmanovitch and Williamson33 examined the divitrification of Eastern and Western, North American coal-type ashes quenched over a period of several hours. Their results show that the crystallization is well represented by the CaO-FeO-Alz03SiOZ quaternary. Huffman et 01.~~ examined the high-temperature behavior of coal ashes using Mossbauer analysis, SEM-AIA analysis, and X-ray diffraction. Huffman’s samples were heated in a modified ash fusion apparatus and quenched for 5510s from temperatures ranging from several hundred degrees Centigrade below the initial deformation temperature, to the fluid temperature. They found partial melts as low as 200-400°C below the initial deformation temperature. Under reducing conditions, melting was greatly accelerated and controlled by the iron-rich corner of the phase diagram. The iron in the ferrous glass below 900°C was reported to originate from the mineral illite in the coal. Under oxidizing conditions, potassium appeared to be the most important low-temperature fluxing agent as the percentage of glass in samples quenched from temperatures below 1100°C was proportional to the potassium-bearing mineral illite. Figure 9 illustrates the percentage of total iron contained in the glass as a function of quenching temperature under oxidizing and reducing conditions. Unfortunately, the data do not treat the percent glass formed parametrically with quenching rate to illustrate the impact of reduced rates of migration of the fly ash to

I o

PYRITE

A

IRON

o

MARCASITE

SUBSTRATE

POWDER

TEMPERATURE

(‘CI

Fig. 10. Sticking behavior of iron-containiy,g minerals on oxidized medium carbon steel. the tube surface or the increased residence time at deposit surface temperature on crystallization. Once cooled below T,,, partial crystallization of the slag alters the composition of the glassy phase and hence, its viscosity. Lower temperatures and increased crystalline concentration account for a rapid increase in viscosity. Hough32 showed that a phase equilibria computer code, employing the Fereday-Watt correlation, could be used to predict the measured viscosity of the slag below T,,, for four slag samples. Hoy et al. 24 indicate the effect on viscosity of change in chemical composition of the liquid slag, resulting from fully buoyant crystals, is larger than the effect of their presence as inert solids dispersed throughout the liquid. However, when the crystal concentration is sufficiently large to permit contact between individual crystals, slag behavior may no longer be Newtonian and may be both quasi-viscous and thixotropic. Plastic behavior is not normally experienced unless the solids content of the slag is greater than 50%.24 discrepancies in Moza and Austin35 recognized conventional techniques for measuring ash fluidity and therefore, developed a drop-tube quenching test to elucidate precise mechanisms for initiation and growth of slag masses formed by molten slag subjected to cooling. Synthetic ash, mineral matter, and low-temperature ash were subjected to the quenching tests. The mineral matter was heated until it fused, melted, and eventually dropped freely through a hole in the top of a furnace containing a

42

R. W. Bryers Temperature, 600

0

II00

OC

650

700

1200

1300

76

1400

Temperature, F

Fig. 11. Corrosion of chrome-nickel alloy (TP-321) by molten trisulfates.39.40 target, representing different substrates. The temperature of the furnace was controlled between ambient and 600°C. The substrate temperature was controlled independently. Adherence to the substrate was observed and the contact angle and adhering force measured. Pyrites and mixtures of pyrites and quartz, and pyrites and illite, were found to have sticking temperatures as low as 250-300°C. The adhesion force for pyrite is illustrated in Fig. 10. The sink 2.85 gravity fractions of three bituminous coals showed a drop in sticking temperature of almost lOO”C, compared to ash from the parent coal. The quenching tests indicate sticky substances deposit at temperatures much below the initial deformation temperature. High-speed photography showed that the molten drop adhered with rapid freezing of the contact area, followed by a zone of freezing moving up into the drop. The material next to the interface was glassy and supercooled. Crystalline nucleation occurred higher in the drop.35 Consequently, coalash-slag viscosity based on composite ash chemistry is not necessarily a good index for the slagging potential of a coal. One must deal with specific chemistry of an ash particle, the chemistry of the ash deposited, the temperature and location within the slag, supercooling of glassy phases, crystallization, and cooling rates to avoid significant error. Correlations for viscosity were originally developed to predict the flow of molten ash in high-temperature cyclones and wet-bottom furnaces where the ash was 100% molten. Although interest in slagging-type combustors has diminished because of high maintenance cost and high NO, levels generated at the very high operating temperatures, viscosity remains as an indicator of slagging in dry-bottom furnaces by virtue of a relationship between the softening temperature of the coal ash and the critical viscosity T,, has been used as an of slag. Consequently, indicator of the fluidity of deposited ash or the plasticity of particulate impacting on tube surfaces. Slag formed in dry-bottom furnaces is initiated by a sintering process which accounts for the greatest loss

in absorption. The sintering process, however, may be due to viscous flow of surface layers, solid-state diffusion or contacting particles, or chemical reactions between gas and solid. The plastic or fluid layer of a deposit includes only the fireside portion of the deposit. Deposits formed by high-moisture coals may be entirely sintered. The fluid properties of greatest interest in dry-bottom furnaces fall within the plastic range and are dependent upon quenching rates of crystallization, crystal concentration, and changes in fluid chemical composition. Consequently, the use of viscosity-a measure of slag fluidity-as an index for slagging in dry-bottom, low-heat release furnaces can produce misleading results. 3.3. High-Temperature Corrosion Due to Sulfur With the increase in demand for electricity in the 1940s and 195Os, thermal efficiency climbed from 27% in 1948 to 38% in 1963, with a project increase to 40% in 1965. To meet the higher efficiencies, the steam temperature increased from 454°C in 1948 to 565°C in 1963. Materials of construction of tubes went from mild steel (i.e., 1% Cr-4% MO) to 2$% Cr1% MO. Steam generator design had approached the threshold for design for some fuels with enriched concentrations of minor elements of sodium, chlorine, and sulfur, while exceeding the limits in some instances3’ The increased consumption of Illinois Basin coals, known to be enriched with sulfur, chlorine, and sodium as organically-bound chlorine and sodium chloride, resulted in wastage of furnace walls and finishing superheater tubes as well as fouling of the finishing superheater at the entrance to the heat recovery zone. Corrosion by alkali trisulfates quickly became a problem. As early as 1945, Corey et aL3’ identified the presence of alkali pyrosulfates in furnace wall deposits and alkali-iron trisulfates on the leading edge of the final superheater tube surface as being responsible for tube wastage. In both cases the deposit

Steam-raising

43

fuels

+ Fe, 0,

( 50 wt. % )

600

“0

I .o

2.0

3.0

4.0

5.0

Probe Distance Above Surface, mm Fig. 12. SO1 profiles above mixtures

containing

consisted of a hard, white enamel-like material and corrosion was dependent on the existence of a molten phase.37.3* Pyrosulfates were found to be molten at temperatures exceeding 398°C for sodium pyrosulfate (Na2S207) and 454°C for potassium pyrosulfate. Coats et al9 showed in Fig. 3 that K2SZ07 could be formed with as little as 150 ppm SO3 at 404”C, whereas NazSz07 required at least 2000ppm SO3 to form at temperatures of 398°C. Maintenance of their stability on furnace wall tubes whose surfaces reached 482°C requires at least 1OOOppm SO3 for K&O,, and at least 2% SO3 for NazS207.8 Consequently, the formation of pyrosulfates would be confined to furnace walls and economizer tubes. They would not be expected to form on superheater tubes. Corey et d.37~38, Nelson3’ and Cain” reported the alkali-iron trisulfates require at least 250 ppm SO3 to form. The alkali-iron trisulfates are molten at 894°K and decompose above 525°C. The bell-shaped curve reported by Cain in Fig. 11 illustrates the range in which they are molten, stable in the presence of S03, and corrosive.40 Cain and Nelson39s40 found that mixtures of sodium and potassium alkali trisulfates could lower the melting point of the pure constituents to 477°C. Sulfur trioxide in flue gas can be attributed to reactions within the flame, oxidation of SOZ, and decomposition of sulfates. The latter makes a very minimum contribution as the quantity of sulfate found in coal is insignificant. Hedly explained that although SO3 can form in the flame, the reaction rates are too slow and the residence time much too slow for significant SO3 to form.4’ Manning42 reported that equilibrium conversion of SO2 increased rapidly below 982°C and reached 100% by 426°C. At reduced levels of 02, as found in flue gas, the conversion is substantially reduced at the higher temperatures, However, it has very little impact on achieving 100% conversion by 426°C. Hedly4’ found the conversion of SO3 rarely exceeds l&2% in a steam

FezOi at 593°C in 5.8% S02_‘x~44

generator. The difference is due to insufficient residence time for the reaction to occur. Krause et al.43 recognized such levels of SO3 would be insufficient for the alkali trisulfates to form. They also found that conversion of SO? to SO3 within the boundary layer can be 10 times that occurring in the bulk stream due to the catalytic activity of Fe203 and sufficient residence time.43 These data appear in Fig. 12. Levy and MerrimanM indicated the catalytic behavior of Fe203 was substantially greater than that of Fe304. Two observations made of full-scale deposits led to two diametrically opposed conclusions. ln one case. Reid’ and others observed the formation of a white. thin layer enriched with alkali-bearing compounds shortly after immersing the heat-transfer surface into a hot flue gas stream, suggesting direct formation 01 alkali sulfates on the tube surface. Anderson, Jackson, Crossley. and Rayner45-49 all observed on occasions the absence of inner layers of deposit beneath thin accumulations of alkali-sulfateenriched fly ash accumulations, suggesting that the inner layer was formed as a result of migration of the alkali metals to the cooler surface. Jackson474x presented a very comprehensive summary of vapor and decomposition pressures, shown in Fig. 13, that are essential to the understanding of the formation and stability of alkali compounds as they migrate to the cooler heat-transfer surface. Assuming a permeable deposit experiencing a marked temperature gradient under equilibrium SO3 concentrations, it was proposed that the volatilized sodium hydroxide and potassium hydroxide released during combustion reacted with aluminosilicate, which migrate to the cooler surface and react with SO3 to form sulfates. The sulfates interact with Fez03 in the ash and SO3 present at the tube surface to form alkali trisulfates. as shown by Eq. (11). Between 565 and 676°C the trisulfates are molten. Tube iron goes into solution breaking down the trisulfates and forming iron

R. W. Bryers

44

6

IO

7

‘1’

II .’

_

I2

1.1

LI0UID.90LI0

’

lo’

I

-s >

L

IQ%CO*_

\

4

\

IO2

\\

\

EC

TEMPERATLIIE;C FLUE

-

GAS c--

DEWSIT-

;;;;;ACE

..----. __-_ -..-

PRESSME OECOMKlSl7l0N PfiESSuRE PARTIAL PRESSURE OF SO3 TENTATIVE OATA EFFECTIVE VAaA ?RESSLWE

-‘-

S03FORMED

Tenpcraturc

Range

VAWUR

FcSO4

Deconporl

tton

537’C

znso4

Decolposi

t Ion

646’C

CdT~LYTICALLY

Fig. 13. Vapor and decomposition pressures of sodium, potassium, iron, zinc, and lead compounds and partial pressures of sulfur trioxide.47,48

blue

gistme* cl0

to

10% -s;

Into

silica10

mmrrrl

Diffusion pWlOd

r@rldLlO

SWhC8 phase

at 1.000

SWf8C~

Alkali capture ( rhOed

)

-1,200 ?? c Fig. 14. Reaction of alkali metals at surface of fused silicate particles!s

Steam-raising

sulfide, which migrates to the deposit corrosion product interface and reforms alkali trisulfates to complete the cycle, as shown below.

1

*ni”a’FeSz(coal) Source

Fe+K3Fe(S04)3-(Fe20i+K2S04)+FeS 1 + 02 ‘L

SO, P

SO, + Fe,O, (11)

They assumed that the alkalis arrived at the tube surface as a result of a chemical reaction of sodium hydroxide with the surface of aluminosilicates, forming a sticky surface and permitting attachment to other deposits on the tube surface. Figure 14 illustrates the counter-diffusion of sodium and potassium through the illite fly ash particle and their effect on the melting temperature of the particle surface. In the absence of aluminosilicates and pyritic iron but in the presence of high SO3 levels, as in the case of oil, sodium sulfate will form a molten deposit at the tube surface by passing the first step and causing severe fouling while avoiding corrosion. Research on alkali trisulfates has taught us that high-temperature corrosion of furnace wall and superheater surface can be avoided by ensuring an oxidizing environment and avoiding flame impingement on furnace walls using ‘air belting’ in the flame basket, limiting steam temperature to approximately 537°C using shielding to raise exposed surfaces to temperatures exceeding the melting temperature of the alkali trisulfates and restricting the sulfur (i.e., iron level) and alkali level (i.e., KzO, NazO) of high alkali bituminous coal:s.

3.4. High-Temperature

Corrosion Due to Chlorine

Chlorine became a problem for furnace walls and superheaters while firing pulverized coal. In the U.K. during the late 1950s where the Cl content ran high as 0.8% with an average of 0.25%, experience taught operators they could operate corrosion-free if they did not allow the Cl concentration in the coal to exceed 0.3% on a dry-coal basis.5’ Most coals in North America contain less than 0.3% Cl on a drycoal basis. However, there are substantial reserves of coal in North America that contain high levels of Cl (i.e., >0.3% on a dry-coal basis). These reserves consist of deep mines confined to the Illinois Basin. Until recently, the abundance of high-sulfur/lowchlorine coals provided an economic alternative to using a coal with a corrosion potential. Although the abundance of high-chlorine fuel is localized, the increase in restrictions on the use of high-sulfur coal has created an incentive is developing for exploiting these reserves. The forms in which the chlorine occur are important as they determine the mineral transformation

fuels

45

during combustion, which ultimately affects the fireside behavior of the species. The forms in which chlorine occur also determine their potential for removal during the fuel preparation as a remedial measure for fireside problems. Three major forms for the occurrence of chlorine in coal have been proposed: inorganic chlorides, organo-chlorine compounds, and chloride ions in brines and other waters associated with coa1.s0-54 For a good many years it was believed that chlorine was present as a chloride of sodium, potassium, and calcium due to the intrusion of saline water-a position initially supported by Gluskoter of the Illinois Geological Survey with regard to Central Illinois coals.52,53 Aqueous leaching tests generally gave a good correlation between the chloride and alkali ions with which they were associated as an organic alkali compound.5’ Recent aqueous leaching experiments by Gluskoter et al. and Chen et a1.53-s8 have led these investigators to change their original thinking about Cl in North American coals from chlorine occurring predominantly as inorganic alkali chlorides, to a mixture of soluble chlorides and unspecified ‘organic’ chlorides. Electron probe analysis of an Upper Freeport bituminous showed chlorine to be homogeneously distributed throughout a given maceral. No correlation existed with other inorganic species presents8 X-ray Absorption Fine Structure (XAFS) spectroscopy performed by Huggins and Huffman on various rank coals whose chlorine ran between 0.04 and 0.84% indicated the chlorine in a majority of coals investigated was present as a chloride anion in the moisture associated with the microcracks and pores in the coal. A second form of occurrence was identified as crystalline NaCl, which presumably precipitated from the chloride-rich solution as the coal dried.54 Chou59 of the Illinois Geological Survey reported similar results. With the chlorine occurring primarily as anions of moisture in fine cracks and pores, it is not difficult to rationalize the differences in results reported earlier using various analytical procedures.54 It would also appear that a high level of removal could be achieved by fine grinding and aqueous removal. Gibb” recently demonstrated that extremely rapid dechlorination of coal occurs in the early stages of devolatilization releasing chlorine as Cl or HCl, which he attributed to the presence of chlorine as weakly bonded ions in the coal matter. Equilibria for Cl and HCl favors the formation of HCl at the lowtemperature oxidizing conditions to which furnace wall deposits are normally exposed. Engdahl et aL6’ and Krause et aL6* indicate Cl has never been detected in incinerating steam generating flue gases, thereby substantiating equilibria predictions-at least as applied to incinerators. The accuracy of equilibria calculations depends, of course, on the reaction kinetics and the system residence time. It appears that system residence times are sufficient in the

R. W. Bryers

46

2oa

I NO

:

IS00

CONDENSATION

:

;

/

:, ,

800

600

so

90 PEP hno

Sal,

100 CENT

sodlvm

120

110

OF

THEORETICAL

compounds,

130 PER

AIR

510s

CENT

OF THEORETICAL

AIR

prw8nl

Fig. 15. Thermodynamic equilibrium for sodium compounds in flue gas for off-stoichiometric conditions and varying SiOz levels while burning a low Cl coal.63

100

200

I

I

200

300

400

Temperature, “C 300 400

I

500

500

600

I

I

I

9QO

700

900

900

000

1100

Metal Temperature, OF

Fig. 16. Corrosion rates of carbon steel in chlorine and HCl as a function of temperature.68

immediate environs of the tube surface for the reactions to go to completion. Crystals of sodium chloride present within the coal structure should not volatilize until significant decomposition of the char has taken place. At temperatures in excess of 648”C, the alkali chlorides would be expected to volatilize. Depending on the composition of the local environment, the chlorides would decompose at somewhat higher temperatures, as illustrated earlier in Fig. 13 prepared by Jackson4’ The potential for direct deposition of NaCl by passing the combustion process would then exist as well. English and North American coals rich in chlorides

also contain high levels of sulfur. Cutler et a1.,64 Boll and Pate1,63 Wibberley and Wa11,65 Halstead and Hart,66 and Wal?’ examined the thermodynamic equilibria of chlorides and sulfates at levels normally encountered in the high-chlorine coals, as well as alkali partitioning under reducing conditions, in an attempt to understand the formation of alkali chlorides or sulfates on tube surfaces. An example of Boll’s results appearing in Fig. 15 indicates that chlorides will deposit on tube surfaces under reducing conditions in the absence of a ‘getter’ such as quartz. Under oxidizing conditions, sulfates should form preferentially. Cycling environmental

41

Steam-raising fuels

.w...-.

400 pwn HCI

--A-

TIME.

- 2000pprnHCI

h

Fig. 17. Metal loss data for mild steel in NzO-10%C0-10%N~0~0.5%S0~

with HCI concentration of 0.

400, and 2000ppm at 500”C.76 conditions should provide the worst conditions as Cl would be released periodically. Chlorine has a secondary effect on potassium normally found in coal as a stable silicate such as a feldspar or illite. The potassium may be displaced by sodium or chlorine at high temperatures, forming a chloride or sulfate and thereby producing a second alkali capable of engaging in the fireside corrosion problem. Cutler et a,!.64 and Raask” propose the following reactions: mK20 - XSi02 e YA1203 (Condensed) + 2NaCl (gas) + (m - 1)K20 - K20. Na20XSiOz x YA120s (Condensed) + 2KCl (gas) 2KCl+

SO3 + O2 Fr! K2S04 (gas) + HCl.

(12) (13)

Illite is known to be sticky down to temperatures as low as 950-1000°C and hence, provides the mechanism for transporting potassium from the flue gas to the leading edge of convective bank tubes where it is retained as a semi-molten deposit providing a source for K,S04 or KCl. Investigations by Brown et al.,68 as well as by Battelle, indicate chlorine attack begins at 204°C whereas hydrogen chloride attack does not start until 426°C as shown in Fig. 16.68-70 Since equilibria does not favor the formation of Cl* in the flue gas at furnace walls under oxidizing conditions, any attack by Cl2 must occur beneath a deposit or under reducing conditions. The first step of high-temperature corrosion begins with the formation of a protective film on the tube surface. In the second step, the Cl2 released by the coal, generated from the decomposition of HCl under reducing conditions or the release of HCl from alkali salts in the presence of SO2 and 02, penetrates the oxide scale and reacts with the elemental iron to form

FeCl,, physically separating the protective coating and corrosion product from the tube surface. Thus, Cl* + Fe + FeCl?.

(14)

The FeC12 vapor diffuses outward through the oxide scale and on its way is oxidized to Fe304 and FezO,, releasing fresh Cl1 for attack of base meta1.72-74 The process can be cyclic. Thus, 4FeClz + 302 = 2Fe20A + 4C12 T 4Fe 2.

(15)

Reducing conditions are essential to releasing the initial Cl,; oxidizing conditions are required for the corrosion to proceed. Consequently, cycling oxidizing conditions promoted by flame impingement produce the worst conditions. Lee reports that FeC12 has a specific volume of 11 times that of the reacted metal. Its formation will induce non-protective porous scales. Linear corrosion rates could result from repeated scale spalding.” Krause et ~1.” found FeClz as a corrosion product in the metal temperature range of 148-260°C. Although FeC12 is somewhat volatile, Krause74 reports catastrophic corrosion does not occur until its melting temperature is reached at 508°C. The volatility and melting temperatures of FeC12 and FeCl, appear in the extended version of Jackson’s48 curves for vapor pressure of alkali metal salts. On low-temperature surfaces, HCl may react with the protective coating of Fe203 to form FeC13. Thus, 1/3Fe20s + 2HCl=

2/3FeCls + H20.

(16)

Experiments by Mayer and Manulesco” indicated 0.2 volume percent can make an Fe20s layer porous while 0.8 volume percent will completely disintegrate it. The FeCls is extremely volatile and will not remain on tube surfaces in most steam generators.”

48

R. W. Bryers

Fig. 18. Analysis of gaseous and solid species samples on the rear wall of a 120-MW boiler. Reducing conditions are found in areas of high corrosion. Excess oxygen is found in areas of low corrosion. Chlorine content of coal is 0.5-0.6%.” Brooks and Meadowcroft and Clarke and Morris” investigated the combined effect of CO and HCl in the laboratory and compared the results to field experience. The addition of HCl to the simulated combustion gases caused a transition from protective parabolic oxidation curve to linear kinetics, which increase by a factor of three as the temperature increases from 400 to 5Oo”C, as illustrated in Fig. 17.76 At the higher temperature, the corrosion rate appeared to be independent of HCl above 400ppm.

The saturation effect may have been due to a limit imposed on scale porosity formation by the rate of volatilization of Fez03. The laboratory-scale morphology and corrosion rates were comparable to that of full-scale operation. In all cases, the scales were multi-layered, consisting of a loosely adherent, large grained, porous outer layer of FeS on top of inner bands of FeS and Fe304. The occasional evidence of intergranular penetrations in full-scale furnace wall tubes was missing in the laboratory. There was no

Steam-raising

INFLUENCE OF COMBUSTlON AT A FIXED CHLORINE CONCENTRATION

fuels

49

INFLUENCE OF CHLORINE IN THE ABSENCE OF SIGNIFICANT VARIATION IN COMBUSTION OR BOILER OPERATION

WEIGHTED MEAN CHLORINE CONCENTRATION DURING TIME ‘1’

Fig. 19. Rationalization of plant corrosion rate data with respect to chlorine concentration and flame proximity. The degree of attack at a given chlorine level increases as the reducing conditions at the wall become more severe. Each number represents a different power station.” evidence of chlorides at the scale metal interface. Lee illustrates the impact of reducing environment on sulfidation-chloride enhanced attack in full-scale furnaces in Fig. 18.” Lee and Whitehead’s” plot of corrosion rate with respect to mean chlorine concentration (shown in Fig. 19) provides an empirical quantification of the chloride-enhanced attack of furnace walls. The role of chlorine is simply believed to be one of destroying the protective oxide layers of the tube.7’X72 Experiments simulating superheater surface indicate that under oxidizing conditions, chlorine does not engage directly in the metal wastage mechanism. In sodium-free fuels, it was found that chlorine released potassium, which then engaged in the more traditional sulfidation corrosion.” From these and other experiments by Gibb and Angus,” it was concluded that chlorine, not sodium, was the controlling factor in releasing volatile potassium essential for alkali sulfate attack. Quantitative data appearing in Fig. 17 indicate superheater attack is a linear function of the chlorine content of the coal containing

more than 0.15% chlorine. Since the role of chlorine is one of making potassium available in a corrosive form rather than engaging directly in the corrosion mechanism, it is unfortunate potassium was not considered a parameter in the empirical correlation. Despite the demonstrated dependency of corrosion associated with chlorine on the presence of CO, the empirical index of 0.3% Cl in coal is still used as a means of selecting alternate coals to avoid chloride attack. 3.5. Fouling of Convective Heat Recovery Surface Corrosion-free fouling of convective heat-transfer surfaces did not become a problem until the late 1960s when low-sulfur/high-sodium lignite became a viable fuel for generating electricity from steam. The cooling surfaces of a steam generator are designed such that the flue gases leaving the radiant heat exchange portion of the combustion zone are cooled to a temperature slightly below the initial deformation temperature of the coal ash. In this

50

R. W. Bryers

,

A 8 C D E F G

30 15 7 7.5 5 5 5

1148 1062 985 876 806 755 562

31 31 38 39 43 44 37

’ S, is the- centerline space between tubes in the lane perpendicular to gas flow

Fig. 20. A typical convection bank design for subbituminous coals.‘* Table 5. A comparison of boiler design for variously ranked coals and evolutionary changes to mitigate fouling and slagging”’ Plant

Public Service Indiana

Commanche

Belle River

Colestrip 3, 4

Antelope Valley

Fuel

Bituminous

Subbituminous

Subbituminous

Subbituminous

Lignite

Start-up/capacity (gross) Furnace dimensions, ft: width depth height volume Design heat release furnace volume plant area Design furnace exit, “C

1982/68 41 _ 519,600 12,000 2.07 1315

1973/360 45 40 IO 238,000 12,540 2.10 _

l984-85/678 82 51 184 678,000 9,480 1.70

1984-851776 97 46 217 745,000 9,500

1.83

1984/440 65 54 273 858,000 9,500 1.83

II21

1037

1065

54 24

125 25

None _

Tube spacing: side-to-side, in. Radiant panels and/or platens

first second

35

119 I7

Conventional bank

first second third fourth fifth sixth seventh

17 8 4.375 4.375 5.375 _

8.5 6.4 4.0 4.0 _

24.0 12.6 9.0 4.5 4.5 4.5 5.8

10.0 5.0 4.5 4.5

24.5 11.0 9.0 6.0 6.0 _

_

Slagging limits load

Occasional

Occasional

Occasional

Never

Never

Fouling limits load

Occasional

Occasional

Never

Never

Never

Steam-raising fuels

Formation of White Layer: - Vapor Phase and Small Particle Diiusion,

possibly by van der Waals and electrostatic forces.

