Comments on the paper by W. T. Reid

170

W.T. REID

C O M M E N T S O N THE PAPER BY W. T. REID N. GAT (TRW)

CHARLES L. WAGONER (B&W, R&D)

There seems to be a gap between research on mineral composition and its effect on viscosity of the slag and the application of this information. The question is what is done with the information on slag viscosity? It is possible to perform slag-dynamic analyses (similar to fluid-dynamics), in which the local slag properties vary with temperature as a function of the slag-layer depth. The driving forces are both gravity and the shear due to gas flow. This analysis will yield slag-layer thickness, as well as heat-transfer rates and areas of slag build up; viscosity models in the non-Newtonian regimes must be used, as well as the local thermal conductivity of the slag. Such an analysis may give clues to a proper design of hardware and proper operating conditions for a given geometry of the hardware. Although ash fouling is determined by various parameters and not every particle impacting on a surface sticks to it, some control over the impaction efficiency may be exercised. The impaction parameter is a function of the Stokes number or the stopping distance l = (pd2/18p)V for low Reynolds number. By modifying the combustion process, one may modify the ash-size distribution; similarly, the flow velocity in the boiler may be controlled to obtain a desired value for the specified parameter.

Slag can exist in the form of sticky glass particles at temperatures significantly below levels measured for initial deformation using the ASTM standard test method for fusibility of coal ash. For example, Fig. 1 shows values of apparent viscosity (for what is probably pseudoplastic, non-Newtonian flow) measured for coal D at temperature below IT, the initial deformation temperature. 1 Coal E did not show this behavior. The temperature span corresponding to the plastic range and the related slagging index (Rv~)were not predictable from measurements of ash-fusion temperatures. This observation supports earlier work showing that no correlation was found between the initial deformation temperature and the flow properties of slag. 2 Unpublished data acquired during slagviscosity measurements made at B & W since the Sage/Mcllroy paper 3 in 1959 shows that the measurement of slag viscosity provides far more reliable information than ash-fusion testing. Also, many of these measured viscosities could not have been predicted accurately using published correlations based on elemental analyses, ash fusion, etc. For lignitic ashes,* there is virtually no correlation between our

20,000

• %. COAL D N &

*Ashes with (CaO+MgO)/Fe203 > 1 and (CaO+ MgO + Fe203) > 20 ~o of the ASTM elemental ash analysis.

@ ~COAL E \

10,000 - -

uJ

5,000 - -

Io~ O k-

2.000 - -

O Z a O

_z

1,000

500

-

== o

200

I

I

COALE----q

C RANGE,600- 20.000 PO, ES,

100

COAL°

O

I

PLASTIC RANGE O

50

5

IT FUSION, COAL E I

ST HT I I COAL DI IT

i 20 - -

10 2200

I

I 2300

I

I 2400

I ST

I

I 2500

FT I • FT

I HT

I 2600

TEMPERATURE °F

FIG. l. An example showing that ash-fusion temperatures do not correlate with measured viscosity characteristics.

Mineral composition and combustion viscosity measurements and correlations published to date. E. K. Diehl of BCR has reported similar findings. Satisfactory proprietary correlations have been developed at Babcock and Wilcox. These correlations are improved periodically as the data base continues to expand. Differences between ash fusions and slag viscosities can be caused by a number of factors, including measurements of melting behavior of a mixture of highly oxidized and sulfated heterogeneous mineral particles heated at 15°F/min during the ashfusion test or measurements of apparent viscositytemperature data using a sample of virtually homogeneous, molten and glass-like slagt that is essentially sulfate-free with an oxidation state (including ferric compounds) near thermodynamic equilibrium as the slag is cooled at a rate < l°F/min during the slag-viscosity test. In summary, our experience shows that neither measured ash fusibility nor calculated viscosity using published correlations can provide consistent and predictable accuracy for evaluation of slag-flow characteristics. Indices including the base to acid ratio, silica ratio and silica to alumina ratio are widely used to relate elemental analyses of ASTM coal ash to slag-viscosity characteristics and fouling and slagging potentials. Although these indices are empirical, they do have a fundamental relationship to viscosity if ionic potential is considered. K. S. Vorres 4 has described ionic potential as the ratio of valence to ionic radius or the ratio of cation charge to ion size. He has published the following values for ionic potentials: Low range K +1=0.75 Na +1 = 1.1 Ca +z = 2.0 Fe +1 = 2.7 Mg +z = 3.0