Tg= 1360 K (1093 Vg = 10 Mlsec

Difksion EERC 5807307

Fig. 21. Formation of initial deposit in fouling deposits79 (courtesy of Dr. S. Benson. University of North Dakota).

manner the designer tries to generate dry, innocuous fly ash before it enters the convective heat recovery zone employing tightly-spaced tube bundles immersed in the flue gas perpendicular to the direction of flow. To further minimize the potential for deposition, the convection bank is preceded by a screen section of cooled water which acts as a barrier to radiant heat exchange and cools the passing flue gas laden with particles-an additional 50°F. The first rows of tubes are generally arranged on wide tube spaces to minimize the collection efficiency. As the flue gas is cooled and the potential for deposition diminished, the spacing between tubes is reduced to improve heat transfer, as shown in Fig. 20 and tabulated in Table 5.‘,“’ During the early stages of pulverized-coal firing when steam pressures were low, a large portion of the enthalpy extracted from the flue gas was used to generate steam. Consequently, the duty of the convective heat recovery zones was low and all economizer superheater and reheat surfaces could be installed at flue gas temperatures below the initial deformation of the coal ash. With an increase in steam pressures, the energy required to superheat and reheat steam became a much larger portion of the total duty of the steam generator. The surface-tovolume ratio of the furnaces also decreased. This made it necessary to immerse some convective heating surface in the furnace cavity where it would be subject to radiant heat exchange if the ash melting temperatures were to be maintained as the criteria for the furnace exit temperature. Consequently, very widelyspaced pendant superheated surface was hung in the

upper furnace as boiler capacity and steam pressure increased. Deposition of ash, no matter what its physical state on convection heat-transfer surfaces shielded from furnace radiation, became known as fouling and the ash accumulations were referred to as bonded deposits. Bonded deposits are characterized as having an inner layer, composed primarily of sulfate, conforming to the profile of the tube and an outer layer contoured by the flowing gas and having a composition approaching that of the entrained fly ash. as shown in Figs 21-23.79 The transitional layer may be composed of a fused molten ash or lightly sintered particles an order of magnitude larger, usually enriched with one of the minor basic constituents in the coal ash, as shown in Fig. 22.79 The outer layer may be semi-fused or sintered, as illustrated in Fig. 23. Frequently the bonding material wetting the fly ash trapped in the outer layer is composed of the same material found in the initiating layer. Bonded deposits are most frequently formed by the alkalis, calcium, potassium, and silica-each creating its own type of deposit peculiar to a specific temperature regime in the heat recovery section. Fouling by bonded deposits was first observed in low-pressure stokers. Silica, the only acidic constituent in coal ash, deposited as the result of a fume formed by the volatilization of SiO? under hightemperature reducing conditions found on the grate of stokers.“,” Padia et al.” have shown in drop tube experiments that as much as 4% of the silica may be released at temperatures of 1557°C as SiO: and 25530% may be released in the presence of

R. W. Bryers

52

Transitionfrom White to Sinter Layer: - Inertial Impaction - Vapor Phase Deposition of - Adherence of Particles -C of Particle Stickiness and Formation of Liquid in the Deposit - Surface Tension Forces

Fly Ash and Products of Combustion

Rebounding Particles

Liquid in Deposit Due to Temperature increase Fig. 22. Formation

of transitional

deposit layers in fouling deposits79 (courtesy of North Dakota).

of Dr. S. Benson, University

Formationof the Outer Sinter Layer:, - Inertial Impaction Prime Mode of Transport - Vapor Phase Deposition Decrease Due to Higher Temperature - CarWe Surface Collects All lipacting Particles

Liquid Phase -I

Fig. 23. Formation

I

I

of outer layers in a fouling deposit79 (courtesy Dakota).

graphite at temperatures of 1694”C.s2,s3 Makowskyx5 demonstrated similar results in crucible tests in which she reported a white fume of submicron particles.84X85 In a laboratory pilot plant, Ulrich et al.86 indicated that submicron fly ash formed by vaporization of fly ash may account for 1% of the fly ash but may represent 99.5% of the total number of ash particles. However, silicon has not been associated with fouling problems in pulverized-coal-fired boilers, possibly due to the improved access of combustibles to combustion air. Phosphorous, found primarily as the minor

of Dr. S. Benson,

University

of North

mineral constituent fluorapatite [Ca,oFl(PO)4)6], has on occasion been responsible for deposits in superheaters and economizers of stoker-fired boilers. Under reducing conditions at temperature exceeding 1593°C. such as may exist in deep fuel beds found in stokers, the fluorapatite decomposes releasing elementary phosphorous. Oxidation of the phosphorous in the presence of moisture downstream of the combustion process produces phosphoric acid, which may condense on heat recovery surfaces and react with fly ash to form hard, insoluble crystalline deposits have not phosphate.snSx4 Phosphate-bonded

Steam-raising

Fig. 24. SEM photomicrograph illustrating heterogeneous condensation of CaO on silaceous fly ash spheres in the vicinity of the tube surface and subsequent CaS04 bonding.

been a problem in pulverized-coal-fired boilers due to the rarity of a significant concentration of fluorapatite in coal, lower flame temperatures, and improved distribution of air. Calcium-sulfate-bonded deposits generally form in the cooler portion of the heat recovery zone dedicated to heat exchange between the flue gas and the primary superheater, reheater, and economizer. The deposits are associated with coals high in calcium and low in alkalis. Bonding of the fly ash is accomplished by crystals of calcium sulfate, commonly attached to sites on adjacent fly ash particles as shown in Fig. 24.x7X88The calcium-sulfate crystals grow as the result of sulfidation of a calcium fume formed by submicron particles scavenged by fly ash particles. Sulfidation is Ii- .ited to those particles depositing on the tube surface where the residence time is sufficient and the S02/S03 levels are high enough for the sulfidation reaction to go to completion. Calcium-sulfate-bonded deposits are frequently initiated by a molten phase separated from the tube surface by a minute layer of submicron particles of CaS04. The fused layer is probably due to sulfidation of the submicron calcium particles in a zone where convective heat transfer ‘is insufficient to dissipate the heat of reaction.88X89 With continued growth of the deposit, fireside layers may become fused as the CaS04 decomposes in the presence of quartz and clays at temperatures exceeding 9544982”C.88-93 Generally, calcium-sulfate deposits are associated with coals rich in organically-bound calcium and low in sulfur, which most frequently include the low-rank coals. Details of the formation of CaS04-bonded deposits will be deferred to the section on mechanistic approach. Alkali-bonded deposits occur at high flue gas temperatures in the vicinity of the entrance to the convective heat-transfer pass (i.e., 982-1260°C).

fuels

53

Although the deposits were observed in stoker-fired boilers, they first became a nuisance while firing highsodium bituminous in Central Illinois, the U.K., and Australia. They became a major problem when the North American lignite reserves opened in the late 1960s. The older bituminous coals responsible for alkalitrisulfate deposits generally contain high levels of potassium (1.553% by weight K20 in the ash) and sulfur (l&4% in the coal) in addition to sodium chloride and organically-bound chloride. The deposits were initiated by a thin layer of alkali sulfates or trisulfates, depending upon the SO3 level at the tube surface. During the process of combustion, the volatile sodium released from the decomposition of sodium chloride and feldspars interacting with chlorine formed several volatile species (i.e., Na, NaOH, Na,O), which subsequently reacted with SO3 concentrated in the flue gas boundary layer at the tube surface and condensed as Na2S04. Chlorine occurring in significant concentrations, released potassium from the mineral feldspar, creating a second alkali species that also formed a sulfate at the tube surface. With continued growth of the deposit and retention of fly ash on the stick alkali-sulfate surfaces, the deposit fireside surface temperature increased and equilibrium once again favored the formation of silicates with the release of S03. The molten, viscous surface of the fly ash particles enriched with alkalis wetting contacting surfaces of adjacent particles and froze as the local temperature dropped with an increase in deposit thickness and a reduction in heat flux. This left a sintered or bonded deposit and confirmed the Reid et al. theory of direct deposition of alkali sulfates on tube surfaces.’ Utilization of Australian and North American lignites in the 1960s for generating steam introduced severe-to-catastrophic fouling also due to sodiumsulfate-bonded deposits. Unlike bituminous, the lignites contain very little sulfur, chlorine or potassium; however, they are rich in calcium and sodium appearing as organically-bound mineral matter. Calcium and sodium in this form volatilize during combustion forming a fume at some point in the postcombustion process composed of NaOH vapor andi or submicron particles of CaO, which condense on heat-transfer surfaces making it receptive to further deposition. Benson et al. 79 illustrate in Table 6 how the composition of the deposit changes with growth. Numerous empirical approaches were taken to quantify alkali-bonded deposit fouling. It was recognized that the initiating mechanism was dependent upon condensation of a volatile species and consequently was also dependent upon concentration of the alkali in the flue gas and thus level of concentration of the alkali in the fuel. Michel and Wilcoxson reported fouling became a problem once the alkali content of the coal exceeded 0.8% by weight. Attig and Duz~~~ introduced a range of concentrations of sodium reflecting different levels of

54

R. W. Bryers Table 6. Crystalline phases found in deposits and fly ash of beulah lignite79 Deposit

Fly ash

Inner sintered layer

Outer sintered layer

CaS04 Na$04/NazCa(S04)2 FezOJFe304/MgO Akennanite Hauyne Quartz

CaS04 Melilite Hauyne Fe203/Fe304/MgO Quartz Plagioclase Na2S04 Na&a(SO&

Quartz Fe203/Fe304/MgO Na2S04 CaSOI Na2Ca(S04)2 CaO AlzSiOS KSO, Amorphous phases

Table 7. Summary of key empirical correlations for slagging’02 Values

Test/fuel/index 1. Ash fusion temperature

Used to select maximum furnace exit temperature

Application: all fuels Index: initial deformation softening (Sph.) Softening (hem.) fluid 2. T Aiplication: all fuels Index: T,, x Tsocten

Temperature at which viscosity changes from Bingham Plastic to Newtonian

3.

Dry bottom low medium high severe

T250

Application: all fuels Index: temperature at 250 poise-the critical limit for slag tapping

Wet bottom

T250

>2325 T 2250-2100 2275-2050 <2200 0.5 max. for dry bottom >0.27 min. for wet bottom

Tzso < 2600°F

4. Base-to-acid ratio (B/A) Application: all fuels Index: C %(Fe203 + CaO + MgO + Naz + &O) C %(Si02 + A1203 + TiOz) 5. RS-slagging Application: eastern bituminous Index: (B/A) (%S on dry coal)

6. Slagging index T, Application: all fuels Index--T,: (TFT - Ts.T.) +~/~(TI.D.) 7. Slagging index R,, Application: all fuels Index: R,, = [TZso(oxid.) - T1om(red.)975.

xfsl

<0.6 0.6-2.0 2.0-2.6 >2.6

Degree of slagging low medium high severe

T, 2450-2250 2250-2100 <2100

Degree of slagging medium high severe

R”,

Degree of slagging medium high severe

4

0.5-0.99 1.o- I .99 >2

where: f, is a correlating factor corresponding to main temperature between oxidizing and reducing viscosity 8. Iron/calcium ratio Application: all fuels

Eutectics formed between 0.3 and 3.0 increase slagging

Index: Fe203/Ca0

fouling by sodium in bituminous coal. They also made a distinction between volatile and non-volatile sodium minerals by determining the water-soluble portion of the total sodium found in the coa1.95 Duzy and co-workers,95,96 Grondhoud et aL9’ and Sedor et aL9’ extended the concept to lignites. Borio et aL9’ modified characterization of the alkalis in coal ash to

include the acid-soluble components. This was again modified at a later date by Miller and Given,loo and Benson and Helm”’ to include water- and acidicsoluble ammonium acetate portions as determined by a chemical fractionation process. Winegartner”’ summarized all of the fouling and slagging indices in a lexicon and published the results as an ASME

Steam-raising

Table 8. Summary

55

fuels

of key empirical

correlations

for fouling”” Values

Test/fuel/index RF,

Rk

Application: eastern bituminous only Index: RF = (B/A) [%Na*O (ASTM ash)] R; = (B/A) [% water soluble NazO in LTA ash]

Sodium content of ASTM coal ash Application: all coals Index: % NazO in the ash

G

Degree of fouling

>l.O

0.7

medium high severe

Lignitic % NazO

Bituminous % NazO

Degree of fouling

<2.0 226 668 8

<0.5 0.551.0 I .O-2.5 >2.5

low medium high severe

<0.2 020.5 0.5 1.0

Alkali content of ASTM coal ash Application: all coals Index: C (% NazO + 0.6589 % KzO) Alkali content of coal Application: eastern bituminous coal Index: C (% NazO + 0.6589 % KzO) (% ash/IOO)

Ash sintering strength Application: all coals Index: Compression strength prepared fly ash

of specially

Chlorine Application: all coals Index: % Cl in coal

booklet. These indices have been accepted as an industrial standard and are presently used as a guideline for characterizing fouling and slagging. Winegartner’s results are summarized in Tables 7 and 8.“’ The slagging and fouling indices were developed from field and pilot plant data using specific coals. Extrapolation of this data to coals from other sources may be subject to considerable error. An evaluation of the indices by Barrett of Battelle Institute by means of a survey of 130 boiler operators in the U.S. and Europe indicate they were only 50% accurate.‘03”04 One of the main flaws with the indices was that other than furnace exit temperature, sometimes poorly defined, they did not include operating or design parameters. A second survey of design practices by four major boiler manufacturers for a set of six coals of varying rank indicated the manufacturers did not agree on the impact of coal impurities on boiler design. An examination of the indices indicates they are simply based on ash fusion temperatures, viscosity, and ash chemistry which have already been shown to be loosely related. In the case of fouling, there are no

IOW

Same as 2. above % Alkali

Degree of fouling

<0.3 0.330.45 0.45-0.6 >0.6

low medium high severe

Strength of fly ash at 1700°F IO00 1000-5000 5000- 16 000 >I600

Degree of fouling

% Cl

Degree of fouling

<0.2 0.220.3 0.330.5 >os

low medium high severe

low medium high severe

corrections to compensate for major concentrations of other elements such as calcium, and ash concentration is overlooked. The latter should impact on total surface available for absorption of volatile species as well as impact on the quantity of ash impacting on a surface and its collection efficiency. It has already been shown that slagging and fouling manifest themselves in a variety of ways depending upon local environmental conditions, dominating modes of transport, and local boiler geometry--giving rise to varying degrees of deposition and deposit chemistry. The simplified indices do not address operating and design parameters or variation in mineral contents. 3.6. Sintering Tests Subjecting fly ash deposited on tube surfaces to high temperatures for long periods of time causes particle-to-particle bonding at temperatures well below the onset of melting, as defined by the ASTM initial deformation temperature. This sintering has been observed to occur over a range of temperatures as low as 750-1000°C for Eastern bituminous coal,

R. W. Bryers

56

Temperature,

“C

800

900

1000

I

I

I

Sintering

1100

I

Tempercture,F

Fig. 25. Effect of alkali content in coal on strength of sintered fly ash from Illinois coal as determined by Barnhardt.‘06s’07 producing compressive strengths of 40,000 psi.‘05,‘06 As already noted in the discussion on types of bonded deposits, the bonding mechanism may be due to a chemical reaction as in the case of sulfidation or interparticle wetting by the viscous flow of a multicomponent system. Kuczynski,“’ as early as 1949, indicated that at least four different mechanisms could be responsible for particle-to-particle bonding, including viscous flow, chemical reaction, diffusion, and surface tension. Barnhart and co-workers106~‘07introduced a sintering test as an index of ash fouling tendency of Eastern bituminous coals as early as 1956. The test was performed on fly ash generated in a very specific laboratory combustor simulating residence time and temperature in full-scale steam generators. The fly ash was ignited to 490°C to remove carbon and then crushed to a specific size and pelletized to conform to a standard. The pellets were heated in a furnace at sintering temperature for 15 h. The pellets were dressed and subjected to compression testing in a standard tensile machine. Figure 25 illustrates the results for Illinois coals.‘o63’o7The procedure must be followed precisely, including the generation of the initial fly ash. The results provide a guideline for fouling that is restricted to high-alkali Eastern bituminous coals responsible for sodium- and potassium-bonded deposits dependent on viscous flow of complex sodium and potassium silicates and whose fly ash includes previously formed complex alkali-silicates. The curves in Fig. 26 indicate the sintering strength increases exponentially with temperature and can be represented by the Arrhenius

equation: C = AemEIRT.

(17)

Consequently, Barnhart’s data can be illustrated on a semi-log plot vs the reciprocal of the temperature, as shown in Fig. 26. In this way, comparisons can be easily made with those of other investigators. Barnhart and Williams”06 sintering tests spawned application of the procedure on new fuels and development of new techniques involving conductance, ash shrinkage, etc. Other investigators have found there is a lack of consistence between measurements performed on field-generated fly ash, fly ash generated in the laboratory, and ASTM ash. This stands to reason as the sintering process must reproduce the precise manner in which the ash is deposited if the results are to be representative of fouling in the convection bank. At least two different types of sintering have already been identified as being responsible for fouling in the heat-recovery zones of steam generators. The same sintering test could not be expected to simulate both types of fouling, particularly if they occur simultaneously or in series. Application of Barnhart’s sintering test to lignites indicated the criteria for fouling established by Barnhart did not apply.“’ However, the correlation of sintering strength with temperature appears to agree reasonably well with correlations of deposit weight on field probes and pilot plant probes with temperature. The results appear to be more favorable from a mechanistic approach than an empirical approach. In subsequent investigations, measurements of

57

Steam-raising fuels

BECOMPOSITION Decomposition

I

of CaSO,

Decomposition of

’ --_----’ Pure CaSO,

in the Presence of Illite, etc.

I

MELTING

TEMPERATURES 4

Na-Al-Silicates Na,SO, *

ESo4)2 1600°F

ZOOO'F

1800°F

2200°F

24OO'F

100,000 Jnvestiqatar Barnhart

0.75

Forms of Sodium Silicates

Gibbs

Silicates & Chlorides

Walchuk

Organic

CClMl

organic

Selle

Organic

Cunning

Silicates & Chlorides

Yilu

Silicates

0.65

0.70

0.60

Sintering Temperature, Fig. 26. Summary

of sintering

test data for various

resistivity and ash shrinkage were used as indicators of the formation of a molten phase in a homogeneous, compacted, granulated ash as the temperature was increased, thus indicating the onset of the viscous flow sintering process. The initial tests were performed

l/T

0.45

0.50

0.55

0.40

x IOOO-‘F

coals compared

to Barnhart’s

original

data

at temperatures exceeding those used to determine sintering rates for slag Particle size and alkali concentration effect on the rate of sintering. The same principles were used to

by Barnhart applications. had a strong evaluate

the

58

R. W. Bryers

impact of temperature on sintering. In most cases, sintering measured by resistivity was coupled with tests on shrinkage and compressibility. Conn and Jones”” test was applied to lignitic ashes rich in sulfates and calcium. Sintering was observed slightly below the melting temperature in ASTM ashes (i.e., @SC) and between 700-800°C for fly ash.“’ The mineral species hauyne nosean [(Na2Ca), . 5AlSi04(S04)], whose melting temperature falls in this range, was found to be present. The sintering temperatures defined by Conn and Jones coincidentally agreed very well with melting points defined by differential thermal analysis endotherms reported by Bryers8’ for the decomposition of sulfates in the presence of silicates. The mechanism for sulfatic bonding, however, is quite different. The compression test appears to evaluate the fouling tendency very well and agree with Barnhart and Williams”09 test although the ash composition was different. The results are somewhat low for predicting actual fouling in steam generators based on bulk gas-stream temperatures. Cummingeral.“O and Yilu”’ investigatingsintering due to bituminous and anthracite ashes from England, North America and Chinese, all report sintering in the ‘non-fouling range’ reported by Barnhart. Cumming et cd.“’ explained sintering using the ‘necking’ theory proposed by Raaskz2 in which the neck formed between adjacent particles was the source of ash shrinkage and simultaneous change in resistivity. The neck is driven by surface tension forces and increased inter-particle content areas reducing the ash resistance rather than increasing ionic migration due to the molten phase.“’ Several investigators evaluated additives synthesized in the laboratory rather than generated during combustion and indeed found a reduction in sintering.“2”3 Stallmann and Neave1114 investigated sulfides under reducing conditions as a means of examining deposits found in pyrolyzers with melting temperatures well below the predicted ASTM initial deformation temperatures. The ashes were found to sinter between 537-777°C. The data may be applicable to low-rank coals fired with a deficiency of air.‘14 Sintering via viscous flow is a process dependent on time and surface tension and inversely proportional to the square root of the viscosity and particle size. The viscosity decreases exponentially with temperature. The change in surface tension with temperature is small. The sintering process in situ in the steam generator occurs as a result of condensation of alkalis on silicates. Consequently, as proposed by Jackson, the alkalis are concentrated at the particle surface rather than occurring as a homogeneous spheroid of fly ash.47 As the temperature increases, the alkalis flow as silicates away from the sphere forming necks between adjacent spheres as well as diffusing inwardly. The composition of the surface changes with time and temperature as more silica goes into solution and alkali flow away,

unless the alkalis are replaced at a comparable condensation rate. The latter is not simulated in the laboratory testing, nor is the difference in composition between fly ash and deposited ash. Fly ash, be it generated in the laboratory or the field, has a decidedly different composition from deposited ash, particularly in regard to the surface concentrations of sulfur and alkalis, as the latter react with the silicates in the immediate environs of the tube surface where residence time is long. Rindt et ~l.,“~ Benson,‘15 Bryers,87 etc., have reported that the mellites form at the tube surface as a result of decomposition of sulfates in the presence of silicates. Furthermore, Rindt et ~1.“~ reported a significant change in composition of sodium silicates as one proceeds from the tube surface to the fireside surface of the deposit. The lower melting hauyne noseans are found near the cooler tube surface along with sulfates, whereas the higher melting gehlenites are concentrated in the outer layers. Finally, not only does the composition of fly ash change with rank, but so does the sintering mechanisms. It may be that viscous flow sintering only applies to the outer portion of deposits formed by higher rank coals. The sintering tests are highly empirical and should not be extrapolated from one fuel type to another unless the mechanisms of fouling are clearly understood, the local environmental conditions under which fouling occurs are well defined, and the sintering procedures are upgraded to properly represent the sintering mechanism taking place in the steam generator. 3.7. Pilot Plant In the early 196Os, investigators began to use pilot plants firing coal at a rate of about l,OOO,OOO B.t.u./h to evaluate the fireside behavior of coal ash. Presently, there are 60 or more combustors owned and operated by industry, government, and academia dedicated to the examination of fouling and slagging. Most of the combustors have been catalogued in a register organized by Higgins and Morley”’ as a project sponsored by the ASME Research Committee on Ash Deposits and Corrosion Due to Impurities in Combustion Gases. Although the thrust of the combustor work is directed at evaluating fouling, some investigators have been successful at evaluation of slagging. The combustors have been fired vertically up, down, and sideways. Most use a refractory-lined chamber with and without supplemental cooling. As one reduces the size to fit the economic constraints of the laboratory, the surface-to-volume ratio increases to a level that prohibits water cooling if the desired furnace exit temperature is to be met in a reasonable residence time. Slagging must be evaluated on a semiquantitative basis. To match absorption rates, gas temperature, and surface temperatures, it is desirable to immerse more than one probe in the furnace.

59

Steam-raising fuels 8000

YALLOURN COAL BOILER PERFORMANCE-COAL

700

A

-

QUALITY RELATIONSHIP

YALLOURN ‘C’ STATION

- 40 weeks MORWELL SOILERS s 4 s

6000

-

5ooo

_30

V E F

weeks

Y In 9 p

4ooo-

f I 9 a

- 20 weeks 3000

-

2000

-

z B $!j

- 10 weeks

1000

YALLOURN X MORWELL

0 0

I

I

I

I

I

0.1

0.2

0.3

0.4

0.5

A MORWELL COAL 0.6

FOULING INDEX

Fig. 27. Fouling index vs boiler availability.“9

Probes oriented parallel and perpendicular to the direction of flow should be included to monitor the difference in slag formation by coarse particulate depositing by inertial impact and fine particulate depositing by thermophoresis or diffusivity. Fouling probes are usually installed in the convection pass at the furnace exit where they are shielded from direct flame radiation. Characterizing the fouling potential of a coal ash is the primary objective of each pilot plant. Although fouling probe tube size, spacing and orientation as a bundle should simulate thermal and fluid simultude to full-scale bundles, the arrangements vary considerably from facility to facility. Few fouling probes are shielded from refractory sidewall radiation. All probes should be monitored for axial and circumferential temperature and have access to continual visual inspection. Rates of deposition are difficult to evaluate on a continuously operating facility except by continuous recording of heat flux. Changes in deposit composition with temperature or time can only be monitored by using multiple probes or performing numerous tests using time as a parameter. Since mineral species vary with rank and geological origin of the coal and each rank of coal

has its own rate of deposit accumulation. Hence, the testing interval must be geared to specific fuels. In essence, each combustor is an empirically calibrated instrument for evaluating slagging and fouling of a specific fuel type and must have a substantial data base to be of value. Deposits should be analyzed in depth to understand the mechanism involved in depositing ash in the combustor. By so doing, there is some hope for translating the deposition to other operating conditions beyond the scope of the combustor. The combustor built by the State Electricity Commission of Victoria is an example of the ultimate empirical use of a combustor for evaluating fouling of coal from a dedicated source.“8.“9 Garner ‘I8 performed a statistical analysis on the rate of deposit formation as a function of ash chemistry. Two ways of expressing the relation sought were explored--the first took the form of a linear expression:

“i’ = a[A] + h[B] + c[C] + d [D] + I + K

(18)

where y is the weight of deposit, [A], [B], [Cl, [D], etc. are the various inorganic constituents in the coal, and a, h, c, d, etc. are coefficients to which the

60

R. W. Bryers

System Temperatures 10. Combustor Bottom Refractory 1. Primary Combustion Air 2. 3. 4. 5. 6. 7. 8. 9.

Secondary Combustion Air Tertiary Combustion Air Tertiary Combustion Air Comb&or Exit Probe Bank 1 Exit Probe Bank 2 Exit Probe Bank 3 Inlet ESP Inlet

11. Combustor Middle Refractory 12. Combustor Top Refractory 13. Probe Bank 2 Exit Refractory Stack

PI

Sample

Cyclone v

> /I Pulverized Coal Feeder

Draft Fan

Bypass Cyclone Electrostatic Precipitator and Control

u

EERCJGl1331.CDR

Combustor

Fig. 28. University of North Dakota upfired furnace.‘*’ multiple linear regression analysis describes a level of significance; the value K is a constant. The second form was related more closely to the way in which chemical reaction rates may be expressed: y’ = K[A] : [B] b [D] ‘!