High range Fe + 3 = 4 . 7 AI +3 = 5,9 Ti +4 = 5.9 Si +4 = 9.5

Low ionic potentials normally produce low viscosity melts (polymer breakers) and high potentials produce high viscosity (polymer formers). Recently, I discussed relationships among indices, viscosities and ionic potentials. 5 For example, B A

Fe203 + CaO + MgO + Na20 + K 2 0 SiO 2 + A120 3 + TiO 2 '

substituting ionic potentials for the cations, B A

=

2.7 to 4.7,* 2.0, 3.0, 1.1, 0.75 9.5, 5.9, 5.9 '

where the asterisk denotes a value that depends on ferric percentage. Thus, we find a high correlation between low ionic potentials and basic constituents, t The term glass used here is not meant to imply a straight-line relation between log viscosity and reciprocal temperature or the complete absence of any solid phase. Also, two immiscible liquid phases (layers) are often observed with melts from lignitic ashes.

171

which b,zlps explain why the B / A ratio is important. Also, the influence of ferric percentage on slag viscosity is apparent when the ionic potential increases from 2.7 to 4.7 as ferric percentage (oxidation state) increases. The B / A ratio is used only for Eastern U.S. coals and does not differentiate between the relatively higher potentials of Ca 2 +, Fe 2÷ and Mg 2 ÷ compared with the much lower values for Na ÷ and K ÷. A. F. Duzy 6 has proposed a dolomite ratio to explain the rapid increase in ash fusion temperatures associated with increasingly large quantities of CaO and MgO found in many Western U.S. coals. Use of the dolomite ratio (100(CaO + MgO)/(CaO + MgO + Fe203 +NazO+K20) improves correlations significantly for predicting ashfusion temperature or slag viscosities for those samples with a basic content above 40%. This improvement is consistent with the observation that, excluding the influence of the term for Fe203, the dolomite ratio is responsive to changes in the ratio 2.0 3.0 ionic potentials for Ca 3+ and Mg 2 0.75 1.1 potentials for K + a n d N a + REFERENCES

l. ATTIG, R. C. and DuzY, A. F., Coal Ash Deposition Studies and Application to Boiler Design,presented at the American Power Conference, April 1969, and published as B & W paper No. BR-899/TP9-4. 2. REID, W. T. and COHEN,P., The Flow-Characteristics of Coal-Ash Slags in the Solidification Range, presented at the Annual Meeting of The American Society of MechanicalEngineers, November29 December 3, 1943. 3. SAGE,W. L. and MClLROY,J. B., Relationship of CoalAsh Viscosity to Chemical Composition, presented at Joint AIME-ASME Solid Fuels Conference, October 26 29, 1959. ASME paper No. 59-Fu4. 4. VORRES,K. S., Chemistry of Coal Ash Melting in Gasification and Combustion, Am. Chem. Soc. Div. Fuel Chem. Prepr., 24, No. 2, 247-254 (1979). 5. WAGOYER,C. L., Slag Viscosity, presented to Coal Gasification Modeling Workshop, Entrained-Flow Gasi-fication/Combustion Session, January 20, 1982 and published in the Proceedings, DOE/METC/82-24, UC90c, pp. 314-319, July 1982. 6. DuzY, A. F., Fusibility-Viscosity of Lignite-Type Ash, ASME paper No. 65-Wa/Fu-7 (1965L

R. C. FLAGAN(CIT)and A. F. SAROFIM(MIT) The processes of slagging, fouling and erosion are extraordinarily complex and depend upon a sequence of events involving transformations of mineral matter in coal during combustion, the transport of residual ash, fumes and vapor to the tubes, and the reactions at the tube surfaces. An impressive accumulation of empirical information has been used to develop correlations between fouling and slagging behavior and certain physical and chemical characteristics of the ash in coal. These have been well described by Reid in his review. These empirical correlations have proven

172

W.T. REiD HEAT TRANSFER SURFACES

MINERAL

,.TERNAL

,NO

REOOC,NG

//'VAPORS

ENVIRONMENT//

'

/ ~ ; III X,

ANC

=

<.,,

.