(19)

This form could be evaluated by the multiple linear regression analysis when expressed in the form: logy’ = log K + a log [A] + b log [B] + clog [C] + d log [D]

(20)

The coefficients a, b, c, d, etc. in this equation being the exponents in Eq. (19). The equations developed by this statistical analysis give excellent correlation between calculated values for the weight of ash deposited and the actual weights measured in the furnace. The equations developed are as follows: y = 0.030 [TiO*] + 0.99 [Fez03] + 0.061 [CaO] + 0.264 [MgO] + 0.423 [Na20] - 10.6

(21)

and y’ = 0.044 [TiOz] “op4[FezOx] o“?99[CaO] “?*’ [Mg()] ‘? [Na20]0.728~ Sodium

was found

(22)

to be the most significant

constituent. Magnesium was surprisingly high on the list of significant constituents; its role, however, appeared to depend upon the presence of CaO and Na2Q118,119

The double salt MgS04. Na2S04 melting at 680°C was identified in the deposits from the Morwel coal. Magnesium sulfate was also reported but in small proportions compared to the MgO. The utility of Gardner’s correlation is illustrated in Fig. 27, showing how the expression was used to predict operating time between outages. The fouling index was also used to flag core samples in the mine field to permit blending of the coal and to maintain a low sodium level.“’ The correlations, however, are empirical and do not apply to any other coals. The University of North Dakota has probably accumulated the largest data base on firing low-rank coals. Their tests also included an assortment of other rank fuels and petroleum coke. Over 750 tests have been performed in this facility, backed up by extensive analytical capabilities for characterizing coal and deposits including CCSEM, chemical fractionation, SEMPC, X-ray fluorescence and X-ray diffraction.12’ The facility, illustrated in Fig. 28, has been used to quantitatively compare the fouling potentials of Western coals due to organically-bound sodium.12’ Careful, detailed analysis of ash deposits from the pilot plant as well as from the field have identified the role of sodium in high-temperature fouling. They

Steam-raising

61

fuels

Percent Ash ln the ioal 1 1

9- _---_

a._--__

Fig. 29. The impact

2

_

3

4

5

6

2

3

4

5

:

789

6789

,I(

-

.LSW

of ash and sodium

level on western coals on fouling laboratory combustor.

have shown, in Fig. 29, that the rate of deposit after 5 h increases parabolically with total sodium in the coal, and exponentially with total ash at a constant sodium level.“’ Unfortunately, the time in all tests is limited to 5 h-a serious limitation in all pilot plant testing. Examination of the deposit indicated an initial layer was formed consisting of NazS04 CaS04. Subsequent crystallization gives the deposit its strength. With increased growth in the deposit, the sodium and calcium mellilites and gehlenite form.‘2’ Sintering is not purely condensation or gas-to-solid

determined

empirically

in a

bonding, but includes viscous flow. Ultimately, gehlenite is formed at a level of which viscous flow becomes the bonding mechanism. Numerous investigators have shown that the alkali-bearing silicates formed as a result of the decomposition of CaS04 and/or Na2S04 in the presence of quartz and clays.86.“‘.‘23 Honea’ showed how an increase in deposit temperature implemented by an increase in tube surface temperature results in a decrease in concentration of the inner white alkali-sulfate layer while increasing the formation of gehlenite. Benson rf

62

R. W. Bryers

al.” reported the mineral composition of the inner layer as well as identified some low-melting minerals in intermediate layers in Table 6. Bryers and Walchuk ‘24 found in experiments with bituminous coals that the initial layer laid down on slagging probes was enriched with potassium originating as the mineral illite in the coal. Surfaces subjected to direct impingement of flue gases or high levels of turbulence were highly enriched with iron originating as coarse liberated pyrites. Thin films of iron were laid down by an iron fume on surfaces subjected to zones of highest heat release, suggesting fluxing of silicates by iron took place at the slag surface. He also found pilot plants are not always successful in simulating full-scale fouling or slagging. Pilot plants did not always accurately portray the fouling potential of the coal ash on heat-transfer surfaces. In the case of Texas lignites, hightemperature surfaces such as the refractory in pilot plants were prone to severe fouling, obscuring the fouling behavior of cooler probe surfaces and producing misleading results. Fouling was due to the formation of a low-melting silicate entrained in the flue gas, which was prone to deposit on heated surfaces. Cooler heat-transfer surfaces in full-scale boilers were not subject to fouling. Consequently, one must understand mechanisms by which ash deposits and differences in simultude between fullscale operation and pilot plants to effectively and accurately interpret results. Drop tubes have been used to examine the transformation of minerals during combustion and to study the impact of an isolated low-melting species in the fly ash on fouling. Moza and Austin3’ used a drop tube to examine the sticking potential and adherence of various mineral species on a substrate at temperatures and surface conditions simulating heattransfer surfaces in a steam generator. The apparent contact angle and adhesion force of the deposited particle to the substrate surface for various substrate temperatures and surface conditions was examined by Moza and Austin.35 They found that iron-rich glass, probably originating as the 2.95 pyritic-rich gravity fraction, and illite melt more thoroughly and remain fluid longer as a super-cooled glass contributing to greater adhesion. Partially decomposed pyrites gave good wetting and high adhesion strength at a substrate temperature as low as 3OO”C,as shown earlier in Fig. 10. 3.8. Summary of Empirical Approach Empirical procedures used to evaluate slagging or fouling are usually limited to a small range of fuels. Consequently, they work best for captive coal situations where variations in mineral matter are small, mechanisms of deposition are consistent, and design and operating variables remain constant. Most empirical indices assume a homogeneous mineral composition and distribution, which is not always

Table 9. Minerals occurring in coa1’7~48 Shale group Species: muscovite, illite, bravaisite, montmorillonite General formula: (K, Na, H30, Ca), (Al, Mg, Fe, Ti)4 (Al, SiOsOz@H, I%) Kaolin group Species: kaolinite, livesite, metahalloysite Formula: Alz(Si205)(OH), Sulphide group Species: pyrites, marcasite Formula: Fe& Carbonate group Species: ankerite, ankeritic calcite, ankeritic dolomite, ankeritic chalybite General formula: (Ca, Mg, Fe, Mn)C03 Chloride group Species: sylvite, halite Formula: KCI, NaCl Accessory minerals group Quartz Felspar Garnet Hornblende Gypsum Apatite Zircon Epidote Biotite Augite Prochlorite Diaspore Lepidocrocite Magnetite Hyanite Staurolite Topaz Tourmaline Hematite Penninite

SiOz (K, Na)zO-A1203 -6Si02 3CaO - Al,O, - 3SiOz CaO .3FeO -4Si02 CaS04. 2Hz0 9CaO. 3PzOs - CaFz ZrSi04 4CaO. 3A1203. 6Si02. Hz0 K20 - MgO - AllO . 3Si02. Hz0 CaO . MgO . 2Si02 2FeO. 2MgO. Al,03 - 2Si02. 2H2 A1203. Hz0 FezOx - Hz0 Fe304 A1,03 - SiOz 2FeO. 5A1203 - 4SiO2 . Hz0 (AlF)$i04 H9A13(BOH)2%019 Fe203 5MgO - A1203 ’3Si02 - 2H20

the case. The results are generated in the laboratory under heating conditions, whereas slagging and fouling occur under quenching conditions. In the

case of sintering, more than one mode of sintering may be taking place simultaneously or in series as the deposit develops. Consequently, one must know which sintering process dominates so that the operating conditions may be duplicated. Deposited ash or fly ash identical to that generated in a full-scale facility must be used. Despite numerous innovations in combustion system technology and substantial increases in combustion system capacity over the past 70 years, steam generators are still designed and coal is still traded on the basis of fusibility generated by techniques developed outside the technology and applied to stoker-fired boilers. Over 4000 individual investigations have been made to extend the range of applicability of the fusibility data, develop highly empirical indices, or examine the fundamentals of deposition and corrosion to obtain a better, more accurate procedure for characterizing the behavior of coal ash. The fundamental studies, although directed

Steam-raising fuels MAG. 3000x

MAG. 150X

MAG. 15X

MAG. 15X

A 6 C D -

Finely Dispersed Pyrite Crystals Undefined Aluminosilicate Clay Inclusion Quartz Inclusion lllite Inclusion

Fig. 30. SEM photomicrographs

of various

forms of pyrites,

at very specific situations, have made it possible to place limitations on gas or metal temperatures for specific categories of coal. All the research efforts, however, have been directed at establishing the potential for coal to create a fouling, slagging or corrosive situation. During this period of time, steam generators were designed for specific fuels and, for the most part, operated on a captive fuel source or longterm contract. Since the early 1980s the situation has changed drastically. Steam generators are expected to operate on a variable fuel supply, including coals of different rank containing decidedly different mineral composition. Fuels from new reserves with decidedly different mineral compositions are being introduced to the marketplace. In addition, fuels are being traded on the international market which contain mineral compositions whose fireside behavior is unproven. To meet these new demands, fuels must be characterized in greater depth and the fundamentals of mineral transformation, mineral composition, and mineral deposition must be more clearly understood by operators and designers.

E - Pyrite Veins F -- Pyrite Fracture S - Calcite

quartz.

Filling

and calcite in Illinois no. 5. coal

4. MECHANISTIC APPROACH

The empirical approaches to fireside behavior of impurities in coal frequently fall short of satisfactorily predicting fouling and slagging because they do not adequately characterize the fuel and its impurities, and they do not properly model the chemical processes taking place during combustion and post-combustion quenching. Coal is a heterogeneous substance composed of an array of mineral and coal components whose individual thermal and chemical behavior may be quite different upon heating. In the previous discussion, it was observed that partitioning of the elements due to volatilization occurs in the combustion process. In addition, there is a difference in the way minerals occur and are associated with carbon and other minerals within the total coal mass, which affect the true melting and solidifying temperature of each fly ash species entering the steam generator, as well as the quantity of mineral volatilized. Consequently, coals must be characterized to account for variations in mineral occurrence and

R. W. Bryers

64

COMBUSTION TRENDS Moisture

w

Decreases

>

Flame Temperature

>

Increases

)

X Ash

)

Increases

c

Volatility of Inorganic

4

Decreases _ Increases

)

c

Decreases

c

. Slagging Potential Fouling Potential GENESIS OF COAL

A PORTION OF Ca LEAVES SYSTEM

Greatest ORGANO-METALLIC COMPOUNDS

COMPOUNDS

MINERALS

I

* Silicates Si,Al,K.Na,Fe

Ca,Na,Fe

Na,Fe,Cad

SOLUBLE SALTS

MINERALS

??

*

Silicates S,Al.K

*

*

Carbonates Ca,Mg,Fe,Ba

Na,Ca

Si.Al,Ca,Fe.K,Ba. SURFACE AND GROUND WATER Na,Fe,Ca,Ba,S,Cl

BITUMINOUS

SUBBITUMINOUS

LIGNITE

PEAT SWAMP

*

Silicates r

*

Carbonates

*

:::xv

*

Sulfates Ca,Ba

F$;i!j' Carbonates Ca,Mg,Fe

SOLUBLE SALTS Na,Ca

Least '"'"""l'4~~1~"~~~~ WATERS-

(T;

= =

Major source of elements Traces

Fig. 3 1. Transformation

of mineral matter during coalification.

potentialpartitioningofmineralspeciestosuccessfully predict slagging, fouling, and/or corrosion.

4.1. Mineral Matter in Coal

Mineral matter in coal is generally considered to be the sum of all the discrete inorganic mineral phases and the organically-bound inorganic elements found in coa1.‘25V’26 There are over 125 different minerals found in coal. However, only the 25 reported in Table 9 have been recognized as occurring in significant amounts.‘7~48~‘27The minerals in coal occur as discrete grains, flakes, or aggregates in one of five modes: ?? Microscopically-disseminated inclusions with macerals. ?? Layers or partings wherein fine-grained clay minerals usually predominate.

Nodules including lenticular and spherical concretions. Fissures including cleat and other fractures or void fillings. Rock fragments, megascopic masses of rock material found within the coal bed as a result of faulting, slumping, or related disturbances.” Figure 30 illustrates some of these mineral forms. Harvey and Ruch’26 explain an understanding of the genetic classification of mineral matter inclusions in coal during the coalification process, provides a means for predicting the quality of coal in areas ahead of mining and exploration, as well as the degree to which the coal can be altered by beneficiation and mineral transformations predicted during combustion. The ancient coal seams comprising our present coal reserves were formed from vast peat swamps. The

Steam-raising fuels MAG. 3000x

MAG. 3200X

MAG. 4000X

Fig. 32. Various types of pyrite crystals found in different bituminous. SiO,

\

Number of Sample Locations

Appalachian

Basin

Western

Interior

Northern

Great Plains

Rocky Mountains

L-Fig. 33. Variation

\

”

50

AP, of principal

inorganic

Fe,O,

+ CaO + MgO

constituents in coal ash from regions.“’

swamps were located on coastal plains often adjacent to river deltas. Over periods of many centuries, the swamps subsided while layer upon layer of fallen vegetation covered the peat. This vegetation contained minerals in its tissues that had been drawn into the plant life during its growing stage. The fallen vegetation introduced the first genetic group of minerals into the swamp: detrital-deposited minerals formed by crystallization of inorganic elements incorporated into the tissues of living plants (inherent mineral matter > 1% of the coal ash). Detrital deposition also includes minerals introduced into the swamp as silt from discharging rivers, dust from wind storms, or volcanic eruptions. Ultimately, the swamps were drowned by discharging rivers or sea water; coastal swamps invaded by sea water were injected with sulfate. The life of the swamp repeated itself numerous times., each contributing to a new layer of residue. The second generic: group, syngenetic minerals, were formed during the peat stage by bacterial

various

U.S. coal basins

and

reduction of aqueous sulfates resulting in crystal growth in microscopic pores within plant remains. The second genetic group includes mineralization around nuclei or other centers forming nodules. These include coal balls and other plant replacement forms. The third group, epigenesis, includes minerals formed after the peat was identified as fillings in fissures and voids and as products of weathering or oxidation.‘26-‘30 The process of coalification is a slow, complex process involving increasing pressure and heat with time, driving moisture out. Methane and CO1 are generated and trapped within the pores. Several chemical processes take place, described as petrification, humidification, gelification, and vitrification. resulting in the transformation of vegetation into vitrinite-the primary maceral group. Partial oxidation creates char or fusinized debris classified as inertinite. With increased coalification, all macerals become enriched with carbon, eventually forming anthracite.‘26m I30

66

R. W. Bryers

During coalification, mineral matter undergoes transformation. The concentration of silicates and clays increases due to an increase in coal density. As the chemical bonds are destroyed, the alkali metals are transformed from organo-metallic compounds to dissolved salts and eventually silicates, as shown in Fig. 31.‘29,‘30 Organically-bound iron and iron carbonates are precipitated as a sulfide in voids, fissures and fractures, particularly in coal beds invaded at some point in time by sea water. Calcium precipitates as a carbonate or sulfate and in time, form calcite or react with silicates to form feldspars. Iron, which exists in some instances as carbonate in Iignites, is precipitated as a sulfide in bituminous coals in voids, fissures, and fractures. Consequently, the ratio of pyritic iron to total iron in the ash increases to nearly 100% as coal is aged from the lower ranks to bituminous. Figure 30 shows how iron and calcium are transformed from inherent ash and organometallic compounds at location A to precipitates in coal fissures at locations E, F and S within a bituminous coal. Figure 32 is an example of pyrites existing in a crystalline form, possibly generated during the syngenetic stage. The characteristics of the mineral composition varies decidedly with coal rank and with its original exposure to either fresh or sea water intrusion producing decidedly different fireside behavior. Glick and Davis13’ show the expected variability of the five key elements by geological basin and region in Fig. 33. Minerals occurring in aged coal may be classified into five main groups. These include shale, clay, sulfur, and carbonates. The fifth group includes accessory minerals such as quartz and minor constituents such as feldspar.16 Shale, usually the result of the consolidation of mud, silt and clay, consists of many minerals including illite and muscovite-these are forms of mica. Kaolinite is the most common clay minerals.‘4”6 The sulfur minerals include pyrites with some marcasite. Marcasite has the same chemical composition as pyrites but a different mineralogical structure. Sulfur is also present as organic matter and occasionally as sulfate. The latter usually occurs in weathered coal such as in outcrops. The amount of sulfate-sulfur in coal is generally less than 0.01%. Generally, 60% of the sulfur in coal is pyritic, particularly when the sulfur concentration is low. At higher concentrations, it may run as high as 70-90%. Pyrite occurs in coal in discrete particles in a wide variety of shapes and sizes. The principal forms are:‘4,‘32 ?? Rounded masses called sulfur balls or nodules an inch or more in size. ?? Lens-shaped masses which are thought to be flattened sulfur balls. ?? Vertical, inclined veins, or fissures filled with pyrite ranging in thickness from thin flakes up to several inches.

Small, discontinuous veinlets or pyrite, a number of which may sometimes radiate from a common center. ?? Small particles, greater than 2~, or veinlets disseminated in the coal. Microscopic pyrite occurs in five basic morphological types: -framboids; -isolated, well-defined crystals; -non-spherical aggregates of euhedral crystals; -irregular shapes; and -fracture fillings. The term ‘framboid’ is derived from the French word for raspberry and thus refers to naturally occurring spheroidal clusters of hundreds of cubic or octahedronal crystals.133Y135 All coals contain some of the third and fifth forms of pyrites, and some coals contain all five of the principal forms.125,134~‘40 The carbonates are mainly calcite, dolomite, or siderite. The occurrence of calcite is frequently bimodal. Some calcite occurs as inherent ash, while other calcite appears as thin layers in cleats and fissures. Iron can be present in small quantities as hematite, ankorite, and in some of the clay minerals such as illite. In addition to the more common minerals, silica is present sometimes as sand particles or quartz. The alkalis are sometimes found as chlorides or as sulfates, but probably most often as feldspars, typically orthoclase, and albite. In the case of lignites, unlike bituminous and sub-bituminous, sodium is not present as a mineral but is probably distributed throughout the lignite as the sodium salt of a hydroxyl group or a carboxylic acid group in humic acid. Calcium, like sodium, is bound organically to humic acid. Therefore, it too is uniformly distributed in the sample.101,‘36 The mineral composition of Australian coals differ substantially from that of North American and European coals. The Black coals contain from 10 to 17% ash and are low in moisture. The Victorian Brown coals contain less than 5% ash and more than 60% moisture. Virtually all Australian coals contain less than 1.3% sulfur and very little chlorine. Unlike Northern Hemisphere coals in which iron appears primarily as pyrites and marcasite, the iron in Australian coals occurs as siderite and ankerite. In the low-rank Brown coals, iron is also found as organically-bound mineral matter and frequently occurs in concentrations exceeding that of aluminum-not a trend usually found in Northern Hemisphere coals. Calcium is generally low and usually appears as a carbonate in the older coals and organically bound in the low-rank Brown coals. Magnesium, sodium, and significant portions of the potassium present also appear as organically-bound mineral matter. The highest concentration of sodium is found in the low-rank coals.‘4’ Despite the apparent order in mineral formation by geological origin, anomalies do occur. Some lignites ??

Steam-raising

Table 10. Chemical Initial (pg/g dry coat) Beulah lignite Na Mg Al Si K Ca Fe San Miguel lignite Na Mg Al Si K Ca Fe Eagle Butte subbituminous Na Mg Al Si K Ca Fe

fuels

fraction

% Removed by Ha0

67

results”’ % Removed by NH_,OAc

5700 2600 5180 7230 0 11,500 10,650

42

0 0

74

19,700 6200 11,030 348.800 0 26,300 12,700

28 0

1000 3300 6700 10,500 400 17.700 4000

% Remaining

1 2 22 2 *

0 0 77 93 *

I

26 27

0 72

*

72 16 0 2 *

0 0 7 0 *

0 84 89 98 *

0 0

65 0

25 49

10 45

60 9 0 0 75 7 8

40 55 0 0 25 34 0

0 30 37 0 0 58 78

0 6 63 100 0 2 I5

I

57 97

% Removed by HCI

0

I

1

4 *

*

have been found enriched with pyrites, which could be responsible for otherwise unpredicted slagging. In some instances, minerals are reported that are peculiar to a particular coal source. Zeolites are the leading sodium source in Texas lignites. Finkelman’42”43 reports high sodium levels in Utah, Wasatch Plateau coals as the mineral anaclime (NaA1Si0206 . H20), which has no record of fouling. 4.2. Mineral Anal,vsis Characterization of minerals in coal has been summarized for combustion engineers by Harvey and Ruch,‘26 and Finkelman and Gluskoter.‘43 They indicate sample preparation is crucial in characterizing the mineral content of coal. Physical separation, leaching, thin and polished sectioning, and low-temperature ashing are the four procedures commonly used in preparing coal for mineral characterization. Physical separation is a long, laborious procedure that is inefficient in releasing minerals from the coal, particularly in the small size ranges. In the interest of time and cost, partitioning of minerals and coal are performed on a very small size and gravity matrix producing results subjected to considerable error. Numerous solvents have been used to dissolve either the organics or inorganics in leaching processes. Problems have been encountered with destruction of minerals by organic solvents and significant hide-out of inorganics in the coal such as clays, pyrites, quartz and carbonates due to encapsulation of carbonaceous species. Thin polishing has been used for mineral identification using petrographic techniques. The low reflectivity of most

minerals in coal makes them difficult to recognize and identify. Low-temperature ashing by passing a stream of oxygen through an electromagnetic field produced by a radio frequency oscillator at temperatures less than 150°C which completely oxidizes the carbon leaving an unaltered residue of minerals. Unfortunately, artifacts such as bassinite are generated from the mineral matter comprised of organically-bound calcium. Similar artifacts are generated from organically-bound sodium, magnesium, and iron. They must be recognized as artifacts and not minerals indigenous to the coal. Numerous techniques are now available for characterizing minerals in the prepared samples. X-ray diffraction performed on low-temperature ash is a powerful tool and the most widely used technique for qualitatively identifying the presence of minerals in their crystalline form in concentrations of a few weight percent or greater. Samples as small as individual particles a few microns in diameter, to samples as large as several grains can be analyzed. The X-ray diffraction pattern is a signature analysis of the minerals, permitting positive identification of the minerals present. Although quantitative analysis of mineral concentrations in LTA is frequently reported, the technique requires further perfection.‘43 Thermal techniques such as different thermal analysis (DTA) and thermogravimetric analysis (TGA) have also been used as a signature analysis based on changes in physical properties with temperature. The properties monitored include temperature weight and energy. Although the system may work well pure mineral species, the signatures generated may be altered or masked

68

R. W. Bryers

substantially if the mixtures exist and minerals cannot be isolated. ‘43 Microanalytical techniques including scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX), electron probe microanalysis equipped with EDX, transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) have very good resolution to very small particle sizes. SEM can magnify from 10x to 20 000 x with a great depth of Focus and secondary electron image resolution to 150 A. STEM has spatial resolution in the order of Angstroms (lop4 pm). The techniques can be applied to raw coal samples as well as low-temperature ash, and provide visual images of the morphology of the mineral structure. The techniques are particularly useful in examining very fine particulate. These techniques only provide partial analysis for elements with atomic numbers 11 or greater. They are somewhat insensitive to sodium in low concentrations-a key element in the fouling/ slagging process. Prediction of the minerals present is strictly by inference. Several groups of minerals have very similar or even identical elemental composition, but decidedly different physical properties (i.e., pyrite and marcasite are both described chemically as FeSz). It is also difficult to predict quantitative mineral composition of a bulk coal sample from a microanalysis. This is partially being overcome by using a computer-controlled scan of a sample by SEM (CCSEM). One should understand, however, that the final mineral analysis is still by inference.143 Extended X-ray analyses (fine structure) [XANE] is another signature analytical technique most recently applied to coal and cokes for specific identification of the crystalline forms of minerals present.54 Mossbauer spectroscopy has been widely used for characterizing the mineral forms of iron in coal as well as in slags and deposits.‘43 4.3. Mineral Matter Analysis The inorganic elements occurring in coals as complexes ion-exchangeable coordination and cations rather than discrete mineral phases are determined by chemical fractionation-a process whereby the inorganic constituents are selectively extracted based on their bonding in the coal. The coal is first extracted with water to determine the quantity of water-soluble species. The residue from the water extraction is then mixed with 1 M ammonium acetate (NH40Ac). The ammonium acetate extracts the elements associated with coal as ion-exchangeable cations present, primarily as the salts of organic acids. The remaining residue from the ammonium acetate extraction is then stirred with 1 M hydrochloric acid to remove the elements as acid-soluble minerals such as carbonates and oxides. The elements remaining in the coal after the extractions are associated with coal as silicates, aluminosilicates, sulfates and oxides. Table 10 is an example of the application of chemical

Table I I. CCSEM mineral categories’50 Quartz Aluminosihcate K-aluminosilicate Ca-aluminosilicate Fe-aluminosihcate Iron oxide Spine1 Aluminium oxide Calcium oxide Dolomite Ankerite Rutile Calcium silicate Apatite Pyrite Gypsum Barite Gypsum/barite Aluminosilicate/gypsum Calcium aluminate Iron suIfate/pyrrhotite Calcium-rich Silicon-rich Periclase Unknown

fractionation to several ranks of coal to illustrate the expected variation in organically-bound or watersoluble mineral species.‘50 It is important to note that

a comparison of the elemental analysis performed by ASTM ashing and chemical fractionation reveals a deficiency in sodium level of the ashed coal. The levels of sodium in the chemically fractionated coal may range from 40% to several orders of magnitude higher than found in ASTM ash, possibly accounting for some discrepancies in the empirical fouling indices.lU 4.4. Mineral Distribution Complete characterization of the mineral content of coal requires some understanding of how the minerals are associated in the coal with each other as well as the carbonaceous fraction. Mineral associations will determine the chemistry and physical state of the individual coal fly ash particles during combustion and post-combustion quenching. Their association with carbon will influence partitioning during combustion by forming more volatile species as a result of their reduced oxide state as well as the maximum temperature they will achieve. The distribution of minerals between inherent and extraneous ash will also influence the extent to which coal can be beneficiated as well as the impact of beneficiation on the fireside characteristics of the deposited ash. Several investigators have used simple size and gravity partitioning techniques to examine the mineral distribution in coa1.“,‘3-‘5 The procedures are extremely laborious and full of pitfalls when dealing with pulverized coal, particularly in the fine size range. Samples are contaminated by the

Steam-raising

Table 12. CCSEM

analysis

69

fuels

of coals, results expressed

as weight percent

on a mineral

basis15’

Size range, km I .o--2.2 Be&h Quartz Iron oxide Rutile Kaolin&e Alumina Montmorillonite K Al-silicate Fe Al-silicate Ca Al-silicate Aluminosilicate Mixed Al-silica Fe silicate Pyrite Pyrrhotite Oxid. pyrrhotite Gypsum Barite Gypsum/barite Gypsum/Al-silica Si-rich Ca-rich Unknown Totals* *Totals

2.2-4.6

4.6- 10.0

10.0-22.0

22.0-46.0

46.0&100

Totals

0.0 0.0 0.3 0.3 0.4 0.0 0.2 0.0 0.0 0.1 0.5 0.1 0.1 0.2 0.0 3.0

2.1 0.0 0.1 0.0 13.2 1.4 0.5 0.0 0.1 0.1 0.1 0.2 0.3 0.1 0.0 0.2 0.3 0.1 0.0 0.0 0.1 1.9

2.8 0.0 0.4 6.4 0.0 0.2 0.2 0.0 0.0 0.0 0.0 1.7 0.0 0.0 0.8 0.8 0.0 0.0 0.0 0.0 0.0 1.2

3.8 0.0 0.0 0.1 2.5 0.5 0.0 0.0 0.0 0.0 0.0 0.0 3.8 0.0 0.4 0.5 0.0 0.0 0.0 0.3 0.0 I .o

4.2 0.5 0.0 0.0 3.0 0.2 0.0 0.0 0.0 0.1 0.0 0.0 13.7 0.4 0.0 0.4 0.0 0.0 0.0 0.1 0.0 0.4

3.3 0.8 0.0 0.0 0.3 0.3 0.0 0.0 0.0 0.0 0.0 0.0 5.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

17.4 1.4 0.2 0.4 35.1 4.6 0.8 0.2 0.4 0.6 0.5 0.2 25.1 0.5 0.1 1.9 1.7 0.2 0.1 0.7 0. I 7.5

18.7

20.9

14.5

12.7

23.1

10.1

100.0

1.2 0.0 0.1 0.0 9.7 2.1

may not be correct

due to rounding errors.