LNo,As,SbJ

~

~NUCLEATION ~'~ I ~

I

~

lll~

W~"..:.;' ~ ;~" "-:" k~COAGUL~TION \\

METAL OR SOBOXIDE

ASH

~'~

,_, ~

~

CHAR

//

OOo SCAVENGING

IMPACTION ~ / PULVERIZED COAL PARTICLE 50/u.m

l]

\\

VAPORS

RESIDUAL FLY

~'-"--~'--/~'/-~ SURFACES

PARTICLE

(

1

ASH

- 50 ~ m )

FIG. 1. Schematic of ash-particle formation and deposition mechanics. to be very useful to the designer and indeed provide the only basis for design at present. Inasmuch as the correlations do not take into account the details of the distribution of the mineral matter in coal, the effect of combustion conditions on the form of the mineral matter in the furnace, and the transport mechanisms in furnaces, it is not surprising that discrepancies are often encountered between the predictions given by the correlations and furnace experience. Elimination of these discrepancies will require a much more detailed description of the transformation and transport of the mineral matter in furnaces, as is described briefly below.

within the porous char, a point is reached where the pores merge and undermine segments of the partially burned char which are released as fragments. Methods for analyzing this problem quantitatively have been applied to coal combustion only recently. 6 The particle-size distribution of the residual ash particles is further modified by the formation of cenospheres as a result of gas evolution within the molten ash particles.l'7 The ash particles are distributed over a broad range of particle sizes by these physical transformations.

x 1. Physical Transformation of Mineral Matter in Coal during Combustion A schematic of the fate of mineral matter during combustion is shown in Fig. 1. Most of the mineral matter in coal will end up in the residual ash produced by the coalescence of the fused mineral inclusions in coal during char burnout. The particle size of the residual ash depends on factors which are incompletely understood. A single coal particle may fragment during combustion and each fragment may produce an ash particle. The size of ash particles will depend upon the initial coal particle diameter, its mineral matter content and the uniformity of its distribution, and the number and size of fragments produced during combustion.'-a The particle-size distribution of coal is a parameter that is under partial control of the operator. The uniformity of the mineral-matter distribution can be determined by the low-temperature ashing of density-graded sizefractionated coal, a time-consuming process. 'U The fragmentation of the coal during combustion is the least understood process. As reaction occurs in depth

[

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Impoctor ------Irnp0ctor Upper Bound [// I /

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, I iI,,,il 10

DIAMETER

D(,um)

FiG. 2. Particle-size distribution of the coal ash aerosol entering the electrostatic precipitator at a pulverized coal fired utility boiler; reproduced from Ref. 8.

Mineral composition and combustion With the exception of the few cenospheres, the largest ash residue particles are somewhat smaller than the parent coal particles from which they were formed. The mass distribution falls off toward the fine particle end due to the limited quantity of ash present in the smaller fuel particles or fragments and the discrete size of the mineral inclusions in the coal. Figure 2 shows a typical ash particle size distribution obtained at the outlet of a coal fired utility boiler. 8 Similar results have been obtained in numerous laboratory studies? 11 The vast majority of the ash is in the large particle mode in the size distribution. These particles are the ash residue. A much smaller mass is found in very small particles, between 0.01 and 0.2 pm diameter. These very fine particles are smaller than can reasonably be explained by the fragmentation of coal particles during combustion. The source of this fume is volatilized ash which nucleates homogeneously as the vapors diffuse from the hot reducing atmosphere near the surface of burning char particles into the cooler oxidizing atmosphere far from the particle surfaces. 1z'13 The nuclei are initially very small but grow rapidly by condensation of additional ash vapor and by coagulation, producing the observed narrow peak in the submicron size distribution, its size and size distribution are well described by coagulation theory.Z,lo

constituents in size-fractionated ash samples are plotted vs particle size.9 For this relatively low-temperature laboratory combustion experiment, the fume consists primarily of iron oxides. The concentrations of Si and A1 in the fume are virtually undetectable. The quantity and composition of the fine particles is highly dependent on coal composition and combustor operating conditions. Finely dispersed elements, such as those organically bound in the coal, will vaporize to a larger extent than those found in micron-size mineral inclusions. 14 Alkali metals and other high vapor pressure compounds will vaporize preferentially. Elements such as Si, Ca, Mg and Fe, the oxides of which have a low vapor pressure, will vaporize by reduction of the oxide to a suboxide or metal by reactions of the type MO.+CO~.~-MO.

Because the fine particles are formed entirely from volatilized ash, their compositions may differ substantially from that of the bulk ash. This is illustrated in Fig. 3, where the concentrations of the major ash

b3 C3 0 b0

0.1 __

O.4

I

AI

~ + C O 2.

The CO is provided in the locally reducing atmosphere of the particle. Vaporization will be enhanced by high temperature or by long residence times at reducing conditions. The indicated effects of coal composition are illustrated by the results in Table 1 on the composition and amount of fume produced during the combustion of 20 coals under identical conditions in a laboratory furnace.17 The highest fractional vaporization of ash is observed for the North Dakota lignite with a high sodium content. The lignites as a group have a higher concentration of ion-exchangeable Ca and Mg than the bituminous and subbituminous coals and this is evident in the composition and amount of fumes produced during their combustion. Minor and trace components of the ash are also redistributed with respect to particle size by the vaporization/condensa-

2. Chemical Transformation of Ash during Combustion

0;2

173

Si

Ca

_o

1.0

0 -l-a II o.oi I-0 l--

o.1

I I ~.o

10

I

!

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l

I

I

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,, I -I, o.1 1.o

STOKES EQUIVALENT

] I O~ II I 10 0.01 0.1 1.0 DIAMETER (microns)

I 10

FIG. 3. Variation of Ash composition with particle size determined by a-particle-induced x-ray emission (DIXE) analysis of aerosol samples, which were size classified on-line using a low-pressure cascade impactor. Two sets of measurements from similar experiments are presented. Dashed lines indicate the abundance of the constituent in the bulk ash; reproduced from Ref. 9.

174

W.T. REIb

tion mechanism. Enrichment of the smaller ash residue particles was first reported by Natusch and coworkers 18'19 nearly a decade ago. Studies of the microstructure of ash particles have provided clear evidence of surface enrichment due to the condensation of volatilized ash. 2° This surface enrichment is observed on both the residual ash and the fume, as would be expected from the schematic mechanism in Fig. 1. The concentration of selected elements as obtained in the fume produced during the combustion of a Montana lignite, has been measured by Auger spectroscopy on the original aerosol which had a diameter of 150/L and on the aerosol after 25A and 75A had been ion-milled off the surface. 12 The decrease in concentration of alkali metals and increase in concentration of iron is indicative of the enrichment at the surface of the former and in the core of the latter. Particularly important is the high concentration of alkali salts on the surface of the ash since these will influence the bonding of particles to each other and to tube surfaces. The amount of sodium that is vaporized may range from 20 to 100 ~o, with the larger percentages corresponding to coals in which the sodium is primarily present in an ion-exchangeable form or sodium chloride. The lower percentages correspond to coals in which the sodium is tied up in sodium aluminosilicates. The sodium is normally tied up in sodium sulfate. The effect of combustion conditions is shown in Fig. 4, which presents the fractional K I00

-~

3500 ~

3000 I

2750 ,

2500 ,

2250 ,

2000

I0

\

- '

" zo

w

_N >

"

7;

z

9

Co

.OI

I

I

3.0

4.0 I

Tp

x 10 4

] 5.0 K "1

FIG. 4. Rates of vaporization of selected coal elements as a function of particle burning temperature T for a Montana lignite; 4~ is the fractional vaporization of elements in coal divided by the pulverized coal-particle burning time tb.

vaporization of different elements in a lignite as a function of temperature. The extent of vaporization increases rapidly above temperatures of 1500 K, most rapidly for the case of the volatile elements.

3. Deposition The deposition of ash constituents on boiler walls and tubes will be governed by the state or size of the ash and the aerodynamics in the vicinity of the tubes. Vaporized ash will condense on the boiler surface and form bonded deposits. Ash particles, depending on their size, can be deposited on surfaces by thermophoresis or diffusion augmented by turbulence. The mechanisms for surface deposition are relatively well understood21 24 but the models available for calculating deposition rate have received little application since information on the ash-particle size distribution is often missing. Moreover, while the rate at which particles of a given size will reach a surface may be calculated readily using our present understanding of aerosol transport, the probability that a particle will adhere to a surface is not now known.