partitioning fluid. Separation of the gravity range of the partitioning fluid is extremely poor. Separation of small particles is interferred with by many extraneous body forces approaching or exceeding that of gravity. The scanning electron microscope and energy dispersive X-ray, coupled with the computer (i.e., CCSEM) are presently being examined as a means of determining the distribution of minerals in coal by size. The micro-analyzer can examine many epoxymounted particles in a reasonably short time. A direct comparison can be made between raw coal, char and fly ash samples using the same technique and set of assumptions. Computer automation of the scanning electron microscope (SEM) and energy dispersive X-ray (EDAX)-commonly referred to as computercontrolled scanning electron microscopy (CCSEM)is being developed to locate dispersed inorganic particles in coal and quantify their size, shape, and composition. When coupled with automated digital image analysis, morphological data can be stored, examined, and used to determine differences between the included and excluded mineral matter as well as species sharing partially common boundaries (i.e., juxtaposition of minerals with coal). The two systems may be applied to raw coal, partially spent char, and fly ash or deposited ash, in effect revealing changes in mineral chemistry at various stages of combustion.145-‘48 SEM and EDAX analyses can be applied manually, directly on raw coal samples, deposited ash, or fly ash. The results are limited to exposed surfaces and hence the subjective surface selection and interpretation by

the investigator: Secondary electron emission is limited to 100 A penetration while back-scattered electron emission is limited to 300A penetration, limiting EDAX analysis to surface composition. Automation of the SEM/EDAX analysis requires the pulverized coal, ash deposit or fly ash specimen to be mounted in epoxy and polished to obtain a more objective quantitative analysis. Automated methods employ SEM and the electron microprobe analyzer (EMPC) equipped with an energy dispersive X-ray spectrometer (EDS or EDAX), to which are applied the CCSEM and scanning electron point count techniques (SEMPC). With the addition of automated image analysis technique, morphological data can be collected, stored, and analyzed as well.‘45 The CCSEM analysis employs back-scattered electron imaging as the back-scattered electrons provide a significant contrast in the image as their intensity varies with average atomic number. The differences in contrast can be used to distinguish between epoxy, coal, and mineral as the electron beam scans the surface. In the beginning of the procedure a scan is made and the image stored in the computer for future image analysis. The electron beam is programmed to scan the field of view. Upon finding a bright inclusion, defined as a mineral inclusion, the beam performs eight diameter measurements in order to locate the center. An EDAX scan is performed at the center for twelve elements including; Na, Mg, Al, Si, P, S, Cl, K, Ca, Fe, Ba, and T.‘45.‘46 An associate computer then analyzes the data for size area perimeter and chemical composition. Empirical correlations for area-to-volume relationships

70

R. W. Bryers

EAGLE BUTTE SUBBITUMINOUS

Ca 0.0

0.1

0.2

0.3

0.4

OS

0.6

0.7

0.6

0.9

Al 1.0 B3m7a

(A) 0.2 5 Dp < 10 I(M, 869/1092 Si

A

Al

CA-AL-~

0.0

0.1

(8)

10

0.2 s

0.3

D,, (

0.4

0.5

0.6

0.7

0.6

0.9

1.0 eso3?b

40 PM, 295/344 PARTICLES

ASH COMPOSITIONFOR CROSS-SECTIONEAGLE BUTTE

ASH

BY CCSEM AT UND (1500 K, 21% 02) Fig. 34. Ca-AI-Si ash composition ternary diagrams for CCSEM of Beulah lignite and Eagle Butte subbituminous at 1500K 21% Oz. Each point represents ash particles with CA + Al + Si >80moIe percent (courtesy of Dr. J. J. Helble).

developed from standards permits particle size identification. The elements defined in the EDAX spectra and the individual elemental count is compared to those of an extensive list of mineral categories appearing in Table 11 for identification of the minerals in the particle. Unidentifiable minerals, by virtue of unrecognizable elemental composition or complexity of association (as in the case of a

mixture of aluminosilicates of similar composition) are listed as K-aluminosilicates, Ca-aluminosilicates, or unknown. The density of the identified mineral is used to calculate the weight percent from the volume. A subsequent program counts the identified mineral species by area and/or weight and composition, and summarizes the results in weight distribution form illustrated in Table 12. Once the data is stored. the

Steam-raising

71

fuels

45-07 CCSEM Data particles 14.6 microns

Si

Ca Fig. 35. CCSEM #data presented on a ternary diagram using a third dimension to express concentration specific size range (courtesy

computer becomes a powerful tool for analyzing, characterizing, and displaying the mineral composition of the coal. As an example, the data can be illustrated on a ternary diagram that can be used to visually examine pertinent physicochemical properties, as shown in Fig. 34.1M To unmask overlaid data points in congested areas encompassing large concentrations of data, a third dimension may be used to illustrate the frequency of occurrence of a specific mineral species, as shown in Fig. 35.‘48 Such a diagram quickly identifies levels of concentrations of low-melting species formed by association of one or more minerals within the coal. The technique also permits direct comparison of the mineral composition of fly ash and deposits with that of the raw minerals in coal, revealing transformations during combustion and identifying species responsible for fouling as illustrated in Table 13.“’ Possibly the oldest application of CCSEM is identifying particle size. With CCSEM one can make a direct comparison of fly ash size distribution to mineral size distribution, providing the first step in predicting char and mineral fracturing and agglomeration during combustion. The procedure is not without flaws or weaknesses.

for a

of Dr. J. J. Helble).

Prediction of particle size and shape from a plane intersecting the particle, which may or may not include the particles true center, introduces potential error. The magnitude of the error might be reduced by expanding the data base. Assuming mineral composition from percentage elemental X-ray count is extremely difficult for particles composed of more than one clay species, creating large concentrations of unclassified or possibly mislabeled species. The technology is developing rapidly and these problems will no doubt be overcome. The SEMPC technique is primarily applied to fly ash and deposits as a means of quantifying the chemistry of various phases. In the SEMPC technique, the computer-controlled stage is moved in a raster pattern over the area of sample to be examined. The stage is programmed to move in regular intervals so as to collect a grid of data encompassing the area of concern. At each interval (i.e., lOOpm), the stage is halted and an EDAX spectrum is collected. A total count is taken over a very short span of time to identify the presence of sample. If a large count is recorded, then ash has been detected and the count is resumed and EDAX data collected. The process is repeated over a minimum of 230 points. The collected

72

R. W. Bryers Table 13. CCSEM

analysis

of coals, results expressed

as weight percent

on a mineral

basis15’ (Con/.)

Size range, pm 1.o-2.2 Eagle Butte Quartz Iron oxide Rutile Calcite Kaolinite Montmorillonite K Al-silicate Fe Al-silicate Ca Al-silicate Na Al-silicate Aluminosihcate Mixed Al-silica Ca silica Pyrite Pyrrhotite

Oxid. pyrrhotite Gypsum Barite Apatite Ca-AI-P Gypsum/barite Gypsum/Al-silica Si-rich Ca-rich Ca-Si rich Unknown Total*

2.2-4.6

4.6-10.0

4.5 0.1 0.2 0.5 2.4 0.3 0.2 0.1 0.4 0.2 0.2 0.2 0.1 0.2 0.0

8.0 0.0 0.2 1.4 3.8 0.3 0.2 0.0 0.5 0.1 0.3 0.1 0.0 0.2 0.0

6.2 0.0 0.0 0.0 8.0 0.0 0.5 0.0 0.0 0.3 0.8 0.0 0.0 0.0 0.0

0.0 0.1 0.6 0.0 1.3 0.2 0.1 0.3 0.7 0.0 3.8

0.2 0.0 0.6 0.2 3.8 0.2 0.0 0.0 0.4 0.1 3.9

0.0 0.0 0.0 0.3 1.2 0.0 0.0 0.0 0.0 0.0 4.9

0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.3 0.0 0.0 1.9

16.5

24.5

22.2

20.4

*Totals may not be correct due to rounding

10.0-22.0

10.0 0.0 0.9 0.3 4.1 0.0 0.0 0.0 0.3 0.0 0.3 0.0 0.0 0.9 0.4

22.0-46.0

10.5 0.0 0.0 0.0 0.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

46.0-100

Totals

2.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

41.1 0.1 1.3 2.2 19.4 0.6 0.9 0.1 1.1 0.3 1.3 0.2 0.1 1.2 0.7

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 3.2

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.2 0.1 1.2 0.5 6.4 0.1 0.6 1.0 0.1 17.8

14.3

2.2

100.0

errors.

data is converted to quantitative chemical analysis. Oxygen is calculated by difference restricting analysis to completely oxidized deposits. The composition at each point is compared to the chemical composition of 43 phases commonly found in deposits as well as X-ray diffraction patterns for the deposit. Those compositions not appearing in the standard list are identified as amorphic phases. The procedure appears to be limited by the standard list and accuracy of the X-ray diffraction and may not be able to identify glassy forms or crystalline phases not reported in the standards. However, once incorporated with an automated image analysis procedure, differences in chemistry between amorphic and crystalline phases may become readily apparent, improving diagnostics for the sintering process.‘4g,‘50 4.5. Thermal Behavior of Minerals

Minerals analyzed by gravity and size fractionated coal, as well as CCSEM, have indicated the ash generated during combustion of pulverized coal is heterogeneous. Consequently, the physicochemical properties of the individual particles influencing their thermal behavior during combustion do not necessarily conform to the values determined by analysis of a composite sample but are related to the individual pure mineral species in the coal. These properties may be further altered by association with carbon or other mineral associations. Consequently, the combustion

engineer must be concerned about the thermal behavior of the pure mineral species as well as its occurrence as inherent or extraneous mineral matter. The thermal behavior of minerals commonly found in coal has been examined by mineralogists for years using differential thermal analyzers under air at slow heating rates.” A large portion of these investigations were summarized in an extensive survey of thermal behavior of coal minerals by Watt.” The transformations experienced by several minerals most commonly found in coal have been summarized by Jackson4’ and Bryers and Walchuk’24 in Fig. 36. The results must be used only as a guideline for predicting the thermal behavior, as thermal shocking during combustion in the presence of carbon and other mineral forms will alter some of these transformations and may defer others until post-combustion deposition on heat-transfer surfaces. More recently, Mitchell and Gluskoter”’ examined minerals released from coal by means of low-temperature ashing (LTA) (i.e., oxidizing carbon at temperatures of 150°C under an oxygen plasma without disturbing the mineral structure) by slow heating the released minerals on a hot stage quenching and analyzing. The analyses were performed at temperature, and after quenching the results were summarized in Fig. 37.15’ Most transformations have been examined at a slow heating rate. Consequently, there remained some uncertainty as to whether they were chemically or kinetically driven, Stinespring et a1.‘52 repeated the

73

Steam-raising fuels

3 P 950%

allmdwa

9bd.

quart3

(sol)

Cakik

T

IOOP to lloooc

r’ FsS, Fep(SO&,FeO (partial

525-Z’

melt)

---c

S, SO,

F%Os liquid

1--s,

970%

HIS

III FI

1600%

Fig. 36. Phase transformation of minerals commonly found in coal. experiments with low-temperature ash in a combustor at high heating rates. With the exception of pyrites, he confirmed the transformation proposed by Gluskoter and others, indicating the transformations were thermodynamically controlled for all minerals except pyrite. The decomposition of pyrite was kinetically controlled and thus time dependent. The investigations did not include phase separation due to volatilization of mineral species reduced to a low oxide state by included carbon. Ulrich et ~1.~~ examined the transformation of volatile species generated during combustion under reducing conditions using JANAF tables to predict vapor equilibrium compositions at high temperatures under varying degrees of oxygen. Their data shows that substantial amounts of silicon, aluminum, iron, calcium, magnesium, sodium, and potassium can be expected to volatilize under reducing conditions. Aluminum, iron, calcium, and magnesium evaporate as pure metals, whereas sodium and potassium appear as pure metal cyanides depending upon oxygen levels. The volatile species are formed at the carbon flue gas interface. The degree to which they are released is diffusion controlled. The vapors are oxidized and precipitated during the migration process forming submicron particles. Nucleation generally occurs rapidly, forming a cloud of active particles that

coagulate at a decreasing rate. Brownian collision and coalescence appear to be a major growth phenomena. Growth ceases in the cases of oxides and metals when the temperature drops below the fusion point. Impaction caused by turbulence and fluid motion is generally insignificant for submicron particles.86 4.5.1. Quartz Quartz is the most abundant mineral found in coal. It melts at 1723°C and boils at 2230°C. Under reducing conditions, silica monoxide forms which melts at 1420°C and boils at 2600°C while releasing an SiO vapor. The vapor pressure of silicon is low in the range of temperatures experienced during combustion. Honig153 reports values ranging from 0.1 mHg at 1157°C to 1 mHg pressure at 1852°C (see also Ref. 84). In the presence of carbon and other mineral species, the vapor pressure is altered substantially. When a mixture of aluminosilicate and graphite is heated, the volatilization of silicon monoxide begins at about 1150°C and reaches a maximum at 1400°C. Mackowsky’54 reports that volatilization of silicon monoxide starts at about 1649°C in the presence of carbonates and clays, and reaches a maximum at 1704°C. In the presence of pyrites or metallic iron, volatilization begins at about 1560°C and continues at

R. W. Bryers

Amorphous

14

b

K(Si,AI)AI,O,,(OH) Pyrite ’

’

FeS,

Hematite t Fe,O,

Calcite

Anorthite

CaCO,

CaAI,Si,O, Mullite

Metakaolinite

Kaolinite

+

4

AI,Si,O,,

Al2Si0,2Hr,O Gypsum

b

Anhydrite 4

-

%aSO;2H,O

b

CaO Gehlenite

b

C%AI,SiO,

:

Quartz 4

Note:

Amorphous b

4

SiO,

Analysis performed at temperature

Fig. 37. Summary of mineral transformations due to heating low-temperature ashed coal by Gluskoter and Mitchell.“’ a rapid rate as the temperature rises until practically all the silica in the mineral is volatilized. The melting temperature of quartz and its volatility are very important aspects of the thermal behavior of quartz. However, it is the solidification temperature and the impact of other mineral contaminants on it that affect the sticking potential of quartz. It is quite apparent that the role of quartz in fouling is multi-faceted (i.e., condensed vapor, an absorbent of alkalis, and captured aggregate), and consequently, it may contribute to deposition within a given combustor in several different ways. 4.52. Kaolinite Kaolinite is the second major source of silica found in coal. It is most frequently found in the partings of the seam and as a finely-dispersed particle in the coal (i.e.,
flame temperature encountered during combustion. Therefore, the fused spheroidal fly ash generated from the mineral matter in coal primarily forms as a result of the fluxing action between pure minerals contained in a given particle. Illite and biotite appear to be an exception. Both minerals contain small concentrations of iron and potassium and form a glassy phase at 950°C and llOO”C, respectively. Depending upon its fluidity, this glassy phase could be responsible for surface deformation at a relatively low temperature and provide the necessary sticking potential to prevent reentrainment on hot surfaces. These minerals are stable to high temperatures; however, Cl in the environment will release the potassium as a volatile species. 45.4. Pyrite Pyrite is the primary source of iron in Northern Hemisphere coals. It has a specific gravity of 5.00. Once liberated from coal or other minerals, it should be easily separated from non-pyritic matter by gravity separation at a Sp.Gr. of 2.85. Except for calcite and illite, few minerals commonly found in coal have gravities in excess of 2.70. Calcite has a gravity of 3.00, as does illite. Heavier gravity fractions generally include large concentrations of pyrites with varying degrees of contamination by illite, quartz, or calcite.

Steam-raising

fuels

75

Table 14. Potential reactions during oxidation of FeS2’56 Primary reactions AH FeS KCal

Reaction

1. 2. 3. 4. 5. 6. 7. 8. 9. IO. II. 12. 13.

I SO2 + 8Hz0 = 2Fe203 + H2S04 2FeS2 + 702 + 2H20 = 2Fe203 + HzS04 2FeS, + 702 + 2Hz0 = 2Fez03 + H,SO, 2FeSz + 702 + 2H20 = 2Fez03 + 2Hz + 2SOx FeSz + 302 = 2FezS04 t SO2

4FeSz +

4FeS2 + 1302 = 2Fe203 + 4SO2 + 4SOj 6FeSz + 1902 = 2Fe20d + 6S02 + 6S0, 4FeSz + I IO2 = 2Fe201 + 8SOl 2FeSz + SO2 = Fe304 Fe& + 202 = 2FeO $- 4SO? FeS2+302=Fe+2SOz 4FeSz + 302 = 2Fez01 + 8S~a 2FeSz + 202 = 2FeS -t 2S02

Secondary

Il. 12. 13. 14. 15.

02

KCal/Mol

1309.20 629.78 608.76 546.46

327.30 314.89 304.38 273.23

87.28 89.96 86.96 78.06

248.94 882.00 1271.30 799.73 575.14 326.20 99.40 222.92 102.32

248.94 220.50 211.88 197.66 191.71 163.10 99.40 55.73 51.16

82.98 67.85 66.91 71.88 71.89 65.24 49.70 74.32 51.16

reactions AH KCal/Mol

2FeS03 = FezO1 + SO2 + SO, 4FeS04 = 2Fe203 + 4S02 + O2 3FeS0, = Fe304 + 3S02 + O2 Fe& + 5Fe203 = 11 Fe0 + 2S02 FeS2 + 16Fe20, = 11 FexOp + 2S02 10FezOi + FeS = 7Fe304 + SO? 5FeS04 + FeS = 2Fe304 + 6S02 3FeS + 2S02 = Fe304 + 5s FeS+3S03 =FeO+4S02 2FeS04 + 3H20 = 2Fe(OH)3 + 2S02 + 1/202 FeS04 = Fe0 + SO3 3FeS04 + 1/202 = FexOp + 3S03 2FeS04 + 1/202 = Fe20s + 2S03 3FeS2 + SO2 + O2 = Fe304 + 7s 2FeSz + SO2 + 1/202 = Fe201 + 5s

Pyrite is a combustible mineral with a heating value of 3000 B.t.u./lb, whose decomposition is kinetically dependent. The rate of decomposition is governed by reaction rates, pore diffusion, and bulk or stream diffusion, as well as the presence of adventitious impurities.l7.'55-160 Although the bulk of the particle does not shrink during combustion, some fragmentation occurs altering the mineral size distribution.“’ The combustion process is extremely complex and consequently has been investigated extensively.‘7”55-‘60 FeO, FeS, Fez03, Fe2(S04)3, S, SOZ, and SO3 are listed as some of the products of combustion. Schmukls6 considered thirteen reactions between FeS2 and 02, listed in Table 14. Several of the components in the Fe-O-S system may coexist as an intermediate step in the combustion process producing a low-melting phase illustrated by the phase diagram in Fig. 38.‘.15’ The melting temperature of Fe304 or Fe203, the final product of combustion, is 1540°C and is of little consequence in ash deposition except as aggregate or a fluxing agent to other deposited species. The intermediate phase is of particular interest as temperatures as low as 590°C have been reported. Coarse, slow-burning pyrites

AH

KCal/Mol

Reaction

I. 2. 3. 4. 5. 6. 7. 8. 9. 10.

AH FeSz

-71.88 -211.04 -254.34 -172.18 - 107.58 -47.93 -169.58 -21.24 +41.64 -78.46 -62.40 -111.50 -56.00 +68.47 f40.49

encapsulated in a molten layer of pyrrhotite can be responsible for the formation of very high melting slag (i.e., containing as much as 85% Fe203, as shown earlier in Table 4). Srinivasachar et al.‘60 and Wa1l’59 report the oxidation of pyrrhotite and magnetite crystallization, during which time a melt may exist, accounts for more than 80% of the total decomposition time, as shown in Fig. 39. Extraneous pyrites extracted from coal ignites at temperatures as low as 450°C and proceeds to decompose at a rate slightly better than anthracite.“’ Carbon and/or other mineral species tend to lower the rate of combustion. The presence of a reducing condition, by virtue of carbon, or deficiency of O2 lowers the ignition temperature and slows down the rate of combustion by promoting the formation of pyrrhotite. Although decomposition of inherent pyrite has been investigated, the process is not clearly understood and requires further understanding. At least a portion of the extraneous particles of pyrites fracture and disintegrate when thermally shocked, yielding a fume of 0. l-l pm diameter particles.'2.'59"" Inherent pyrites were found to generate an iron fume according to the following

R. W. Bryers

76

I

I

I

I

I

I

2eooC

-I

2600-

2400-

FeO+melt

-

-

900

1600-

FeO-

FeS eutectic 800

I 20

14006

I 40

I 60

I 80

100

FeO, weight percent

Fig. 38. Phase diagram of system FeO-FeS.‘57 reaction: FeS(1) = Fe(g) + 1/2$(g).

(23)

The extent to which the iron fume originates as inherent and extraneous ash is not entirely resolved. However, the total fume is estimated at 4-7% of the iron in the coal. This portion of the original iron is likely to be responsible for fluxing of silicates in the gas stream or at the surface of the slag. 4.55. SideritelAnkerite Siderite is more commonly found as the mineral form of iron in Southern Hemisphere coals. Nelson et a1.84 report siderite present as nodules and as impure lenses and bands in coal. As in kaolinite and calcite, siderite may occur as fillings in the cavities of fusain and as small veinlets within the coal mass. Siderite has a gravity of 3.6. When liberated from coal, it should report to the heaviest gravity fraction. Decomposition of siderite begins at 1085°F with the release of CO2 and the formation of Fe, FeO, of Fe304 depending upon the partial pressure of Oz, CO, and COz. Inherent siderite most likely acts as a flux for silaceous minerals, while extraneous siderite is innocuous unless in contact with other silaceous mineral species on a hot surface. Ankerite is found in coal on occasions and behaves similarly to siderite. 4.56. Calcite Calcite is one of several carbonaceous

minerals

found in coal. It has a gravity of 2.7 and is found as veins in coal, particularly in the cleats and fractures. Like siderite, it decomposes at a low temperature @lo-920°C) releasing Ca or CO depending upon the partial pressure of Oz, CO, and CO*. Inherent calcite more than likely is a fluxing agent for other inherent silaceous minerals. Extraneous calcite, found as a free species, may become sulfated at lower gas temperatures contributing to calcium-sulfate-bonded deposits. Although Graham et al.‘62 and others have reported calcium fumes, it is uncertain to what extent or which species (i.e., extraneous or inherent calcite) is responsible for the fume generation. 4.6. Thermal Behavior of Mineral Matter Mineral matter includes organically-bound sodium, calcium, magnesium, and trace amounts of aluminum and iron found in low-rank coals. One might also want to include the organically-bound sulfur and most ofchlorine found in coal in this group. Although sodium chloride (strictly speaking) is a mineral, it too might be considered mineral matter on the basis of its thermal behavior. Mineral matter is all volatilized during combustion. Lindner et al.‘63 investigated the release of organically-bound sodium and sodium chloride from coal. The transport of sodium from its original site to the surrounding atmosphere is very complex. Organically-bound sodium is released under reducing conditions as metallic sodium. The rate of release must be dependent upon total surface, pore size, pore diffusivity, and bulk diffusivity. Linder et al. report

71

Steam-raising fuels

,

1800 51600

,

,

,

,

,

,

,

/L

,

#

-

(a) Tg .14SOK

kL/kG=

,

200

0

!

!

10

!

!

20

!

!

30

!

!

35 40

/L

I

40

Residence

Time

I

120

I

1

I

200

(ms)

(b)

10.20

30

40

40

Residence

Time

120

200

(ms)

(cl Fractlonal

Time for Each Stage

Fig. 39. Calculated (a) particle temperature and (b) iron-bearing phase distribution profiles as a function of residence time, (c) fractional time spent in each stage: l-pyrite heatup; 2-decomposition to pyrrhotite; 3-pyrrhotite heating to melting point; 4-pyrrhotite melting; 5-oxidation to molten iron oxide; 6cooling of iron oxide to crystallization temperature; ‘I-magnetite crystallization.‘60

the organically-bound sodium is evaporated as the volatiles are released, intimating that the char is depleted of sodium once the volatiles have escaped. Formation of Na(OH) is completed in the gas stream.‘63 Sodium chlorine melts at 801°C and begins to volatilize at about 750°C. Above 750°C the rate increases significantly. Its rate of escape, as in the case of organically-bound sodium, is dependent upon pore and bulk diffusivity and instant properties of the char affecting either. Lindner et ~21.‘~~found that evaporation of crystals 0.3 pm were pore diffusion controlled. Evaporation rates were comparable to those of devolatilization. Once released, the dominant sodium-bearing gas species is Na(OH). It is free to react with the extraneous silicates present as well as the char and inherent

silicates to form NazSiOs and Na*SOs, as conceptually illustrated by Lindner et al.163in Fig. 40. The reaction rates of the sodium with silica to form silicates takes place in five stages: of gaseous reactants and 1. Transportation products in the char/boundary. 2. Diffusion of gaseous reactants and products through the pores of the char. 3. Reactions at the sodium silicate melt surface. 4. Diffusion through the silicate melt. 5. Sodium silicate forming reactions at the silicate/ silica interface. Experiments by Lindner et ~1.‘~~indicate that the controlling stage changes as the reaction proceeds. Boundary layer transport controls the process initially, followed by diffusion of the sodium within the melt and gaseous hydroxide within the char. Sodium hydroxide servicing the char burnout will

78

R. W. Bryers

GASES

Hot (2O%XS air

Gas Cooling

N2,02,H20.C02

1

S02,NaCI. Na

SO2

75um coal part+, containing 10°~0S102 and NaCl

Ash Spheridisation

Burnout

Devolatilisation

Reactants

NaOH

, NaZCIZ

I

Char, . co;l:amlng

Partially ash

crystals (-100

fused

[-500msl

msl

Spherical

k$e;;ed

ash ( 30pm

Na CI, Na2S04

I

[O.lpml

Fig. 40. Mechanisms of Na ash reactions.‘63 HEAT TRANSFER ///I/ /////I////

CHAR

F

PARTICLE

/ -1

SURFACES

VAPORS MO,V,k --

.a. -1

‘MINERAL INCLUSIONS

EXTRANEOUS

ASH

SP. Gil. > 2.0

A/

“‘=USION

__ -..

HEAT TRANSFER A.un -APrnP”

AND

RE-ENTRANMENT

SURFACES

Fig. 41. Fate of mineral matter in coal during combustion.

form chlorides or silicates depending upon level of concentration of Cl and 02/CO present. At lower temperatures the salts of NaCl and Na2S04 will form a submicron fume.‘633’64Consequently, sodium occurring as mineral matter may contribute to fireside deposits in several ways: as a low-melting silicate responsible primarily for very high-temperature slagging and fouling (i.e., >954”C), and as an initiator of fouling by forming low-melting sodium sulfate just above convection bank superheater/ reheater tube temperature. Organically-bound calcium occurs in substantial

quantities in low-rank coals. Although organicallybound calcium is known to form a fume of reactive submicron CaO as well as calcium silicate cenospheres, its release from the coal has not been examined as sodium has. Organically-bound calcium may also contribute to fireside deposits in several ways. The fume initiates furnace wall deposits as well as low-temperature convection bank heat-transfer surfaces by forming calcium-sulfate-bonded deposits. Calcium silicate may perpetuate the growth of partially developed slag as semi-molten anorthite or gehlenite.

Steam-raising

fuels

,110o Fu)W

!

I

.lOOO

$ 3 r 6 E

FLAME’

I

. 900

g

TEMPERATURE

hOVING

LAYER . 600

Fig. 42. Mode of slag growth 4.1.

Fate of Mineral Matter During Combustion

Dr. Sarofim and others at MIT identified the interaction of the inherent minerals during combustion conceptually.‘62 The diagram was modified by the author to include extraneous and inherent ash as well as including types of deposits formed and mode of transport to the surface, as shown in Fig. 41. The extraneous ash, in this case, is defined as the particulate whose gravity exceed ~2.0 and therefore is virtually free of carbon. The extraneous ash is primarily associated with the older coals (i.e., bituminous and anthracite). It is generally composed of quartz, pyrites, and some of the heavier clays. During combustion this material is heated to temperatures slightly below the flame temperature. The actual temperature level achieved by each individual particle depends upon its particle size and flight path relative to the combustible matter. The minerals melt and form cenospheres. The size and number depend upon the size distribution of the pulverized minerals in the fuel and agglomeration

on furnace

wall

during combustion. Depending upon size and local flow patterns, the fly ash migrates to the surface via eddy diffusivity, thermophoresis, or inertial impact. The fly ash is quenched to a temperature substantially below the melting temperature of the constituents comprising the individual fly ash particles as they move through the boundary layers of flue gas to the bare tube surface. Only those very small particles subjected to very small gravitational forces approaching the surface with little kinetic energy are retained. The retaining force is not clearly understood. Large particles impact and become reentrained. Once a thin layer of dust is formed by particle fl pm in size, the surface of the tube becomes irregular and its temperature increases. Eventually the surface temperature reaches the solidification temperatures of specific fly ash species originating as a mineral in coal and specific particles begin to stick. Melts also begin to form between two high-melting species that share a common low-melting eutectic such as quartz and completely oxidized pyrites. At this point the rate of furnace deposit growth increases and its thickness,

80

R. W. Bryers 300x

600X

MAG. 3001

300x

- ._._- ..- ._North Dakota Lignite

Montana Subbituminous

Utah Subbituminous

Bituminous Upper Freeport

Bituminous Kentucky No. 9

Fig. 43. Initial layers of ash deposited on a slagging surface while firing a lignite, subbituminous, and bituminous. as illustrated by the model in Fig. 42, increases significantly. The thickness of the initial layer depends upon the initial tube temperature, the local heat flux, and the thermal properties of the deposited ash (i.e., thermal conductivity). Particles passing through the boundary layer may also scavenge volatilized mineral matter formed by inherent ash, including SiO, Ca, Fe, Na20, and K20. Eventually the entire slag layer becomes molten. Continued growth may be sustained by direct impact of solid or liquid fly ash and direct condensation of volatilized mineral matter. Equilibrium is achieved when the rate of flow of plastic ash down the tube wall is equivalent to the rate of capture of fly ash. At low flame temperatures and low heat flux, voluminous quantities of sintered slag may form. Thickness approaches an equilibrium as a result of shedding of ‘slag’ when the deposit weight exceeds the bonding strength at the deposit/tube or deposit/ refractory interface. Extraneous ash will not cause fouling unless the furnace exit temperature exceeds the melting temperature of the lowest melting species of fly ash entrained in the flue gas, a reaction takes place with SO3 formed at the tube surface to cause bonding by sulfidation, or a low-melting sulfate deposited on the tube surface increases the stickiness of the tube surface, thus retaining the otherwise innocuous silicates. Partially-spent pure pyrites can also cause severe slagging of the inlet to the convection pass due to a transitory melt formed between FeS and S, or FeS and FeO. Inherent ash includes finely divided, uniformly disseminated particles or crystals of various mineral types and organically-bound mineral matter. Inherent ash is primarily found in low-rank coal. During

combustion these mineral forms are subjected to the highest possible flame temperatures, as well as a reducing environment. Thus quartz, calcite, dolomite, or pyrites are partially reduced to their lower oxidized states. During char burnout the char and solid mineral matter are subjected to fragmentation resulting in size reduction. Minerals retained in the char particle are subjected to melting coalescence and partial vaporization-further altering their size, chemistry, and physical state. Thus quantifying the distribution of mineral matter between volatilized species and the residual solidified or molten particles is extremely difficult. The residual solid and molten particulates are also subject to agglomerationfurther altering their size. At this point, they might be treated as ‘extraneous’ fly ash. The volatilized mineral matter from the reduced mineral species will condense homogeneously via the process of nucleation, or heterogeneously on fly ash on tube surfaces. The temperature at which condensation takes place depends upon whether or not the vapor has been chemically altered (i.e., oxidized or sulfated), its concentration, and its vapor pressure. Nucleation and scavenging may take place in the gas stream or within the tube boundary layer, depending upon the residence time and temperature level required for the phase transformation to take place. Either process may require substantial subcooling for the condensation to occur at a reasonable rate. The volatile contribution from inherent ash may be small as the inherent mineral matter in pulverized coal is generally small. 4.8. Slagging Slagging is defined as the formation

of fused or

Steam-raising fuels sintered deposits on heat-transfer surfaces and refractory in the furnace cavity subjected to radiant heat exchange. The nature, degree, and composition of slags may vary significantly throughout the furnace depending upon surface temperature, flame temperature, absorption rates, direction of gas flow, mineral composition (i.e., coal rank), mineral and coal size distribution, concentration of mineral matter, and oxygen level of the flue gas in contact with the heat-transfer surface. Therefore, an understanding of slagging and where it might occur requires knowledge of a complete temperature and compositional flue gas profile for the furnace. Changes in composition and temperature profile at the tube surface are especially important. Slagging may be the result of direct impingement of unspent flue gas and entrained molten particles on heat-transfer surfaces within the combustion zone due to an excessively high heat release, poor air distribution, poor burner management, or on heat-transfer surfaces immersed perpendicular to the direction in flow of flue gases as in the case of divisions walls and pendant superheaters located upstream of the classic furnace ‘nose’. Under these circumstances, coarse molten particles simply impact on the surface, freeze, and stick until a layer with sufficient thermal resistance develops to form a molten ash. ln most cases the slag will resemble the extraneous coal ash and its slagging potential will depend upon the fluxing capabilities of the basic elements found in the extraneous ash or the combustibility of pyrites when the pyrites are not associated with other mineral species. The latter frequently results in a slag consisting of 85% Fe203, In low-rank coals whose minerals exist primarily as inherent ash, the slag may be rich in silica, which is frequently the dominant coarse species. The deposition may coincide with or be preceded by a layer of calcite laid down as a fume by organically-bound calcium. Quartz enrichment of a completely fused deposit has been found on leading edges of pendant superheaters subjected to direct radiant heat from the flame, while 90” from the stagnation point on cylindrical tube surfaces, the deposit consists of sintered calcium sulfate. Under good combustion conditions where surfaces designed for dry-bottom operation are subject to parallel flow under oxidizing conditions, slagging is initiated by the finer mineral species with the lowest melting potential or fumes formed by organicallybound mineral matter and volatiles released during combustion from the inherent minerals. As illustrated in Fig. 43, the morphology and composition of the initial layers change significantly with coal rank. The initial layer of low-rank coal consists of submicron particles of calcite, calcium sulfate, and clays originating as inherent mineral matter and organically-bound calcium. Particulate found in the innermost layers of slag from older coals are enriched with potassium-bearing silicates originating as illite in

81

the coal, free quartz, and submicron iron scavenged by quartz particles from an iron fume. The inner layer is also sensitive to the size distribution of the minerals in the coal. The size of the particles occupying the inner layers are decidedly smaller for coals containing extremely fine minerals primarily occurring as inherent ash. It is believed there is a substantial reduction in heat absorption as a result of deposition of the initial layer due to its porosity. The total reduction in absorption differs significantly between low- and high-rank coals due to differences in chemistry responsible for major differences in reflectivity and thermal conductivity. Continued development of low- and high-rank coals is also decidedly different. Subsequent layers of high-rank coal develop from random sites on the tube surface, very much like drop-wise condensation. The site probably develops as a result of the deposit of a relatively large particle whose outer half penetrates a thermal boundary layer where the temperature exceeds the melting temperature of any eutectic formed by fluxing agents depositing on the particle. The individual ‘droplets’ grow; new sites develop. Eventually the voids between sites fill in and a molten layer forms. On a macro basis, slag on a full furnace wall develops from patches, also in a ‘drop-wise condensing’ fashion. At elevated surface temperatures, particles deposit as dry fly ash, subcooled glassy phases, and a condensed fume. Fluxing of the dryer material takes place at the slag surface between ~8 15 and 1300°C (lowest eutectic in the SiOz-Al@-Fe0 system). The fused slag is composed of rapidly quenched, subcooled glassy silicates. The material is in the plastic range and flows as a non-Newtonian fluid. Its thermal conductivity is several times higher than the initial layer. Above 1300°C thickness of the deposit will depend primarily upon composition. Slag composed of eutectic mixtures will melt forming a fluid whose thickness will depend on the viscosity of the fluid and equilibrium between the mass deposited and the mass flowing down the wall. Off-eutectic mixtures may grow substantially as the viscosity will depend upon the percent of solid component precipitating from solution. Large concentrations of precipitated material (i.e., large differences between initial deformation or solidification temperature and fluid temperature or melting temperature), extends the permissible temperature range during which the deposit is extremely viscous, thus allowing it to grow. The mixture consists of crystals suspended in a liquid and consequently behaves as a non-Newtonian fluid. The concentration of crystals depends on composition and quenching rate. A slow quenching rate (i.e.. thick deposit with high surface temperature) permits crystallization of the precipitating phase. Figure 44 shows the glassy substrate, precipitation of iron near the slag surface, and deposition of condensed iron fume originating as fragmented pyrites on the surface. Crystals of iron also appear to congregate about gas pockets just below the slag surface. It is believed the

R. W. Bryers

82

Fig. 44. SEM photomicrographs

and EDAX scan profiles of the cross section of slag formed on furnace probes after firing a bituminous coal.

gas pockets were formed by the decomposition of pyrrhotite with the release of SO*. If partitioning of minerals occurs during combustion, an iron-rich slag can be formed by preferential deposition of partially-combusted pure pyrites (probably as pyrrhotite) in a semi-molten state. Oxidation of the pyrrhotite occurs at the tube surface, leaving a solid deposit consisting of 85% FezOX with a melting temperature in excess of 2800°F. Increasing the surface temperature of the furnace wall, as in the case of refractory or a burner throat, reduces the heat flux substantially-see Fig. 42. A combination of low heat flux and off-eutectic mixtures of high-melting and low-melting substances can result in very thick slag deposits. Morphological examination of eyebrow deposits frequently reveal submicron crystals of iron and/or silica, suggesting at least a portion of the deposit arrives as a fume possibly with recirculated gases. This may be due to the excess of carbon in the early stages of combustion and the willingness of small particulate to follow the stream lines of recirculating gas. Coarser material tends to congregate in the high velocity portion of the flowing stream. Uncooled surfaces (as in the case of many refractory-lined burner throats) will readily collect recirculated ash whose temperatures exceed about two-thirds the melting temperature of the lowest melting species (i.e., the Tamman incipient sticking or solidification temperature of

glass). Consequently, burner eyebrows are strongly dependent upon the quartz temperature and the composition of the lowest melting species in the coal ash. This could be illite in high-rank coals. The steps for forming slag with low-rank coals differ slightly from bituminous coal due to the lower inherent flame temperature (i.e., three times as much coal moisture), the presence of calcium and sodium as organically-bound mineral matter instead of iron, the reduced ash level, and the smaller mineral particle size. Calcium fume deposits on virtually all surfaces in the steam generator. At high concentrations of sodium, the initial layer may be contaminated with sodium sulfate. The CaO reacts with the SO3 present to form bonded CaS04. Sintering begins at temperatures as low as 500°C and the strength steadily increases. In the presence of COZ, a carbonate may form, in which case Skrifvars et al.165 report severe sintering between 600 and 750”C.165 Add sodium sulfate and a known melt will occur at ~900°C. Submicron aluminosilicate found as clay in low-rank coals migrate to the surface at 1600-1750°C. They react with the sulfates between 750 and 950°C to form calcium aluminosilicates as gehlenite in the absence of sodium and mellilites in the presence of sodium. They have a melting temperature in the vicinity of 1780- 13OO”C, respectively. A white layer develops that adheres tenaciously to the tube. It appears to be

Steam-raising fuels

1

J 1000

1

I

1250

1500

,

1

1750

Temperature

2000

(OK)

I 2250

1

I

1

L

c

11

1

I

1

300

1250

1500

1750

2000

2250

1000

1250

I500

1750

2000

II1000

1250

I500

1750

2000

2250

1000

I250

1500

1750

2000

Temperature

(OK)

Temperature

(“K)

Fig. 45. Calculated partitioning of sodium under P.F. combustion conditions. Top plots represent oxidizing conditions in the furnace gases; bottom plots represent the extreme reducing conditions probable with the burning char. (a) Lowell; (b) Leigh Creek; and (c) high chlorine-sulfur coa1.65

of uniform thickness, has a low thermal conductivity, and high reflectivity reducing the local absorption rate. A subsequent layer develops randomly over the furnace wall. The rate of developments appears to be greatest at the crown of the membrane wall tubes where the surface temperatures are the highest. At low heat flux and local flame temperatures, a sintered deposit is formed at cenospheres, originating either as clay and quartz in the coal, scavenge CaO or CaS04 from the local flue gas and stick to the surface. Outer layers of slag have been found to be enriched with silica reaching concentrations about twice that found in the coal ash. The surface temperatures of the particulate must be very high as the lowest melting eutectic in the CaO-Als0s-Si02 system is about 1250-1300°C. At high heat flux, the entire particle goes into solution with the CaO at the tube surface forming a mixture of pseudo-wollanstonite and either anorthite or gehlenite. Polished cross sections reveal precipitates of CaO * SiOZ in a matrix of CaO . A1203 - 2Si02, which agrees with predicted mineral composition on a phase diagram. Slag removed from burner throat areas include iron selectively deposited

at the burner despite low concentrations of pyrites in the coal. The melting temperatures of fused low-rank coal ash are about 100°C lower than fused ash formed by minerals in high-rank coals due to the presence of calcium rather than iron. The increased resistance of low-rank coal ash to heat exchange results in thinner fused layers with higher surface temperatures. Since the composition of the ash impacting the wall very closely matches the composition of eutectic mixtures of anorthite, gehlenite and pseudo-wollanstonite, the ash simply melts once the deposit surface temperature reaches the fluid temperature of the slag and runs like water. There appears to be a very limited plastic range. Slag equilibrium is reached rapidly. A third category of coals includes bituminous coals that are low in iron but ‘rich’ in calcium and contain less than 1% potassium as a feldspar. The calcium, in this case, exists as a calcite rather than organicallybound calcium. Both of the minerals identified as being responsible for initiating slag growth are missing. Consequently, the bond for deposits mechanically attached to the wall is weak and the furnace

84

R. W. Bryers

GAS FLOW

NON AOHERENT

POWOER

E

MOST

5

j

-I t

UPSTREAM TUBE

FACE

;

ADHERENT

I

UPSTREAY

TUBE FiCE MASSIVE

DEPOSIT

DEPOSIT

20

x 0

L L

DEPOSITION

Fig.

46.

m

II

I

STAGES

Various stages of ash deposition including I--thermophoresis, condensation, and III-inertial impact.‘68

periodically sloughs a sintered slag creating unusually high proportion of bottom ash.

an

4.9. Fouling Fouling is the deposition of ash in the non-radiant convective heat-transfer portion of the steam generator immersed in flue gases at temperatures below the melting temperatures of the bulk coal ash. If there is a large particle-to-particle variation in composition of fly ash particles resulting in a significant variation in melting temperature of individual particles, some slagging may be experienced at the inlet to the convection pass. Slagging may also be caused by unspent pyrites if they appear in the coal as coarse, liberated minerals and the residence time in the furnace is too short. Fouling, however, is most frequently caused by the condensation and chemical reaction of volatile alkali mineral matter with fly ash either in the gas stream or at the tube surface creating a deposit and making the surface receptive to collection of other species normally found to be innocuous. As discussed in a previous section on empirical approaches, the constituents primarily responsible for fouling include volatile sodium and calcium, chlorine, sulfur, and phosphorous. Potassium may also be included in this group even though it exists as a stable silicate such as illite. Potassium may contribute to fouling in either one of two ways. As a low-melting silicate, unassociated with other minerals, it may be directly responsible for deposition on surfaces immersed in gas streams exceeding ~2000°F.

II--thermophoresis

and

It may also become very troublesome in the presence of Cl and Na, which will displace it from the silicates forming KC1 or K(OH)--the first step in forming low-melting sulfates in the presence of sufficient levels of S03. Silica has also been identified as a source of fouling due to the volatilization of SiO in stoker-fired steam generators under localized reducing conditions. Its occurrence, however, is rare.‘66 Phosphorous has also been reported to be a source of low-temperature phosphate-bonded deposits due to the formation of phosphoric acid (H4P207), which condenses on cooler surfaces capturing impacting fly ash. The unique conditions under which the phosphoric acid forms and the very low concentrations in which it normally occurs make phosphate-bonded deposits a rare experience.84 The bulk of fouling, however, is due to alkali-bonded deposits and calcium-sulfate-bonded deposits. 4.9.1. Alkali-bonded deposits Equilibrium calculations performed by several investigators using existing thermodynamic data based on the minimum free energy of formation concept have predicted that sodium exists at flame temperatures as NaCl, Na(OH), and Na. The weighted distribution of the sodium compounds for a specific case is illustrated in Fig. 45 by Wibberly and Wa11.65Wa11’63reports a qualitative agreement with measurements made using analytical techniques such as flame photometry, mass spectrometry, laserinduced fluorescence, and laser-induced photodissociation. The predictions assume the absence of

Steam-raising fuels elements such as alumina and silica which in reality are not the case. However, compensation can be partially made by assuming a surface availability factor. The parametric analysis should be completed by including the impact of O2 level or reducing conditions on the reactions.‘63 The equilibrium calculations as well as morphological studies of deposited ash indicate fouling may proceed in two slightly different ways: deposition of sodium silicates formed in the gas stream; or sodium, potassium, and calcium sulfate formed at the tube surface. Above 950°C equilibrium favors the formation of sodium silicates as a result of a surface reaction of Na(OH) with quartz and aluminosilicates, creating a slightly sticky surface on fly ash particles. The absence of SOs, except within the tube boundary layer at these temperatures, dismisses the presence of NazS04 as an intermediate phase. The melting temperature of the silicates formed by sodium with free silica may be as low as 8OO”C, whereas the addition of alumina in the presence of clays raises the melting temperatures to that of a feldspar (i.e., llOOC+). As the flue gas temperatures increase above these levels, the quantity of molten phase increases significantly until the initial deformation temperature of the bulk ash is reached.‘67 To avoid catastrophic fouling of the flue gas entering the convection bank, heat-transfer surfaces are limited to a temperature in the immediate vicinity of the initial deformation temperature of the ash. To minimize fouling by the fly ash with sticky surfaces. the initial banks of convective heat transfer are constructed with exceptionally wide tube spacing to minimize ash collection, efficient without completely sacrificing heat-transfer rates according to the schedule appearing in Fig. 20. Within this temperature range, the particle size will play an important role in whether or not the fly ash will deposit. The reaction rates of alkali with large quartz particles is limited by diffusion through the gas boundary layer to the particle surface and ultimately, diffusion through the outer layer to the core. The molten phase is limited to a surface effect illustrated in Fig. 14. Reaction rates for smaller particles are greater. Consequently, equilibrium is reached and the entire particle is molten. The smaller particles are more likely to form a sticky surface at high temperatures. Although the collection efficiency of the larger particles is greater, the ratio of kinetic energy to retaining force will also be greater, encouraging re-entrainment. Small particles, 1 pm or less, deposit at a slow rate by either thermophoresis or Brownian motion and are subject to rapid cooling to temperatures below their melting temperature as they penetrate the boundary layer. Consequently, it is the mid-range sized particles which appear to be most vulnerable to deposition. Fouling also depends upon the receptiveness of the impacted surface. Frozen deposit is not nearly as receptive to trapping impinging solid particles,

85

Although empirical fouling data appears to support deposition by inertial impact by virtue of an exponential increase in fouling rate with time, it may also be indicative of the change in stickiness of deposit surface as the deposit develops and the surface temperature increases. Field experience indicates a period of immunity during which fouling does not proceed exponentially, suggesting deposits are initiated by means other than impaction of sticky particle-see Fig. 46.16’ Morphological as well as chemical analysis of initiating layers indicate the first particles to stick are submicron particles of calcium and sodium originating as a fume in the gas stream, which condense in the vicinity of the tube surface and migrate to the tube surface via thermophoresis. Potassium associated with a low-melting silicate is not found in these layers except in systems containing Cl and substantial quantities of sulfur. Bituminous coals are generally enriched with potassium chlorine and sulfur. Low rank coals contain little potassium or sulfur but are enriched with calcium and sodium. The latter are more prone to fouling. An examination of the high-temperature surface immersed in the flue gas stream between 926 and 1093°C indicates the ash was formed as the result of the chemical reaction of SO3 with NaOH and submicron calcium particulate within the tube boundary layer. Levy et al. have shown that SOsU at the tube surface may be several orders of magnitude of that in the flue gas stream, permitting sulfidation to occur at locations where the bulk gas stream favors the formation of SO1. Analysis of fly ash leaving the steam generator indicates the kinetics of the sulfidation reaction is too slow for sulfation of the calcium in the bulk gas stream. In low-rank coals, submicron CaO provides sites for preferential deposition of Na7SOI over the tube surface--raising the melting temperature of the deposit ash significantly. The tight radius of the smaller particle reduces the subcooling required of the sodium sulfate to deposit on the surface. In the absence of calcium and presence of potassium, as in the case of bituminous, a corrosive layer is formed by direct condensation of supercooled Na2S04 on the tube surface. Deposition of KzS04 lowers the melting temperature further. The surface temperature of the deposit having been elevated above that of the tube, mid-range sized particles of silicates (some of which may already have surfaces enriched with sodium) deposit and remain. The SOs reacts with NOH present in the gas or the sodium at the particle surface to form sulfates and contacting adjacent particles are bound by the crystals of Na2S04 and CaSO+ At a temperature between 750 and 950°C. the CaS04 and sodium sulfate decompose in the presence of silicates forming a mellilite with melting temperatures as low as 752°C depending upon other minerals present. According to the following

R. W Bryers

86

reaction, [xNazS04 + yCaSOd] + A1203 .zSiOz -+ xNazO - yCa0. A1203 - zSiO* + (x + y)SOs

(24)

bonding shifts from a chemical reaction to viscous flow as the surface melts wet adjacent particles. One might expect the rate of deposit to increase as equilibria favors the transformation of sodium sulfate to silicates. To avoid fouling, one must use liberal tube spacing and limit temperatures to the convection bank based on equilibria of conversion of sodium sulfate to silicate and melting temperatures of the mellilites, rather than bulk ash fusion. Dead gas spaces and horizontal tube arrangements above 750°C must be minimized to avoid collection of fly ash that may proceed to sinter as a result of extended residence time at elevated temperatures. Sootblowing must proceed at sufficient intervals to avoid the transition from the weaker sulfate-bonded deposit to the stronger mellilite-bonded deposits. The gas temperature to the convection bank should be based on the melting temperature of mellilites rather than ash fusion temperatures and the quantity of molten phase expected from equilibria calculations.

500

700

900 DEPOSIT

1100

1301

TEMPERATURE

The basic sketch for the sintering of CaO as a function of the deposit temperature. Four casas are illustrated:

I

Sintaring Sintering

in an inert-gase phase: 100% NI with 1000 ppm SO, in tha gas phase

(3)

Sintaring

with

(4)

Total sintering tendency, when both SO, and CO, would be present in the gase phase

(1 (2)

Fig.

47.

10%

CO, in the gas phase

Sintering due to CO* and SO3 at various deposit

outer layer is rich in plagiocase, which he defined as a solid solution series between anorthite and albite.” In the absence of sodium, it would appear that depletion of O2 or the presence of sulfur was required to form a low-melting eutectic with either CaO or CaS04 essential to bonding fly ash particles to each other or the tube surface. 4.9.2. Calcium-sulfate-bonded deposits As early as 1959, Nelson et al.84 summarized Calcium-sulfate-bonded deposits were first reported Sulzer’s89 work on CaS04-bonded deposits formed back in the 1960s by Grant and Weymouth.‘69 They when firing oil. He proposed a mechanism as follows: have recurred on and off since then.‘6g-‘75 They have ‘CaO forms during combustion and collects on metal become increasingly more important in recent years surfaces where it reacts exothermally with the SO3 in with burning coals rich in organically-bound calcium the gas stream to form CaS04. Sulfur dioxide in the and in systems using finely ground limestone for gases may be oxidized catalytically to SO3 by V2O5 sulfur capture. from the oil combustion. Although CaS04 has a Grant and Weymouth’69 described calciumreported melting point of 1448°C Sulzer feels that bonded deposits as containing fine crystals of the heat of formation may raise the temperature calcium sulfate intimately associated with the ash sufficiently to cause a sticky surface. It should be particles in aggregates and effectively binding them pointed out, however, that the dissociation of CaS04 together. Crumley”’ proposed a mechanism for the begins at about lOlO”C, which is significantly below formation of bonded deposits based on the condensathis melting point’.84s9 Recently, Skrifvars et al.‘65 indicated sintering of tion of calcium chloride. Grant and Weymouth,‘69 Jakisch,“’ Zinzen,“* and Lenkewitz’73 all proposed calcium-bonded deposits depends on the SO2 and +L” ,....,*,11:.., ~al~llllll nn,?.:..... “..,C”tn “” l.&..” ,“A ..,‘ ...,t 1 CO* present. ~~‘ A....orPnt’ ;0 a t.,rr\ “+a..l J,alr‘lrl~., tr., LL,G..__,._0” ‘J’“CbJJ 1J rrrv-orti’ 1116~lyaralllllr; DuI,aLea?i ueu,g *L, L11e=111l IGJU‘ of oxidation of calcium sulfide at the tube surface. procedure beginning with sintering by CO2 at They also acknowledged the formation of a melt at temperatures as low as 300°C due to the formation 830°C in the calcium-sulfide/calcium-sulfate system. of carbonates and reaching a maximum of 750°C just Hein’ reported that calcium sulfates formed by before the decomposition of CaCO,. The process begins once again with the formation of CaS04, Rheinish Brown coals are not troublesome except in reaches a maximum at x954-1037°C as the CaSO the presence of sodium sulfate, which forms a lowreacts with quartz or aluminosilicates to form melting eutectic with the calcium sulfate, and causes calcium-see Fig. 47.‘65 sulfatic bridging. Under locally 02-lean conditions, Micro-analysis of a calcium-sulfate-bonded deposit sulfides will form which also create low-melting eutectics and bonding. using SEM reveals an inner layer composed of submicron calcium and sodium sulfate upon which Gibb’75 reported that calcium promotes the initiadevelops a completely molten or fused layer of Ca tion of deposit growth. He also showed that the initial and S, as shown in Fig. 48. The second layer is layer of deposit is anhydrite-rich, whereas the fused

Steam-raising

Mag. 3000X

Mag. 6000X

s

c

a CURSOR (KEY) = 65.166

First

fuels

Fused

EOAX

F NASS CT ALI E Al CURSOR (KEY) = 65.166

EDAX

I_,ayer ot the Deposit

Fig. 48. SEM photomicrographs

and EDAX scans of the finishing superheater deposit surface showing the tube deposit interface.

generally of uniform thickness and may contain submicron aluminosilicate spheroidal inclusions. Sulfated barium has also been frequently identified as an immiscible fused inclusion. The melting temperature of this portion of the deposit is probably in the temperature range Iof 590&65O”C. A number of theories have been postulated to explain the existence of a molten layer. A definite answer has not yet been developed. It has been observed that the deposit is frequently associated with low oxygen levels (i.e., <2.5%) and excess carbon. Consequently, the molten phase could be attributed to a CaSO, - CaS complex; however, CaS has never been identified in the laboratory. The melt could also be caused by sulfidation of submicron CaO in a zone where there is inadequate ventilation for dissipation of the heat of reaction causing temporary local excursions in temperatures to very high levels. Subsequent layers are formed by the sulfidation of

calcium deposited on fly ash particles, as shown in Fig. 48. The calcium sulfate begins as a submicron particle from which large crystals of CaS04 form. The sphere eventually becomes completely encapsulated with the crystals. Ultimately, the bond is achieved as crystals begin to share adjacent particles, illustrated in Fig. 24. As the deposits grow, the temperature rises. At 950°C the CaS04 decomposes in the presence of silica and aluminosilicates forming a fused calcium aluminosilicate, frequently identified as gehlenite or anorthite. The precise temperature of decomposition has been determined by TGA using artificial as well as real deposit ash, as shown in Fig. 49.87 This has also been observed by many other investigators dating back to the 195O~.s’~‘~~-‘~~ Fly ash initiating deposit formation in the temperature range of 510P704”C consisted of submicron particulate of CaO. Above 704”C, crystals of CaS04 appear which are the result of homogeneous

88

R. W. Bryers

Temperalure

(‘C)

Fig. 49. Thermal analysis of tubeside layer and cross section of primary superheater showing lowtemperature melts at ~600°C below the initial deformation temperature of the ash and decomposition of CaS04 in the presence of silicates at ~800°C. of CaO reacting with S03. The fly ash discharged from the steam generator in the flue gas shows little to no signs of sulfidation, indicating CaS04 is formed at the deposit/tube surface interface where residence time is many orders of magnitude higher than in the gas stream. Microanalysis of a number of fly ash samples indicates that in some instances calcium is scavenged by fly ash during combustion or post-combustion as particles of CaO adhering to the surface of aluminosilicates. In other cases, the calcium has gone into solution forming a single sphere. The availability of CaO at the fly ash surface varies with some yet-to-be-determined parameter (possibly flame temperature) which may have an impact on the degree of fouling. Calcium-sulfate-bonded deposits are associated with organically-bound calcium. They are prone to form at O2 levels below 2.5% and on horizontal surfaces in the heat recovery section between 750 and 954°C. Dead gas space and horizontal surfaces should be avoided as much as possible above 750°C. The fouling is coupled to a furnace slagging problem resulting in a loss in absorption and high furnace exit temperatures created by a highly reflective ash.

condensation

4.10. Modeling the Mechanistic Approach The mechanistic approach to an understanding of how and why slagging and fouling occurs has prompted the use of models and computers to seek a solution to fireside problems. An explosion in availability of analytical equipment in the 1970s and

1980s has supported this approach as well as provided the capability to identify minerals present in coal, their size, and association. CCSEM, in particular, has permitted examination of coal mineralogy and the fly ash it generates on a particle-to-particle basis. Thus coals with ash of similar elemental composition, but decidedly different mineral composition or distribution and association, can be examined in light of the differences and variations in ash deposition behavior between steam generators or locations within a given steam generator and can be accounted for or evaluated. Prediction of fly ash size distribution permits evaluation of differences in ash deposition and deposition rates throughout the boiler. For the first time, engineering parameters other than furnace exit may be incorporated into a slagging, fouling and corrosion index. It is reported that several utilities are requiring CCSEM as a routine characterization of fuels. New restrictions on emissions and the availability of the computer has facilitated and promoted detailed analysis and modeling of steam generator furnace fluidynamics and thermodynamics. Add to this the ability to examine mineral matter on a particle-toparticle basis and the availability of fundamentals of mineral transformations; the next logical step would be to attempt modeling of the slagging and fouling process. The bulk of the modeling thus far has been carried out in the U.S. by Brigham Young University (BYU), the Energy and Environmental Research Center (EERC) of the University of North Dakota, PSI

Steam-raising

Table 15. Principal Element

ash-forming

Solubility

Type

89

fuels

elements

in crude oil”’

in oil

Probable

chemical

Aluminum

Inorganic

Insoluble

Complex

Calcium

Organic 1norganic

Soluble Insoluble

Not identified Calcium minerals in suspension; calcium salts in suspension or dissolved in emulsified water

Iron

Organic Inorganic

Soluble Insoluble

Possible iron porphyrin complexes Finely-sized iron oxides in suspension

Magnesium

Organic Inorganic

Soluble Insoluble

Not identified Magnesium salts dissolved in emulsified water or in suspension in microcrystalline state

Nickel

Organic

Soluble

Probably

Silicon

Ilnorganic

Insoluble

Complex silicates and sand in suspension

Sodium

!lnorganic

Insoluble

Largely sodium chloride dissolved in emulsified water or in suspension in microcrystalline state

Vanadium

Organic

Soluble

Vanadium

Zinc

Organic

Soluble

Not identified

Table 16. Analyses Element

California

SiOz

Mid-continent

aluminosilicates

form

porphyrin

porphyrin

in suspension

complexes

complexes

of ash from crude oils (wt%)’

Texas

Pennsylvania

Kansas

Iran No. I

Iran No. 2

38.8

31.7

I.6

0.8

I.0

52.8

12.1

17.3

31.8

8.9

97.5

19.1

13.1

IX.1

8.7

12.6

5.3

0.7

4.8

6.1

12.7

MgO

1.8

4.2

2.5

0.2

MnO

0.3

0.4

0.3

0.2

v2°5

5.1

Tr

1.4

NiO

4.4

0.5

I.5

Na20

9.5

6.9

30.8

Fez03 Al203 TiO. CaO

K2O so3

Chloride

i

_

_

15.0

10.8

_

_

0.1

I.0 42.1 4.6

Technologies, Sandia National Laboratory, MIT, and Reaction Engineering International. The programs presently being developed are directed at specific aspects of the fouling and slagging rather than a comprehensive model. A comprehensive three-dimensional model for simulating combustion systems is being developed at BYU. This program simulates large-scale, steadystate, gaseous and particle laden, reacting and nonreacting systems. It uses advanced numerics which are capable of solving very large computational meshes that are required to simulate temperature and flow fields in large steam generator furnaces. Both convective and radiant heat transfer are modeled. The codes assume equilibrium gas-phase chemistry and couples the turbulent flow field with the chemical reactions by integrating the equation over a probability density function. The program is an essential

9.1

0.2

Tr

1.3

Tr

Tr

0.4

14.0

38.5

0.6

I .4

10.7

7.0

23.6 0.9

0.9

36.4

2.6

_

0.1

_

first step to evaluation of mineral transformation. The model has been validated on small boilers. Measured and predicted temperatures and species concentrations were in good agreement in most regions of the furnace. The largest discrepancies between measured and predicted values generally occurred near the furnace walls. This is the zone most critical to the understanding of slag deposit formation and growth. PSI Technology uses CCSEM data combined with traditional characterization data to predict slagging potential of a coal in specific zones. A fly ash formation model predicts size and composition of ash particles, allowing for char fragmentation and mineral coalescence. A viscosity model is used to calculate particle viscosity. Simplified boiler flow modeling predicts particle transport to the walls. Although it is claimed that deposit amounts and locations can be predicted, no claim is made to predict

90

R. W. Bryers

the impact of a coal on boiler performance. The model is limited to furnace slagging. MIT is currently attempting to integrate a number of computational codes that characterize the coal minerals by CCSEM, predict fly ash size and composition, and the probability of particle impaction and retention. Sandia has developed a reasonably simple model whose objective is to predict the elemental composition of deposited ash. The model is based on firstprinciple derivations and a series of experimental results. Attempts have been made to validate the program on large-scale steam generators. It has been indicated there is a need for rates of condensation and heterogeneous chemical reaction, and deposit properties such as emissivity, thermal conductivity and viscosity. EERC has developed two models known as ATRANI (Ash Transformation Version 1) which predicts size and composition, and LEADER (LowTemperature Engineering Algorithm of Deposition Risk), designed to predict low-temperature fouling. The first is stoichiastic based on random variables. The second is an expert system derived from a knowledge base together with some inferential rules related to ash formation. The modeling is in its infancy. Each is directed at solving only a portion of the total program. There are some who feel a totally mechanistic approach will never be achieved due to the overwhelming complexity of the chemical processes involved. However, when fully developed and applied piecemeal, they may be a valuable tool for diagnosing problems. The largest thrust in the fundamental arena has been directed at understanding mineral transformations and transport mechanisms. Very little attention has been given to deposit formation and growth. As pointed out earlier, the mechanism of deposit formation has not yet been identified in some cases. The mechanistic approach has identified a need for fundamental understanding in several areas as applied to fly ash: . Fragmentation. . Agglomeration. . Coalescence. . Heterogeneous condensation. . Scavenging. . Nucleation. . Crystallization. . Sintering. . Bonding or adhesion (ash to ash, ash to metal). . Re-entrainment. . Deposit properties such as emissivity, thermal conductivity. 5. FIRESIDE PROBLEMS WITH HEAVY OIL

5.1. Introduction and Background Fireside fouling and corrosion

due to impurities

D-

D-

D-

O-

I-

11I

,/_L

11_ 7(

lo

Temperature (“C) Fig. 50. Vapor pressures of fuel oil ash constituents.*“**-‘93

in oil became a problem in the late 1940s/early 1950s as steam temperatures increased and heavy oils from crudes rich in vanadium were introduced to the marketplace. The most thorough study of the problem was performed by Miller et al.179 for the Bureau of Yards and Docks of the Navy in 1963. The survey was based upon many fine surveys previously reported, as well as the Marchwood Conference of 1963 which was organized, in part, to resolve the problem.8‘t>ts0-‘s3 The mineral matter in oil primary responsible for fireside fouling and corrosion is vanadium and sulfur. Sodium is also a major contributor; however, it is a substance that is basically foreign to oil and is introduced during handling as NaCl found in seawater. High-temperature catastrophic corrosion by VzOs is the most important fireside problem associated with heavy oil. Fouling by addition of impurities such as sodium and additives to inhibit corrosion are the secondary problem. Although numerous approaches have been taken to inhibit the corrosion and minimize fouling, limitation of the surface temperature of heat transfer to the threshold level of V205 attack (i.e., melting temperature of VzOs-bearing ash) and the use of 50Cr-50Ni for uncooled metal supports have been the most successful. The use of magnesium as an additive has been successful in inhibiting corrosion of surfaces where neither of the two alternatives are applicable.

Steam-raising

Table 17. Melting

91

fuels

points of possible deposit constituent?

Compound

Melting,

Silica, Si02 Alumina, AlzO, Alummum sulfate, FezO, Ferric oxide, Fe*O, Ferric sulfate, Fe,(S04)3 Ferrosoferric oxide, FelOd Nickel oxide, NiO Nickel sulfate, NiS04 Nickel vanadate, NiO . V20, Magnesium oxide, MgO Magnesium sulfate, MgS04 Calcium oxide, CaO Calcium sulfate, CaSO, Sodium sulfate, Na2S04 Sodium pyrosulfate, NazSzO Vanadium trioxide, VzO, Vanadium tetroxide, V,O1 Vanadium pentoxide, V205 Sodium metavanadate, NaV03 Sodium pyrovanadate, 2Naz0. Vz05 Sodium orthovanadate, Na3V04 Sodium vanadyl vanadates NazO. Vz04. 5V205 5Naz0. V,O, . I lV>O,

Decomposes Decomposes

Decomposes

Decomposes

Decomposes

‘C

1720 2050 at 770 to AlJO 1565 at 480 to Fe?03 1538 2090 at 840 to NiO 900 2500 at 1125to MgO 2572 1450 880 at about 400 1970 1970 675 630 640 850 625

535

c

-ECON.--

Fig. 5 1. Typical

distribution

of deposits

---&Ii

in oil-fired boilers.‘95.‘96

-

R. W. Bryers

92

MAG 1500X - CROSS-SECTION CRYSTALSOF V

--h_

Si

K

Fig. 52. Typical oil ash deposition superheater surface illustrating inner crystalline deposit due to condensation of Na2S04 and VzOs reacting to form Na,O - V204 - 5Vz05.

5.2. Occurrence of Minerals in Oil Thomas’84 identified, by spectrographic analysis, as many as 25 elements intrinsic to the crude. The principle ash-forming constituents in crude oil were reduced and tabulated by Bowden’ in Table 15. Both inorganic and organically-bound oil-soluble forms have been observed for each element. The mineral matter is not destroyed during processing and thus appears in the heavy oil in the same form. Sodium is not intrinsically found in crudes; consequently, its concentration may increase substantially during shipment. However, it should not exceed 60 ppm if properly desalted at the refinery. Asphaltic base crudes are generally rich in vanadium, particularly those from Venezuela, which often contain as much as 900 ppm reported as V205 .ls6 Some crudes from the Middle East and California also contain appreciable amounts of vanadium. Paraffinic base crudes are usually free of vanadium.‘79”81 Typical

analyses of crude ash appear in Table 16.8 Iron is the second-most abundant element in crude oil. In more recent years, nickel has replaced iron as a leading second element in some crudes. 5.3. Combustion of Heavy Oil During combustion of oil, vanadium is volatilized and oxidized to V,O, in the presence of excess air, exceeding 3%. Unlike sulfur, the kinetics of the oxidation process are fast enough to insure complete oxidation at temperatures encountered in oil flames. VzOs has a high vapor pressure, as illustrated in Fig. 50, and remains in a gaseous state to temperatures in excess of its melting temperature (i.e., 675°C) at concentrations in the oil equivalent to ~80ppm.’ Below 3% excess air, the oxidation of vanadium is not complete and V203 and/or VZ04 are formed.‘87-194 The latter have low vapor pressures and very high melting temperatures (i.e., 1970°C). Table 17 lists

Steam-raising

93

fuels

600

IO

0

30

I

I

SO

SO

L 100

20

60 40 50 No,O.mol percent

I

I

I

I

I

I

60

50

40

30

20

0,

,mol

Fig. 53. Phase diagram

Table 18. Melting temperatures of various magnesium and vanadium compounds deposited in steam generators’98

Na2S04 NaV03 NazO. 3V205 Naz0.6VzOct 2MgO. 3MgO.

Melting characteristics Sinter point, ‘C

Initial melt point, “C

Final melt point, “C

_ 528 554 590

590 621 657

885* 621 668 701.6 798* 1073 1243

_

v205

VZ05 V205

704 976

835 1190

Na/V mole ratio

I/O l/l 113 116 O/l _

* Reported in ‘Handbook of Chemistry’, Lang, 7th Edition. t Melting point of Na$. VZ04 reported, which is slightly higher than freezing point of NazO - 6V205; 620°C. the melting temperatures of constituents frequently found in oil ash.8 Consequently, VZOj and VZ04 are innocuous, leaving the combustor as dry suspended matter.

5.4. Deposition on Heat-Transflr

80

70 V,

Compound

70

Surfaces

Vanadium, sodium, and sulfur are the elements in heavy oil primary responsible for ash deposition. The deposition rate is greatest in the superheater and furnace.‘79 Vanadium is the dominant element present in ash responsible for ash deposition when firing oil not contaminated with sodium or additives, as illustrated by Jacklin et a1.,‘95 and Kirsh and Pross,‘96 in Fig. 51. The deposit usually consists of a few very thin layers of yellow-red submicron particulate, upon which develops a thin black crystalline scale, illustrated in Fig. 52. The scale is thin, as VZOs melts at a very precise temperature only 100°C above maximum tube metal surface temperature. Its surface

90

1

10

percent

for Na?O-V,OS

system.*

tension is very low permitting it to wet all surfaces in the immediate vicinity. The deposit is formed as a result of condensation of pure V205 on the tube surface. Oils containing sodium in excess of 40ppm, but little to no vanadium, will produce voluminous deposits. The NaCl volatilizes and decomposes during combustion forming Na(OH), which is subsequently sulfated at the tube surface, forming Na2S04 with a melting temperature of 855°C. Although the bisulfate may not form, pyrosulfates can form since generally sufficient sulfur is available to create the equilibrium condition for forming Na2SZ07 (i.e., 1200ppm). In the presence of other compounds such as CaS04 and MgS04, low-melting eutectics will form. Mixtures of high-melting sulfates such as CaS04 and MgS04 with low-melting eutectics produce thick deposits as the temperature differential between a completely dry ash and a totally liquid slag is large. The intermediate state is a plastic phase with a high viscosity. Oils containing sodium in excess of 40ppm and exceeding 0.3 times the weight percent of vanadium will lower the melting temperature of V205, forming eutectic mixtures with extremely low melting points, as shown in Fig. 53.8 When introduced in sufficient quantities, additives used to inhibit vanadate attack will increase the fusion temperature of the ash significantly creating a fouling problem. Refractory oxides can raise the melting temperature of the ash to well over 1243°C depending upon the formation of intermediate compounds with higher solidification temperatures---see Table 18.197,198 5.5. Vanadium Corrosion Attack

by VZ05 has been associated

with the

94

R. W. Bryers

Hansen and Kessler”’ provided the empirical correlation illustrated in Fig. 54, indicating corrosion became significant once the concentration in the oil exceeded ~SOppm. Babcock Atlantique showed an increase in corrosivity with gas temperature for a given metal temperature.*02 5.6. Additives

0

.

, loo

20; 300 Vanadium

, 400

I

500

600

in Oil, ppm

Fig. 54. Oil ash corrosion.“’

formation of a melt. The molten V205 acts as a fluxing agent for protective oxide coatings of most metals leaving the surface in an exposed, reactive state. Metal wastage is further enhanced by the release of O2 and Na2S04 and V&s react at the surface of the tube to form beta-sodium vanadyl vanadate according to the following reaction: NaO - V204 - 5V205 + 1/202 + NaO - 6VlOs

(25)

and Na20 - 6V20, + Fe + NazO. V204 - 5VzOs + FeO.

(26) O2 formed by this reaction is augmented by 02 absorbed at the surface of the molten slag from the surrounding and diffuses through the molten film to the unprotected surface sustaining the high rate of oxidation. Phillips and Wagoner’99 found the greatest oxygen-absorbing capacity, most severe corrosion, and greatest differences between melting and freezing temperature all occur at approximately the same ash mixture that is at Na20 * v204. 5v205. Experiments by Niles and Sanders19* showed that three intermediate compounds could be formed between V205 and Na2S04, depending upon the ratio of Na2S0 to V20s present. Thus, Na2S04 + V,Os + Na20 * V20s + SO3 Na2S04 -I- 3V205 + Na20 e3v205 f

so3

Buckland et a1.*03performed an exhaustive study of all of the elements in search of an additive to inhibit corrosion and found magnesium to be by far the only choice. Young and Hersey,19’ and numerous others, confirmed the selection of magnesium. Alumina was found to be a good complementary additive for minimizing the impact of fouling, which would inevitably occur with the addition of magnesium to the oil. Niles and Sanders’98 investigated the behavior of the magnesium-vanadium-sulfur system. The results showed that the equilibrium compounds formed in the reactions depended upon whether the sulfur or the oxide was used as a reactant. When magnesium oxide was reacted with vanadium pentoxide, the equilibrium compound identified in the reaction was magnesium orthovanadate (3MgO - V20s): 3MgO + V205 + 3MgO. V205.

(30)

It was further shown that the formation of this compound is not a function of magnesium concentration. When the mole ratio of MgO to V205 was less than 3: 1, the reaction products were 3Mg0.V20S and V2Os. When the ratio was greater than 3 : 1, the 3MgO - V2Os and MgO were found after heating. At equilibrium, the compound identified in the reaction of magnesium sulfate with V205 was magnesium pyrovanadate, 2MgO. V205: 2MgS0 + V20, + 2MgO - V205 + V20, + 2SOs. (31)

(27)

When magnesium sulfate was increased to four times the molar ratio of V20s, the products showed the pyrovanadate and excess magnesium sulfate:

(28)

4MgS04 + V 20 5 + 2MgO * V20s + 2MgS04 + 2S03.

(29)

(32)

Off-stoichiometric ratios produced mixtures of the bracketing compounds. Field experience has indicated, however, that regardless of the sodiumto-vanadium ratio in the oil, only NazO. 5V2Os has been found in ash deposited in steam generators, and either Na2S04 or V205 depending upon which side of stoichiometry the concentration fe1l.2ooThis seems to imply that only a portion of the total vanadium is completely oxidized, maintaining a low level of V205/Na2S04 in the flue gas.

The ash fusion data for the magnesium orthovanadate and pyrovanadate appears in Table 18. The pyrovanadate has a low softening temperature and would appear to cause fouling in most steam generators. The orthovanadate has a somewhat higher softening and fluid temperature and might be expected to cause fouling in boilers with high furnace exit temperatures. In a more complex situation where sodium and magnesium are present in the oil along with the

NazS04 + 6V2O4 + Na20 +6V20s + SOs.

Steam-raising

fuels

95

r

1

0

2

3

4

5

6

7

8

Excers Air Fig. 58. SO? formation

as a function heavy oil).*”

Corrected to 13OO’F Metal Temperature

of excess air (2.5% S

II 15

10

5

0

Excess Alr 38 Fig. 55. Critical excess air for high-temperature

-

corrosion.“”

140

0

2

4 6 8 Excess Air

10

20 Fig. 59. Stack-solids Excess

Fig. 56. Critical

acidity as a function

of excess air.“”

Air

exces’s air vs deposit

formation.*”

* 4.0‘0 : u) ul = s

3.0 -

E _ g

2.0-

2 (D I! 8 l.O-

1100

1200

1200

1400

Tube MOW Tompemtura, OF

Fig.

57. Effect

of excess air and metal temperatures corrosion due to V20,.2’o

2.5 S tiowy Futi Acidity Up to pH 7.5

%

% Q z 0

on Fig. 60. Stack-solids

1

2345670 Excess Alr

formation

as a function

of excess air.“’

96

R. W. Bryers Table 19. Ash analysis of various types of cokes

Elemental composition

Delayed coke

Shot coke

Fluid coke

Flexicoke

Si02

10.1

A12O3

6.9 0.2 5.2 2.2 0.3 1.8 0.8 0.3

13.8 5.9 0.3 4.5 3.6 0.6 0.4 0.3 1.6

23.6 9.4 0.4 31.6 8.9 0.4 0.1 2.0 1.2

28.2 2.9 0.6 2.7 10.3 0.8 0.5 10.0 0.0

12.0 58.2

10.2 57.0

2.9 19.7

4.5 39.9

1598 1598 1598 1598

1436 1598 1598 1598

1378 1386 1439 1473

1216 1322 1314 1537

1373

1259

1095

1281

1425 1431 1432

1429 1471 1471

1155 1183 1224

1386 1473 1431

TiO Fe203 CaO MgO Na20 !A0 Ni

6

v2os

ASTM ash fusion Reducing

I.D. ST. (Sph.) S.T. (Hem.) F.T. Oxidizing

I.D. S.T. (Sph.) S.T. (Hem.) F.T.

2.4

2.2

1.6 1.4

1.2 1 0.8 -

Porphyrm (‘VTPP)

0.6 04 02 0 20

44-I

60

Energy (eV)

Fig. 61. A comparison of the forms of vanadium in delayed and fluid coke with the porphyrins found in oil using X-ray absorption near edge structure (XANES) analysis by Huffman at the University of Kentucky. vanadium, all the constituents will be found as principal components in the ash deposit. When sodium and vanadium are the major components of

an ash, the compound formed will depend upon the ratio of sodium to vanadium. When magnesium is used, SO3 partial pressure will control which vanadium/magnesium compound will form. Therefore, in a boiler where the SO3 partial pressure is

above equilibrium for the formation of MgS04 and magnesium is used as an additive, one would expect

to find MgS04, NaS04, and 2MgO - V205 in the deposits. Although the corrosion problem might be resolved depending upon the properties of sodium sulfate, magnesium sulfate, and vanadium pentoxide present, a deposit problem could occur as a result of the formation of mixtures of sodium sulfate and the

97

Steam-raising fuels pyrovanadates in the deposit that have low ashfusion temperatures. In the presence of MgSO, the Na2S04 ash-fusion temperatures can be lowered substantially. Again, not all investigators are in agreement as to the chemical reactions taking place. Elsmout and Kema204 disagree with Niles and Sanders.‘98 Their investigations showed that upon heating MgO - V205 mixtures, three well-defined magnesium vanadates are formed as expected, rather than the orthovanadate with an excess of MgO or V20s. They also conclude that, ‘in oil-fired boilers, compounds such as 3MgO. V205 and 2MgO - V20s are normally formed only in exceptional cases. even when an excess of MgO is present. The causes of heavy fouling are mostly due to the formation of compounds within the system Na2S04-MgS04 and Na2S04-Fe2(S04)sMgS04.‘04 It seems reasonable to conclude that if magnesium is to be used successfully with a high-vanadium oil, the sodium concentrations of the oil should be restricted or the oil should be washed free of the alkalis. 5.7. Low E.xcess Air

UTILIM

During the process of combustion, vanadium is oxidized to one of three compounds depending upon the availability of 02--V203, Vz04, or V20s. The compounds V203 and V204 have melting points in excess of 1648°C and therefore, if formed, remain dry in their flight through the steam generator. Consequently, reducing O? levels is one means of reducing fouling and corrosion. The approach was first introduced by Glaubitz.205,206 Since then it has been studied and successfully applied by Cardinal and Clevarec;z07,‘08 Tipler;*09 Seigmund and Chaikivsky;*‘” Hedley et al.;*” Brown;“’ Reese et 01.;~‘~ and Lovejoy et ~1.“~ Seigmund and Chaikivsky’s”” experiments indicated the critical level for high-temperature corrosion and deposit formation was 3% excess air, as illustrated in Figs 55 and 56. The effect of metal temperature and a reduction in excess air on tube metal wastage are illustrated in Fig. 57.“’ The impact of low excess air on SO3 is shown in Fig. 58,“’ and the reductions in slack solids acidity illustrated in Fig. 59.“O Total slack solids increase slightly with a reduction in excess air as shown in Fig. 60.“” With the reduction in SO3 level, the cold-end dew point for sulfuric acid drops, permitting a slight increase in boiler efficiency. O’Neal”’ indicates a 1% change in excess air results in a 0.05% change in efficiency. Therefore, a decrease in excess air from 20 to 5% means a 1% increase in boiler efficiency.

6. FIRESIDE PROBLEMS WITH PETROLEUM COKE

Since the late 1950s. an excess of petroleum has created an interest in using this byproduct

coke from

STYLE STEAM GENERATOR SIDE ELEVATION

STAGED ADOITION COMBUSTION AIR

OF

Fig. 62. A typical steam generator design for firing petroleum coke and other solid fuels containing less than 10% volatile. a refining operation as a fuel for steam generators producing steam and electricity. The increased demands for oil products necessitated taking deeper cuts in refining crude, leaving a residual solid coke high in vanadium (i.e., lOOO-lO.OOOppm), iron, and nickel. Table 19 shows a typical analysis of coke for the various types of coking operation. The total concentration and the relative composition are

R. W. Bryers

Fig. 63. Gas recirculation in a coke-fired boiler.

Mag. 1200X

Fig. 64. SEM photomicrograph at 600x of crystalline nickel vanadate formed on the superheater of a petroleum coke-

fired steam generator.

dependent upon the source of crude and, to a lesser extent, the cooking operation. XANE analysis of the coke from various operations performed by Huffman and illustrated in Fig. 61, shows that the vanadium remains as a porphyrin in the coke despite refining process temperature. There are essentially four different types of coke being considered for steam raising: delayed coke, shot coke, Auid coke, and flexicoke. Delayed coking is an endothermic process in which the crude is rapidly heated in a furnace batch-wise and then confined to a reaction zone or coke drum under proper conditions of temperature and pressure until the unvaporized portion of the furnace effluent is converted to vapor and coke. Products of the coke are gas, gasoline, gas oil, and coke. Although the exact mechanism of delayed coking is extremely complex, it can conceptually be broken down into three distinct steps: ?? Partial vaporization and mild cracking of the feed as it passes through the furnace. ?? Cracking of the vapor. ?? Successive cracking and polymerization of the

Steam-raising

OF PURE MOLTEN 2 I=: 5”

fuels

V20s

OUTSIDE

ig kt =w 1

99

LAYER OF DEPOSIT

-

W aa

-AT= l°C

?f

$

-

if t-m

-

ri_

L_

INSIDE LAYER OF DEPOSIT

1 200

I

I 400

I

I 600

I

I

800

TEMPERATURE,

Fig. 65. Differential

thermal

I low

,

‘C

analysis thermogram of pure VzO, compared deposit thermograms.

liquid trapped in the drum until it is converted to vapor and coke.“6.2’.’ The coke produced from the normal feeds is hard, porous, irregular in shape, and occurs as lumps ranging in size from 20 inches down to fine dust. This type of coke is called ‘sponge coke’ because of its appearance. Delayed cokes are literally mined from the coking drum in a roding operation as large chunks, which must be ground and pulverized to 10% <200 mesh for combustion in suspension.“’ When charging the coke with highly aromatic feedstocks at high pressures (i.e., lOOpsig), a coke with needle-like structure can be produced. The needle coke has high strength and a low coefficient of expansion. It is preferred over sponge coke for use in electrode manufacturing because of its lower electrical resistivity and lower coefficient of thermal expansion.“’ A third type of coke is produced unintentionally during operational upsets. probably as a result of low coke drum pressures or temperatures or low API feed gravities. This type of coke is called ‘shot coke’ as it occurs as clusters of shot size pellets. Strangely. shot coke is peculiar to California crudes.“’ Fluid coke, as the name implies, is generated in a fluidized bed reactor operating at 1000°F. Feedstock is sprayed onto a bed of fluidized coke. The thin film of feedstock is vaporized and cracked by steam introduced to the bottom of the reactor as the coke laden with feedstock flows downward and is withdrawn from the bed. The coke is removed from the reactor where it is fed to the fluidized bed burner. Approximately 25% of the coke is spent to raise the temperature of the reactor coke back to 1000°F and thus provide for the endothermic losses due to feedstock vaporization and cracking yields form fluid cokes which are about 75-80% that of delayed coke.

I

to vanadium-

and nickel-rich

ash

The resultant fluid coke withdrawn from the burner is a solid spherical particulate smaller than 8 mesh (i.e., <2380pm). The coke is very abrasive and can have a Hardgrove Index as low as 17. The flexicoke is essentially a fluid coke that includes a gasifier loop for gasifying and heating the coke leaving the fluidized bed burner in the fluid coke system. The latter functions as an intermediate heat exchanger between the reactor and gasifier. Adding the gasifier to the system increases the yield. Flexicoker yields can be 2240% higher than that of delayed coker. At the higher yields, one might expect higher concentrations of ash. The yields may also be limited by the initial concentration of minerals in the feedstock. 6. I. Fireside Behavior qf’hfineral Matter in Petroleum Cokes Petroleum cokes containing vanadium in concentrations as high as 4000ppm have been fired since 1957 in full-scale steam generators, illustrated in Figs 62 and 63, free of corrosion or fouling except for occasional deposits attributed to either an excursion in nickel or sodium concentration or operation resulting in total char burnout. The arch-type furnace is used to ensure flame stability and provides additional flame length for the low volatile, less reactive petroleum coke. The furnace box is partially lined with refractory to increase flame temperature. Air is introduced along the flame path and hot gas is recirculated to permit ignition and flame stability. The deposits, when formed, consisted of nickel vanadates formed as a result of condensation of a nickel compound and VZ05. Their occurrence is usually limited to a narrow flue gas temperature range. Figure 64 illustrates the crystalline nature of the deposit

R. W. Bryers

loo

fly ,::! G

Fig. 66. A typical low-pressure refuse-fired steam generator. formed. A DTA thermogram of the petroleum coke deposit, illustrated in Fig. 65, indicates that two endotherms were encountered during heating-one coincident with the liquidous line for V20s + Ni(V03)r, and the other coincident with the liquidous line of Ni2V207-suggesting the presence of Ni(V03)2 + Ni2V207. The deposits were formed by condensible species on nickel and vanadium that occurred during operating conditions which promoted complete oxidation of the vanadium to VzOs. Slagging has also been encountered on occasion in recent years. Careful analysis of the deposit attributes the deposition to dirt inclusions pickup during handling of the petroleum coke.2’8-22’ When properly handled and fired, petroleum coke can tolerate vanadium levels several orders of magnitude higher than oil due to suppression of the oxidation of vanadium to its highest oxide state. V203 and V204 have very high melting temperatures and are innocuous with regard to the formation of fireside problems. Recent combustion tests have demonstrated that near-complete char burnout resulted in generation of very little V20s.2’o The reduction in rate of oxidation must then be attributed to differences in the way vanadium is released from liquid and solid fuels. In the case of oil, the oxidation

of the carbon and vanadium occurs directly at the surface of the droplet at very high temperatures. Solid fuels burn slowly at lower temperatures. In addition, the vanadium must migrate through an atmosphere enriched with CO and CO2 countercurrent to the flow of oxygen. Exposure to the maximum level of oxygen occurs at lower temperatures in a flame front removed from the surface of the particle. The rate of cooling is faster than the rate of oxidation of vanadium to V205. Recent combustion tests of fluid cokes containing 10,OOOppm for 1OOh in a pilot plant indicated the cokes could be burned free of fouling, slagging and corrosion, despite very high levels of vanadium. Corrosion by vanadium pentoxide is catastrophic and 1OOh is more than adequate to identify its occurrence with carbon steels or super-alloys. The pilot plant on which the tests were performed is designed to reveal moderate to severe fouling in 5 h of operation. Consequently 1000 h provides a good measure for demonstrating extended operation, free of fireside problems. One of the cokes was clean and contained very little accessory mineral species. The second coke was contaminated with dirt, reducing the vanadium concentration to 39% in the ash. The tests indicated,

Steam-raising

fuels

Metal Temperature. 200

C 600

400

o.6x 1400 F Gas Temperature o 1550 F Gas Temperature

0.5

13

t

A 11

9; 2 3i X

ti

X

7%

X

:

*X X

5;

X’

3

OG'

0

200

400

a00

600

Metal Temperature. Fig. 67. Effect of metal and gas temperature

in addition to a low ra1.e of fouling, the nature of the deposits formed were altered by the presence of dirt or accessory mineral species (i.e., SiO, A1?03. CaO, MgO, etc.). Deposits formed by cokes enriched with accessory minerals developed as a result of inertial impact of fly ash composed of accessory mineral species including calcium, iron and nickel which absorbed the small portion of vanadium converted to VZOs as an intermediate phase. Scanning electron microscope (SEM) photomicrographs, energy dispersive X-ray (EDAX) diagrams, and dot maps indicate the fly ash was reasonably coarse and the primary absorbents were calcium, iron and nickel. Silica was present but did not appear to be responsible for absorbing vanadium. The heavy metals and alkaline earths behaved as an additive absorbing what little VZOs formed. Deposits formed by coke ash void of accessory minerals developed as a result of condensation of volatile vanadium and nickel compounds. The ash was uniformly dispersed about the entire periphery of the tube and consisted of a l/64 inches thick crystalline scale. X-ray analysis indicated the material was primarily a solid solution of nickel vanadate (Ni3(V04)2) and vanadium pentoxide as VZOs. Again, less than 2% of the total vanadium fired deposited on the tube. The fly ash captured was composed of vanadyl hydrogen sulfate (V02)H2(S04) and some vanadium pentoxide. The SEM photomicrographs indicate the fly ash contained superfine material. probably generated by nucleation of a fume composed of a sulfate vanadate. Corrosion and serious fouling were avoided as a result of the controlling heat-transfer

1000

1200

F

on the corrosion

rate of carbon

steel.7J

surface temperature to levels below the melting temperature of VZ05, the formation of the lower oxide states of vanadium, and the high-temperature absorption of available V205 by calcium and nickel. It would then be expected that blending of petroleum cokes with coal in an environmentallysound manner would indeed provide sufficient absorbing mineral species to minimize a fouling, slagging and corrosion situation. Indeed, this has been the case for those few reporting operating experience with petroleum coke/coal blends. Formation of vanadium pentoxide during the combustion of vanadium-rich petroleum cokes is inhibited by excess carbon in the fly ash, a reduction in the rate of oxidation of vanadium and/or the presence of additional mineral species such as SiO? or CaO in the ash. Petroleum cokes containing as much as 10.000 ppm VZOs have been fired free of corrosion with minimal fouling. There appears to be sufficient absorbing minerals in coal ash to inhibit fouling, slagging or corrosion when petroleum cokes are blended with coal in an environmentally-sound manner.

7. FIRESIDE BEHAVIOR

OF INORGANIC IMPURITIES MUNICIPAL SOLID WASTE

IN

I. 1. Tecllnicnl Bmkground

The rapid increase in affluence after World War II in North America and Europe led to an increase in both the quantity and quality of refuse. The shortage of landfills and the cost gave rise to increased use of incinerators. In Europe where fuel costs were higher

102

R. W. Bryers

in the U.S., and because of the administrative in the cities, waste-to-energy plants began to emerge in the mid-1960s. Initially they were coupled with steam turbines in power plants. The upper plateau of steam conditions was in the range of 925-2750 psia with saturated temperatures in the range of 280-365°C. Superheat temperatures were as high as 538°C. At the same time, wasteto-energy units began to appear in the U.S. but with steam conditions at lower pressures and temperatures.“” zs4 Shortly after startup, tube wastage was encountered on waterwalls in the furnace and superheater surface of several European unitsZZ4 A close examination of the problem by researchers in the U.S. and Europe indicated that tube metal wastage was caused by chlorine acting independently or in conjunction with heavy metals of lead, zinc, sodium, and potassium. Chlorine, as HCI, was believed to be the primary source of wastage of surfaces when they were exposed to reducing conditions or when erosion appeared to be a secondary source of tube wastage. It was also established that chlorides of the heavy metals could produce melting eutectics that could act as a fluxing agent on the protective deposit on the tube surface, subjecting the tubes to excessive rates of oxidation or chlorination.z23,“4 New designs evolved out of the first decade of operating experience. They involved major changes in furnace chamber geometry to insure complete combustion of the refuse and to establish the following empirical process design parameter to avoid fireside wastage.‘24.2’s All surfaces in the combustion zone were completely protected with refractory. Extensive baffling of one form or another was installed in the combustion chamber to promote turbulence and to insure complete combustion. Combustion zone exit temperatures were limited to 954°C. Radiant furnace exit temperatures of 760°C were recommended. Flue gas temperatures were limited in new installations to about 704°C in a parallel flow chamber prior to being introduced to the heat recovery zone containing the superheater, as illustrated in Fig. 66. Recommended superheat temperatures were reduced to about 443°C. Krause has shown, as illustrated in Fig. 67, that the corrosion rate increases significantly at higher tube metal temperatures for gas temperatures exceeding 760”C.74 Superheaters were redesigned to minimize erosion and to reduce the differential temperature between the flue gas and the metal temperature. A combination of parallel/counterflow heat exchanger surface was used to maximize protection of the high-temperature surface at the superheater outlet. Mechanical wrappers were employed rather than soot blowers to minimize erosion/corrosion problems. While tube wastage appeared in high-pressure/hightemperature units, there were a number of units both in Europe and the U.S. which successfully operated at lower pressures and temperatures. Process pressures than

organization

Table 20. Eutectic compositions Eutectic

mixture,

25 NaCI-75 37 60 70 70 20 55 70 60 58 70 52 72 90 80 49

with low melting points”’

mole %

FeCb

PbCL-63 F&I, SnCI,-40 KCI SnCIl-30 NaCl ZnCl2-30 FeC13 ZnCl?-80 SnClz ZnCIz-45 KCI ZnCl2-30 NaCl KClL42 FeClz NaClL42 FeCl? PbC12-30 NaCl PbCl?p48 KCI PbCI-28 FeCI? PbClzp10 MgC& PbCl>-20 CaClz NaCI-5 I CaCI?

Melting point F

C

313 347 349 361 392 400 446 504 671 698 770 772 790 860 887 932

I56 I75 176 I83 200 204 230 262 355 370 410 411 421 460 475 500

were reduced to about 650 psig and 246°C saturation temperature.‘22 During the mid-1970s interest in waste-to-energy plants in North America increased dramatically. Tube wastage also appeared in some of these units. Some North American designers followed the European concept of protecting the superheater with a doublepass radiant heat exchanger surface discharging at approximately 648°C. Others chose to use a single tall furnace with liberal use of alloy steel with furnace exit temperatures limited to 8 15°C. Superheater outlet temperatures and pressures were limited to 454°C and 650 psig, respectively. As in Europe, the geometry of the lower furnace, in most cases, was designed to promote turbulence and to ensure complete combustion. Although there was a significant improvement in availability (in some cases in excess of 90% over 3 years for the new systems installed for power production at steam conditions as high as 86Opsig, 443°C and some even higher), the problem of metal wastage still persisted. Novel remedial measures were undertaken to cope, including an array of tube covering materials and application methods; less vigorous slag/soot removal methods; avoiding radiant superheaters downstream of some or all of the boiler bank surface; and lowering the gas temperature to which the superheater was exposed. These measures further improve system availability but at significant cost. However, these were reactions to the problem and did not address the cause or its mitigation. As a result of the Sheraton Palm Coast Conference on Incineration of Municipal and Industrial waste, it was learned that the root cause of the metal wastage remained uncertain.“3 Furnace wall wastage could be due to HCI or Cl reacting with the tube surface under reducing conditions, or it could be due to reducing conditions alone. It has not been clearly established whether the corrosion is a gas phase HCI attack in

Steam-raising

Whlte(NoK),SO,.K,Pb(f

SO+& p

Fe,O,.Fe,O,

,(NoK&SO,

PbSO.

fuels

-No,SO,.KOH.SIO,.M~,BO,

Whlte(NoK1,SO, ____

C&O,

Fe,O,,Fep,.(NaK),SO,

4PbO’PbSO,

Amber PbO.PbSO, Fe,O,.Fe,O,,(NaK),SO,

Scale-DeposerSeporol~on

adherent to deposit

Interlace

Mlxed oxide layer adherent to substrate

a

Fe,O..

Red a Fe, 0, powder Fe CI,.FeS Dtscontmws white and blue-block layer 900-l

h

3 Fe,O,.Fe,Q

Fe.0.

IOOT

Fig. 68. Schematic

I

Red Q Fe,O, FeClz Cotmnwus 600

diagram

powder

white layer

Red Q Fe,O,

location

the absence of deposits or Cl generated within the deposits in the absence of Oz. There is some uncertainty as to whether the reducing conditions are created by CO or unspent polyvinyl chlorides. There is also uncertainty as to how the chlorides arrive at the tube surface (i.e.. polyvinyl chlorides or salts of sodium, potassium, zinc, or lead). Some feel the corrosion could be caused by the fluxing action of low-melting eutectics. Some observers have witnessed corrosion in the absence of deposits, while others claim deposits are the source of the problem. The one common denominator is the presence of reducing conditions in one form or another when surface temperatures exceed ~246°C. Good mixing, sufficient residence time, and combustion temperature all influence the furnace wall corrosion. In stoker-fired boilers where a heterogeneous wet fuel such as refuse is fed on a grate cross flow or perpendicular to the direction of the flow of air and flue gas, thereby placing extraordinary local demands on precise metering of air to meet the local stoichiometry requirements of the fuel. Measures such as the introduction of 40% or more excess overfire air and modifications in chamber geometry to ensure sufticient turbulence must be taken to minimize CO levels and carbon loss. Development of a means of monitoring temperature and CO in situ on a

powder

FeCI, Contmuous gold layer 300-

- 000°F

showing

I

5009

of phases identified

by X-ray diffraction.7J”‘

continuous basis, as well as the distribution of air. introduce new technical dimensions directly responsible for the elimination of the corrosion problem and minimizing its economic role in refuse incineration. The process parameters (i.e.. time, temperature, and mixing) are intimately woven into parameters affecting emissions. Mechanistically, on-line corrosion due to chlorine in fuels can be broken down into several categories (i.e., molten salt attack, high-temperature corrosion due to Cl or HCI under oxidizing and reducing conditions, and low-temperature corrosion due to condensation of HCI). Bryers and Kereke? were the first investigators to recognize heavy metal molten salt attack may occur on high-temperature incineration heat-transfer surfaces exceeding temperatures of -443°C. The attack will occur if low-melting eutectics formed by sulfates and chlorides in the heavy metal cations-Pb, Zn, Na, and K-deposit on the tube surfaces. Typical melting temperatures are shown in Table 20.“’ As shown earlier in Fig. 13. the chlorides of the heavy metals have relatively high vapor pressures.47.4” Lead appears to be the most likely candidate for deposition on the tube surface. The presence of the heavy metal chlorides. of course, will depend upon the temperature level of the flue gas at the tube gas interface, the partial pressure of the trace

R. W. Bryers

104

Corrosion

Rate Normalized to Metal Temp 640°C, Gas Temp. 1050°C

O0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

CHLORINE CONTENT OF COAL (% as received)

Fig. 69. The linear dependence

of corrosion

rate on coal chlorine

NOTES o O\

0

Sulplm~o 10n cNorla0

\o _

Ion

I

content

for austenitic

steek.76

ConcmtrrI8ons doIwmMd lrom 0~os11s surlaca

on

metal

14

c f.

40

80

100

METAL

Fig. 70. Plant rates of sulphate

and chloride

ion deposition

elements, and the existence of oxidizing or reducing conditions. Sommerlad et al.*‘* and Krause74 have identified the presence of chlorides in layers of deposit adjacent to the corrosion product, as illustrated in Fig. 68. The molten salts, when present, act as a flux for protective oxides on the tube, thus exposing the surface to continued high-temperature oxidation and/or chlorination. Krause74 also identified the presence of the least volatile iron compound, FeC12, between the corrosion product and the raw metal surface. The corrosion product suppresses volatilization, Its presence in deposits on waterwalls makes Cl or HCl rather

120

140

1EMPERATUAE Oc

measured

after the precipitator.‘-‘”

than chlorinated salts-the most likely candidates responsible for tube metal wastage. In most cases the attack of heat-transfer surfaces by chlorine in fuels is dependent upon the interaction of Cl or HCl with the metallic components, their protective film. or their corrosion product. The aggressiveness of the attack depends upon the partial pressure of Cl in the gas and the equilibria between HCl and Cl under reducing and oxidizing conditions. At low temperatures, equilibria favors HCl; at high temperatures, it favors Cl.” Under oxidizing conditions in the temperature regimes encountered within the tubeside layers of

Steam-raising fuels

BoilerBank

, Cyclone

Fig. 71. Typical

elevation

view of a stoker wood-fired

deposited ash, equilibria favors the formation sulfates with the release of HC1.74 2MC1+

SO2 + l/20?

-+ 2HCI + MzS04.

of

(33)

The corrosion rate of bare iron or mild steel surface in pure HCl typically occurs at a low parabolic reaction rate up to llOO”F, as shown by Brown et u/.,~’ in Fig. 16, because the FeCl? reaction product forms a stable surface layer:

steam generator.

lower temperatures, one must conclude that Cl, which forms preferentially over HCl under reducing conditions, was present at some point in time. Brown et al.“” clearly show that Cl corrodes steel rapidly at the lower temperature.68 Krause74 indicates cyclic reactions can occur beneath the deposit with the chlorine attacking the tube metal to form more FeCI1: 2/3FeC13 + l/20?

Fe + HCl ---t FeClz + H,.

(34)

Although, as shown previously, Fe& is somewhat volatile, catastrophic corrosion will not occur until the melting temperature is reached at 675°C. However, in the presence of sufficient oxygen, the very volatile FeCls will form: FeCI, + HCl + 1/407 + FeCls + 1/2Hz0

(35)

FeCls will also be formed by the action of HCl on the oxide scale which develops on the steel: 1/3Fez0,

+ 2HCl -+ 2/3FeCls

+ HZ0

(36)

Krause,74 Grabke,” and Mayer and Manolesco75 report disruption of protective oxide layer by HCI. The porosity increases with an increase in HCI concentration.” At 2 volume percent, the layers are completely destroyed. Since FeClz is found in the corrosion product at the

I

-+ 1/3Fe203 Fe 2

+ Cl? (37)

Under reducing conditions, Brooks and Meadowcroft76 have shown that the corrosion rate due to CO is enhanced by the presence of chlorine. At concentrations of HCl exceeding 200ppm, there is a transition from parabolic to linear kinetics with little further increase in rate for higher concentrations of HCI-see Fig. 17. The increase in corrosion resulting from the presence of HCI in the reducing atmosphere was attributed to oxidation/ sulfidation promoted by scale disruption through FeCl? formation and volatilization.“5 They show a linear dependency of corrosion rate on coal chloride content for austenitic steels in Fig. 69.76 The cold-end corrosion is due to condensation of aqueous acidic condensates on low-temperatures surfaces. The rates of deposition on cold surfaces of sulfate and chloride ions are shown in Fig. 70 as

1838 1844 1849

24.6 1.8 0.2 2.5 43.2 7.8 2.2 2.6 4.6 10.1 2.5 2.7

Ash analysis SiO, A&t% Ti02 Fe:% cao taco, MgO MnO P@s K?O Na,O so3

-K ’

52.8 6.1 0.0 0.2 38.6 2.3

Ultimate analysis Carbon Hydrogen Sulfur Nitrogen Oxygen Ash

Fusion temperatures, Initial Softening Fluid

17.4 20.0 2.6

Proximate analysis Volatile Fixed carbon Ash

Balsam

1760 1784 _

1.4

0.7 62.9 13.6 6.4

1.1 67.6 11.2 1.7 2.2 2.2 6.2 2.5

1755 1755 1760

2.6 7.3 0.8 2.2

1.2

2.0 0.6 *

52.4 6.4 0.0 0.1 38.4 3.0

72.5 24.0 3.5

White spruce

6.4 1.1 *

52.0 5.8 0.0 0.1 39.7 2.4

14.7 22.5 2.8

Black spruce

1627 1821 1827

7.6 0.0 0.1 3.1 58.4 11.3 4.7 2.0 2.2 5.3 2.0 1.3

52.1 5.9 0.0 0.1 38.6 2.0

72.9 23.7 3.3

Red spruce

1616 1783 1805

16.0 6.3 0.2 5.0 51.6 4.9 5.5 1.6 2.8 4.1 3.1 2.6

53.4 5.9 0.0 0.2 38.9 3.1

74.3 23.6 2.1

Jack pine

1744 1810 1816

0.6 62.3 14.6 1.9 0.3 2.0 7.2 3.9 0.6

1.5 0.5 *

51.8 6.5 0.0 0.3 38.0 3.4

78.9 17.2 2.2

Poplar

Table 2 1. Fuel properties

1760 1766 1771

2.9 58.2 13.0 4.2 4.6 2.9 6.6 1.3 3.2

3.0 0.6 *

57.4 6.7 0.0 0.3 33.8 1.8

bark

1777 1777

_

0.8 54.2 17.8 5.4 1.3 3.8 8.0 1.7 1.3

4.1 0.3 *

54.5 6.4 0.0 0.6 26.2 2.3

76.5 21.0 2.5

Birch yellow

Canadian

80.3 18.0 1.7

Birch white

of eastern

1727 1821 1827

1.7 55.5 1.4 19.4 1.0 1.1 5.8 2.2 1.4

39.5 3.8 *

50.4 5.9 0.0 0.5 39.1 4.1

75.1 19.9 5.0

Maple hard

1660 1738 1744

6.1 3.1 0.1 0.8 60.4 16.7 2.3 0.4 0.3 6.3 0.9 2.0

50.1 5.9 0.0 0.3 40.7 3.0

78.1 18.9 3.0

Maple soft

_ _ _

3.6 0.0 OFI 0.3 67.1 16.3 2.0 0.1 1.3 4.4 0.7 0.8

46.9 5.3 0.0 0.6 39.1 8.1

73.1 18.8 0.1

Elm

1633 1811 1816

1.1 68.3 2.2 11.5 0.4 2.3 2.6 0.9 0.8

12.4 0.0 *

47.5 5.5 0.0 0.6 38.5 7.9

75.2 18.9 7.9

Beech

1505 1533 1560

7.3 8.4 0.1 3.6 50.3 4. I 8.5 3.4 4.7 5.3 3.2 2.6

55.2 9.9 0.0 0.7 31.0 4.2

69.5 26.3 4.2

Tamarack

1788 1744 I799

1.3 53.6 9.7 13.1 1.2 2.1 4.6 I.1 1.9

10.0 2.1 *

2.5

53.6 5.8 0.0 0.2 37.9

72.0 25.5 2.5

Hemlock

5 5

9

Steam-raising fuels

107

Table 22. Chemical composition of biomass ash”’ “’

Bean straw (I) Safflower Rice hulls AlfAlfa Cotton gin trash Barley straw Corn stalks Rice straw Bean straw (II) Wood chips Corn fodder Paper pellets Corn stalks (ex.) Almond shell Corn cobs Manzanita chips Tree pruning Walnut shell Olive pits Almond shells Corn stalks Cotton stalks Rice mix Wheat + corn (I : 3) Rice straw (good bales) Rice straw (decayed bales)

Si02

Fe?0~

MgO

cao

ZnO

KJO

Na20

SO,

P,O,

29.9 20.46 94.6 7.96 23.2 44.7 50.7 75.2 32.7 8.3 55.3 57.2 63.3 22.6 40.3 5.97 9.95 13.6 10.5 18.6 71.7 33.0 75.0 71.7 78.3 78.6

2.7 1.2 0.03 0.51 1.93 2.6 3.14 0.58 3.93 10.0 2.4 4.29 4.72 3.77 4.06 2.86 I .94 2.44 2.2 3.83 7.1 2.8 0.47 3.3 0.36 0.44

0.9 6. I 0.02 2.87 2.87 4.84 3.08 0.83 3.65 6.22 3.32 0.83 4.78 2.49 2.49 4.94 8.29 3.65 3.48 1.99 2.7 6.05 2.5 8.3 2.0 2.0

4.67 10.84 0.25 I I.2 7.18 3.22 3.9 0.72 6.3 18.61 I .05 0.15 0.56 12.27 I.27 24.49 19.87 7.0 25.89 16.0 0.46 3.56 1.1 0.95 0.7 0.88

0.03 0.03 0.00 0.125 0.187 0.125 0.95 0.00 0.15 0. I93 0.087 0.31 0.00 0.05 0.22 0.25 0.06 0.44 0.12 0.23 0.02 0.07 0.00 0.619 0.00 0.00

22.34 30.0 I 2.4 33.97 I3 00 8.01 10.3 1 I.9 25.3 I I.8 9.59 I 85 X 37 I4 14 2.04 10.96 12.66 21 50 3.13 14.7 10.2X 2 I .40 15.85 14.76 13.0 14.50

0.52 0.91 0.135 3.64 1.59 5.25 0.53

4.7 8.36 2.24 4.64 4.24 I.8 I I .08 I.51 2.28 9.0 3.4X 4.0 7.2 X.0 8.74 6.74 19.72 8.48 17.2 17.4X 2.2 6.55 I .o 2.96 1.6’) I .x3

2.29 3.64 0.46 10.46 IO.00 Il.56 10.00 X.87 7.3 6.X7 2.YX 4.46 2.06 5.5 6.87 x.2 4.Yh 4 58 7 56 7.79 0.66 6.4 I.1 I.1 08 0.9

a function of temperature. Latham rr a/.“’ indicate condensation corrosion can be avoided by good thermal design and diligent housekeeping to avoid cold regions.

8. COMBUSTION OF WOOD Wood wastes were uniformly disposed of in tepee incinerators. Growth of the paper and wood products industry and a need for process steam provided the incentive for adapting wood waste to stoker-fired steam generators, as illustrated in Fig. 7 I. In practice. bark waste is usually fired in combination with other fuels which sometimes leads to some unexpected fireside problems.“’ The ash in wood is generally very low (running about l&3%) as shown in Table 21. The ash is very rich in calcium as calcite or carbonate (i.e., 50-60%). with minor amounts of quartz (i.e., 10% or less). Clay exists in trace amounts. Although potassium is found in minor concentrations, its level of concentration is 4 or 5 times that found in coal. The analysis of the ash, however, may be slightly altered during transportation of the log by sand inclusions or the intrusion of NaCI found in seawater. Ash fusion temperatures are generally very high due to the extremely high concentration of calcium. Wood contains no measurable level of sulfur. Consequently, during combustion the minerals commonly found in ,wood form a high-melting. calcite-rich ash slightly contaminated with potassium and silica. Consequently, the combustion of pure bark should not present a problem. However, sand

0.28 0.82

I .32 0.73 5.09 0.47 5.08 1.19 2.85 I .48 I .0X 7.60 5.86 0.33 1.374 0.54 0.54 0.43 0.44

Total 68.05

Xl.65 lO0.l’ 75.46 64.20 XI.11 93.68 99.x9 x2.43 71.61 78.Y5 7x. IY Yl.46 73.90 X5.53 67. I6 x5.93 62.92 77 74 X6.48 95.45 x3.57 97.56 103.64 ‘)7.7x 09.59

inclusions containing quartz partitioned from the calcium-rich wood ash during combustion may react independently with any potassium present as a volatile species. The potassium absorbed as K(OH) on the surface of the quartz produces a low-melting eutectic (i.e., 800 C), which will stick to hot surfaces or heat-transfer surfaces where the shear force produced by flowing gas is less than the adhering force to the tube surface. The presence of NaCI may not present a problem for pure wood bark. However, in the presence of sand particles. the slagging may be compounded by the increased level of alkali concentration and the enhanced volatilization of potassium in the presence of Cl. Blending of wood with any of the sulfur-bearing fuels almost ensures fouling due to calcium-sulfatebonded deposits as the sulfur reacts with the submicron calcite particles at the tube surface. By itself. wood does not appear to be a problem. Blended with fuel containing other minerals or mineral matter species, particularly sulfur-laden gases. may create serious fouling in the form of CaSO?/K,SO,-bonded deposits.

9. COMBUSTION OF BIOMASS

The genesis of the biomass power industry is in the paper and wood products industry, where wood is used to cogenerate heat and power for process

108

R. W. Bryers Table 23. Deformation and fusion temperatures of biomass ash’3’.2” Fuel name

Deformation temperature, “C

Fusion temperature, “C

Bean straw I Safflower Rice hulls Alfalfa Cotton gin trash Barley straw Corn stalks Rice straw Bean straw II Wood chips Corn fodder Paper pellets Corn stalks (ex.) Almond shell (ex.) Corn cobs Manzanita chips Tree prunings Walnut shell Olive pits Almond shell Corn stalks (8 1) Cotton stalks (81) Rice straw mix Corn stalks + wheat (I : 3) Good rice straw Partially decomposed rise straw

900 700 1439 700 1010 925 820 823 940 1050 1010 890 1120 790 900 1080 770 820 850 860 980 1110 985 985 1060 850

1150 1430 >I65 1550 1380 1100 1091 1190 1260 II90 1180 1130 1235 1440 1020 1400 1550 1225 1480 1350 1140 1275 1200 1260 1250 1280

I.D. #

1. 2. 0. 3. 4. 5. 6. 7. 8. 9. 10. I 1. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

m:

Solid Data - Osman’s Data I233.2341 @

Biomass -

Bryrrs

II

I

Ic

03

A

I

n

1600

1500

1400

0

._ % ‘fii 5

1300

1200

2000 0

I 10

I rod 20 a

Percent Bask I% E (

30

I 40

I go

I

I

60

70

I

I

1100

203 + Co0 + MgO + No20 + KS01

0

Fig. 72. Regression analysis of ash softening temperature vs percent basic in the ash for three different types of biomass compared directly with various ranks of coal.

E e

Steam-raising

Table 24. Summary

80% wood/20% wheat straw Almond hulls Almond shells Rice straw Wheat straw Olive pits Wood waste/almond shells Waste paper

of percent

ash determined

Sodium Potassium Calcium Magnesium Iron Aluminum

by various

techniques

Low-temp. ash <25O”C

TGA on LTA

1000”c

Proximate ash -950°C

9.20 9.03 3.57 22.58 9.44 2.26 8.32 8.70

7.6 3.79 2.07 17.1 6.6 1.28 6.6 6.96

7.67 4.87 2.25 19.09 7.35 1.53 6.23 8.56

Table 25. Chemical Chemical fraction

109

fuels

Water

fractionation-olive IM

Ammonium acetate

650 610 170 70 10 40

IM

Hydrochloric acid IJg/g

240 1400 910 450
420 150 197 66 750 400

9.2. Fuel Characterization Wood represents about 80% of the biomass consumed for raising steam. Agricultural waste (i.e., straw, manure, shells, etc.) constitute the remaining 20%. Wood presents very little fireside problems when burned in a steam generator by itself, despite the fact that S-10% of the ash is composed of potassium. The bulk of the ash is composed of calcium, and silica appears to constitute less than 10%. The ash is highly basic with high melting temperatures. Although potassium appears to be present as an organicallybound constituent which is highly volatile during combustion, in the absence of sulfur, it presents little trouble as it remains suspended in the gas stream as

4.0 5.0 2.0 17.5 5.0 1.0 2.5 7.0

pits

pg/g

uses. Wood has been used as a fuel for generating stream over 40 years. By 1989 approximately 6 GWe of biomass energy-based generating capacity was available in the United States-primarily owned and operated by industrial entities. Disposal of wood waste and availability of a cheaper source of fuel were the main reasons for using biomass as a source of energy. In the United States, biomass electric power generation experienced dramatic growth after the Public Utilities Regulatory Policies Act (PURPA) of 1977, which guaranteed small electric producers that utilities would purchase their surplus electricity at a price based on the cost of producing electricity that was avoided by the utilities. From less than 200MWe in 1979, biomass energybased generating capacity in the U.S. grew to approximately 66GWe by 1989. It is estimated that there are 1000 wood-fired plants in the United States, typically in the range of lo-25 MW.230

TGA on biomass 1ooo”c

Remaining residue

2400 245 940 400 112 270

K20 or K(OH) to very low temperatures. A typical wood-fired steam generator is illustrated in Fig. 7 1. Agricultural wastes, on the other hand, are rich in silica and high in potassium. Occasionally sodium occurs in substantial amounts. Calcium occurs in modest concentrations. The solid ash has very high melting temperatures; however, once the silica absorbs the alkalis present, the fly ash particulate surface becomes molten at very low temperatures. Agricultural wastes are generally low sulfur (i.e., ~0.05%) but contain chlorine between 400 and 4000 ppm. Osman and Gross23’.232presents the ash chemistry for an extensive list of agricultural wastes in Table 22. The ash fusion temperatures for the fuels appears in Table 23. To gain a better perspective for the fusibility of the ash as compared to coal, the physicochemical properties have been plotted on a regression analysis for softening temperature vs percent basic for coal in Fig. 72. From the regression analysis, it is quite apparent that there are three different systems: a highsilica/high-potassium/low-calcium ash as found in grasses; a low-silica/high-potassium/high-calcium ash found in wood derivatives, pits and shells; and a highcalcium/high-phosphorus ash found in manure. All, except for wood derivatives, have unusually low melting temperatures. Blending of various groups may produce ash with somewhat unpredictable melting temperatures, particularly when one recognizes a good deal of the mineral matter volatilizes during combustion. An examination of Osman and Gross’23’.232data, as well as undisclosed data by others, reveals a high degree unaccounted for when the fuel is traditionally ashed at 750°C. Ashing via various techniques such

R. W. Bryers

110

Combuatlon

\ SIo&aO,MgO

Ho&gonoouo Condonoatlon, Nucloatlon, Coal*aonco

InorganIc Vapor IY DF- PI\

Sulfldatlon K,C.KWCy’W

2.

i

&SO.

A

CI,K,S /’

-4

oontralnmont viscous Sla Molton Lw* ltlgh S;rt&ohmporaturo

-

-

_ _

- - Radhnt nod ?i-•lsfw

-Low sutfao. Tompemture

Fig. 73. Transformation

as low-temperature

L

Inwilal

1High Tamp. Convmt/vo Ho& ?hn*fof

Surface

of mineral matter in biomass.

ashing (LTA) at <25O”C, thermoanalysis on LTA at variable tem-

Chlorine in biomass appears as a chloride ion and serves the role of balancing charge. Its concenperatures, and high-temperature ashing at lOOO”C, tration is closely related to the nutrient composition summarized in Table 24, has taught us that certain of the soils. The levels of chlorine required for components are lost continuously as the temperature optimal plant growth are usually far less than the increases and are responsible for the unaccounted for. levels made available by nutrients. Therefore, Below ~750”C, the mineral matter components such variations in chlorine levels are usually indicative as potassium form nitrates and perchlorates as of soil conditions rather than plant physiology.234,235 artifacts of the low-temperature oxidation process. Phosphorus exists in its most oxidized form in These compounds decompose between 200-7OO”C, biomass fuels and is not reduced during plant releasing nitrogen and chlorine above 750°C. metabolism. It is primarily introduced in the form Livingston233 has shown a significant loss in the of dihydrogen phosphate ions (HzPO4) and either alkalis. Consequently, care must be taken in the remains in the inorganic form or is incorporated in procedures used to quantify the elements present. A organic structures by forming esters or pyrophostypical analysis by chemical fractionation of olive phates.236s237 pits in Table 25 at room temperature indicates Silicon is introduced by plant by absorption of potassium is organically bound, whereas sodium is silicic acid from the soil solution. It is present in most predominantly tied up as a silicate. Ash fusion plants at macro-nutrient levels (O.l-10% dry basis). temperatures mean very little since too much of the Silicon is deposited as a hydrated oxide (Si02 . nH20), alkali is lost during the ashing process. Evaluation usually in an amorphous form but occasionally in of the fireside behavior of mineral matter in biomass crystalline forms.234,235 then depends on an understanding of the minerals Potassium is the second most prevalent element present and how they behave during combustion. found in straws and grasses. Potassium occurs as a univalent ion (K+) that is highly mobile with little structural function. It is not associated with the silica. 9.3. Mineral Matter Potassium uptake is highly selective and correlates Sulfur can be incorporated into plants both by with plant metabolic activity. Osmotic potentials absorption and assimilation of atmospheric SO2 and across membranes and in the cytoplasm are reguby absorption through the roots. The two principal lated to a large degree by potassium. It plays an forms of sulfur in plants are as sulfates and organic important role in enzyme activation, membrane sulfur. The former increases with increasing sulfate in transport, and stomata1 regulation. Because of these the nutrient supply. The latter is far less sensitive to metabolic and transport roles, potassium is often sulfate supply in most plants.234,235 found in regions where plant growth is most gravimetric

Steam-raising

Mag.

3000X

Ill

fuels

Calcium occurs almost exclusively in the axoplasm. It forms exchangeable bonds with the ceil walls and has significant function in cell wall stiffening and the structural integrity of plants. It helps regulate plant growth.234.235 Aluminum is toxic to most plants and occurs in small quantities. Aluminum concentrations rarely exceed 300ppm on a dry basis. When it does, it is usually considered a marker for contamination via dust, dirt, or other soil inclusions.‘“4,‘35 Iron has two principal roles in plants: formation of chelates that are active in transport, and participation in reversible oxidation/reduction reactions. Iron is concentrated in leaves.236,‘37 9.4. Fireside Behavior c~j’Mineral~

Fig. 74. SEM photomicrographs at 3000x of calcium phosphate fly ash upon which potassium sulfate has deposited forming a voluminous deposit on screen tubes between radiant and convective heat-transfer surfaces.

Mag.

3000X

Fig. 75. SEM photomicrograph at 1500x of thin scale in convective furnace formed by deposition of K2S04.

vigorous. generally

Tree branches and forest wood slash are enriched with potassium compared to core

,,,d,234-236

Sodium is not considered an essential element to plant. In low concentrations (<2%), it may be beneficial to some plants vis substitution for potassium. At higher concentrations, it is generally considered toxic. High sodium concentrations are frequently an indication of intrusion of salt water or a process additive.234,235

During combustion, the mineral matter, generally referred to as ionically or organically-bound inorganic species (i.e., K, Ca, P, Fe, S and Cl), is released as a vapor phase. Depending upon how they are released and during which phase of combustion (i.e.. devolatilization or char), a portion may be absorbed by the minerals present [Le., SiOZ or included extraneous clay (dirt)]. As shown in Fig. 73, the K, P, Ca, and Cl are oxidized, chlorated or sulfated, as the case may be, to form oxides, hydroxides and chlorides. These compounds all have high vapor pressures and remain in a gaseous state until cooled to very low temperatures. Equilibria in the presence of silica, chlorine and sulfur dioxide favors the formation of chlorides at high temperatures, silicates at intermediate temperatures, and sulfates at the lower temperatures, say 1000°C and lower. The compounds formed and their level of concentration depends on the level of concentration of Cl, S, and O2 in the gas stream. Being that Cl concentration is generally low, potassium will be absorbed by the silicates, creating a particle encapsulated by a viscous potassium silicate enriched with potassium. The level at the surface and hence its melting temperature and viscosity depend on the rate of absorption of potassium and counter-diffusion of potassium and silica to and from the core. Potassium may also be absorbed on high-temperature surfaces of welldeveloped deposits or it may homogeneously condense in the presence of SO, or SO3 at lower temperatures as potassium sulfate. If this occurs in the gas stream, nucleation produces submicron particles that are subsequently agglomerated or scavenged by larger fly ash, shown in Fig. 14, deposited on cooler tube surfaces by thermophoresis. or leaves the boiler as a fume. Depending upon time and temperature, the potassium sulfate may also deposit directly on a tube surface, creating a low-melting sticky surface, shown in Fig. 75. Such a deposit will continue to grow until it reaches the dewpoint for sulfate, at which time equilibria is reached and growth ceases. If, during the course of the formation of the deposits, discrete particles impinge upon the surface. they may become

112

R. W. Bryers

,Y-

Economizer


Heater

Superheater

Secondary Superheater

Water-cooled

654°C

Grate Ash Hopper

Fig. 76. Typical

Table 26. Composition SiOz

Al203

TiOz Stoker-l:

Fuel Superheater Upper wall Grate slag

57.58 33.77 5.41 60.75

1.16 9.47 1.63 10.72

0.48 0.50 0.07 0.56

Fuel Superheater Grate slag

63.42 18.62 62.26

1.95 1.12 1.94

0.02 0.02 0.07

Superheater Upper wall Front nosewall

10.64 7.54 49.51

0.99 1.55 2.16

0.02 0.02 0.05

FezOx

\/

’

biomass

8.73 0.52 0.66

steam generator.

of fuel and deposits-stoker-fired CaO

MgO

Wood/20% wheat 3.98 11.29 3.57 14.68 2.74 4.97 3.79 11.25 Stoker-2: 0.66 0.32 0.48

stoker-fired

KzO

P205

SO3

Cl

8.14% 1.07 1.12 0.76 1.28

2.26 16.30 41.90 0.13

0.16

ash = 7.95% 0.83 13.10 0.47 33.40 0.47 17.7

4.96 3.46 3.74

1.95 8.67 0.04

2.40 15.20 0.04

0.30 0.12

6.05 1.56 0.40

0.97 1.12 1.58

1.29 1.06 3.94

3.90 5.99 0.20

26.00 29.20 4.89

0.05 0.16 0.06

-1.69 -2.23 0.45

straw blend, fuel ash = 2.96 3.4 6.89 3.79 4.09 11.80 1.26 9.05 27.90 3.23 2.32 6.01

Wheat straw, fuel 4.20 0.46 14.41 2.45 10.59 2.15

4.80 3.79 15.09

Na20

Stoker-3 0.80 0.08 2.71

boilers

43.50 51.20 18.70

CO*

Undet

0.13 0.91 4.31 -0.04

Undet = Undetermined.

entrapped, permitting interaction of the potassium sulfate with the aggregate and allowing it to grow thicker. Chlorides may form in a similar manner. However, their high vapor pressures, lower concentrations, and equilibria with other contaminants present produce a very narrow window for their deposition and collection. They may be confined to the external surfaces of low-temperature deposits (i.e., in the temperature range of 750-800°C). The minerals in the biomass (i.e., SiOz and Si07(NaCa)(AlSi)40s) melt and agglomerate during combustion. They may be subject to some fragmentation of the mineral or biomass char. Upon cooling, they solidify. Some SiO vapor may be released during reducing conditions at high temperatures. The dry, pure species impact on heat-transfer surfaces and reentrain unless captured mechanically or in a viscous layer. At very high temperatures, some of

the silicates may absorb potassium, forming a lowmelting compound. Whether or not the low-melting fly ash sticks and forms a deposit depends on the surface temperature, local temperature gradient, the level of local kinetic energy, and degree of supercooling of the depositing fly ash. Surface temperatures in excess of ~850”C, such as exposed refractory, will certainly be good collectors of impacting fly ash, particularly if the refractory is exposed to gases with a low level of turbulence. Deposits may grow to great thicknesses on such surfaces due to low heat flux and flat temperature profiles, particularly if the deposit is viscous or in a semi-crystalline state over a wide temperature range. Molten and supercooled fly ash depositing on cool surfaces stick and form either a frozen fused layer or sintered deposit. At high absorption rates (i.e., high flame temperatures), the ash experiences a steep temperature gradient due to the low thermal

Steam-raising fuels conductivity of dry sintered ash. Once the surface temperature reaches the initial deformation temperature of the lowest melting phase or the plastic range of the deposited ash, the thermal conductivity increases and the temperature gradients flatten, allowing the deposit to grow. Growth continues through the plastic range until the fluid temperature is reached. Equilibrium is reached once the fluid temperature is achieved at the surface and the flow away from the surface of molten slag equals the rate of ash deposition. The initial layer of dry sintered ash is primarily responsible for loss in heat transfer. The thickness of the deposit is primarily governed by the temperature range and viscosity of the plastic slag (i.e., the range over which the crystalline phase melts and goes into solution) and the viscosity of the final molten phase. 9.5. Steam Generator Design and Field Experience To overcome slagging and fouling by low-melting fly ash and large concentrations of volatile alkalis in the presence of SOS and Cl, a folded furnace design is used consisting of a radiant chamber in series with a convective chamber, illustrated in Fig. 76. The exit temperature from the radiant chamber is 898°C and the entrance to the convective heat-transfer surface immersed perpendicular to the flow of flue gases is between 654-732°C depending upon cycle efficiency and steam conditions. Surface parallel to the direction of flow may be immersed in the convective furnace. This type of surface will be subjected to scale formation by K2S04 and allowance must be made for a reduction in absorption. Jenkins et al.236 report that a white, reflective, porous scale, similar to that shown in Fig. 75, is laid down on furnace wall tubes and high-temperature convective surfaces composed primarily of sulfates. Slag formed in the lower grates and furnace walls are enriched with silica and depleted of S03. Heavier deposits in the superheater contain silicates but are enriched with sulfates and chlorides, as illustrated in Table 26.236 Some corrosion has been reported; however, the mechanisms remain undefined.

10. SUMMARY

Fireside problems are extremely complex. Disposition of mineral impurities from the fuel as fly ash or deposited ash on heat-transfer surfaces and refractory throughout a combustion system depends on mineral origin not elemental composition, mineral size, mineral association, juxtaposition of minerals with regard to combustibles, availability of oxygen, gas temperature, surface temperature, and orientation of heat transfer with the gas stream. Transformations that take place during combustion and postcombustion quenching may be thermodynamically controlled or kinetically driven depending upon the

113

mineral species and gaseous environment. Once the fuel is combusted, changes in state of the transformed minerals and mineral matter occur under a quenching mode and consequently, are subject to supercooling from a meta-stable state. The degree of cooling depends upon available sites to precipitate the change and the size of the site. Modes of transport include inertial impact and diffusivity in various forms. Dramatic partitioning of mineral species and hence individual elements occurs once the process of combustion begins. Consequently, the elements of iron, calcium, sodium, potassium, and magnesium found in two coals containing ash of the same composition may behave quite differently during the combustion process. The individual elements found in the impurities within a given fuel may also be divided in their behavior since portions of each follow different process paths through the steam generator as a vapor, solid, or liquid depending upon mineral form and subsequent transformations encountered. The deposition of vulnerable species varies throughout the steam generator as the mechanism for deposition varies due to local environmental conditions (i.e., O2 level), gas and surface temperatures, and local flow patterns. Several elements such as calcium and chlorine may play multiple roles in the fireside fouling or corrosion process. Submicron calcium may be providing sites for condensation of alkalis aggravating the fouling problem, while coarse calcium may act as an absorbent and thereby inhibit fouling or minimizing its severity. Chlorine tends to enhance corrosion by CO. It also releases potassium from otherwise stable silicates to form low-melting, potentially corrosive sulfates. Consequently, it is no wonder that simple elemental analysis of fuel ash and the corresponding fusibility based on heating a composite sample of coal ashed to 750°C is only 50% effective in predicting fireside problems. An understanding of the process of fouling, slagging, and corrosion from a mechanistic or phenomenologic point of view is by no means complete. To achieve these objectives, considerable information must be developed on areas such as fragmentation, volatilization, agglomeration, nucleation, etc., to predict fly ash size and composition, and vapor composition of heavy metal species. It may be that the total process is too complex to ever hope to predict explicitly the degree of fouling from a detailed fuel analysis. However, it is very important that we understand the phenomena taking place as best we can so that a correct diagnosis can be made on a fireside problem when it occurs. Too many deposit problems have been studied in the laboratory under conditions not representative of the actual case in the field, resulting in inadequate or erroneous solutions. The technology of fireside slagging, fouling, and corrosion has progressed substantially over the last 100 years. Much has been learned about very specific systems of minerals, their thermal behavior, and the

114

R. W. Bryers

threshold limits beyond which they will cause serious, unmanageable problems. In recent years, analytical and diagnostic capabilities have exploded to a level much beyond their utilization. However, today, steam generators are still constructed based on proprietary extrapolation of the best available prior experience and fuels are still evaluated on the simple elemental analysis of ash and ash fusion temperatures developed for an outdated combustion system. There is a need for bridging the gap between fundamental research and applied engineering-transforming technology from an art into a science. There is a need to apply our advanced analytical diagnostics to real-time fireside problems to determine the source of the problem. Some of the more specific areas requiring attention include: On-line monitoring of both slagging and fouling in order to effectively and efficiently carry out management of slagging and fouling. Continuous on-line monitoring of furnace exit temperatures to control slagging and avoid the creation of a fouling problem. Upgrading of fuel characterization by employing the more advanced analytical techniques and modifying indices to reflect design and operating conditions. An understanding of the impact of slag chemistry on furnace absorption. *,A better understanding of the operating and maintenance cost of slagging and fouling so that alternatives can be realistically assessed. Exploration of the impact fuel variability, blending, switching, and opening of new reserves on fuel characterization fouling, slagging, and corrosion. Establish the role of Cl in fireside problems. Standardize characterization of fuels on an international basis. Improve upon on-line management of deposits and develop new techniques for deposit removal.

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Antonio, Texas, July (1981). Prost, E., Recherches sur les relations existant entre le degre de fusibilite et la composition des centres de houille, Rev. Univ. Min. 31, 87-98 (1895); Monir. Sci. 46, 56-565 (1895); Sur les relations entre la composition et la fusibilite des cendres de houille, Bull. Assoc. Beige Chim. II, 199-226 (1980); The fusibility

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(1968). 10. Moody, A. N. and Langan, D. D., Fusion characteristics of fractionated coal ashes, Combustion October

pp. 15-17 (1933). 11. Gould, C. B. and Brunjes, M. L., Proportions of free fusible material in coal ash as an index of clinker and slag formation, J. Inst. Fuel pp. 226-229 August (1941). 12. Littlejohn, R. F. and Watt, J. D., The distribution of mineral matter in pulverized fuel, in The Mechanism of Corrosion by Fuel Impurities, H. R. Johnson and D. J. Littler (Eds), Butteworths, London (1963). 13. Borio, R. W. and Narcisco, R. R., The use of gravity fractionation techniques for assessing slagging and fouling potential of coal ash, ASME Winter Annual Meeting, Paper 78-WA/CD-3 (1978). 14. Bryers, R. W., Influence of pyrites on furnace fouling of large steam generators, ASME J. Eng. Power 98, 517-527 (1975).

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21. Nicholls, P. and Reid, W. T., Viscosity of coal ash slags, Trans. ASME, ASME 62, 144-153 (1940). 22. Raask, E., Mineral Impurities in Coal Combustion-

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