4. Conclusions The properties of coal and its mineral content have profound effects on the size and composition distributions of the ash and, therefore, on the slagging, fouling and erosion which it may cause. A number of the physical processes which govern ash-particle formation have been identified, but further fundamental studies are needed before qualitative predictive models can be developed. An understanding of the relationship between fuel properties, combustion characteristics, and ash behavior can be expected to yield profound benefits to the designer of combustion systems. Since particle size is a major factor in the impaction of particles on surfaces and, further, since particle impaction on tubes is already fairly wellunderstood, the ability to predict ash-particle size and "stickiness" should allow the designer to specify the size distribution of the fuel particles and combustion conditions in order to minimize fouling or erosion. Such knowledge is also important if one is to anticipate the effects of fuel beneficiation or other changes. Solutions to the fouling problems may not be the most obvious. While the rate at which particles reach the surface may be reduced by reducing the ashparticle size, this will not reduce the deposition rate if only the small particles stick upon impact. Large particles may even be desirable if they scour deposits from surfaces rather than adhering. An understanding of the relationship between coal-particle size and the size of the ash and of the deposition mechanisms will be required before these questions can be answered in general. Ash behavior may be further complicated by differences in temperatures and rates of combustion of various fuel-particle sizes. While empirical correlations on the effect of ash composition on fouling and slagging will, of necessity, be used for some time,

Mineral composition and combustion only a mechanistic u n d e r s t a n d i n g of the processes which govern the particle evolution a n d deposition can be expected to resolve the m a n y u n a n s w e r e d questions of effect of coal type, coal beneficiation, a n d c o m b u s t i o n c o n d i t i o n s o n the p r o b l e m s associated with coal ash in furnaces. REFERENCES

1. SAROFIM,A. F., HOWARD,J. B. and PADIA,A. S., Combust. Sci. Teehnol., 16, 187-204 (1977). 2. FLAGA~q,R. C. and FRIEDLANDER,S. K., Particle Formation in Pulverized Coal Combustion--A Review, in Recent Developments in Aerosol Science, D. T. Shaw (Ed.), Wiley-lnterscience, New York, pp. 25-59 (1978). 3. QUANN, R. J. and SAROFIM, A. F., A Scanning Electron Microscopy Study of Transformations of Organically Bound Metals during Lignite Combustion, submitted to Fuel, (1983). 4. BORIO, R. W. and NARCISCO, R. R., Jr., The Use of Gravity Fractionation Techniques for Assessing Slagging and Fouling Potential of Coal Ash, d. Enqn9 Pwr, 101, 500 505 (1979). 5. BRYERS,R. W., Influence of the Distribution of Mineral Matter in Coal on Fireside Ash Deposition, J. Engnq Pwr, 101,506 515 (1979). 6. KERSTEIN,A. R. and NIKSA, S., Prediction and Measurement of the Critical Porosity for Fragmentation during Char Conversion, Proc. 1983 Int. Conf. on Coal Science, 743 746, sponsored by International Energy, Pittsburgh, PA, August 15 19, 1983. 7. RAASK,E., Cenospheres in Pulverized Fuel Ash, J. Inst. Fuel, 43, 339-344 (19681. 8. MARKOWSKI, G. R., ENSOR, D. S., HOOPER, R. G. and CARR, R. C., A Submicron Aerosol Made in Flue Gas from a Pulverized Coal Utility Boiler, Envir. Sci. Technol., 14, 1400 1402 (1980). 9. TAYLER, D. D. and FLAGAN, R. C., Aerosols from a Laboratory Pulverized Coal Combustor, in ACS Symposium Series No. 167, Atmospheric Aerosol: Source~Air Quality Relationships, E. S. Macias and P. K. Hopke (Eds.L American Chemical Society, pp. 157-172 (1981).

1PE/;S UJ:2-G

175

10. NEVILLE, M., QUANN, R. J., HAYNES, B. S. and SAROFIM, A. F., Eighteenth Symposium (International) on Combustion, The Combustion Institute, pp. 1267 1274, Pittsburgh, Pa. (1981). 11. TAYLOR, D. D. and FLAGAN, R. C., Aerosol Sci. Tech., I, 103 117 (1982). 12. NEVILLE, M. and SAROFIM,A. F., Nineteenth Symposium (International) on Combustion, The Combustion Institute, pp. 1441 1449, Pittsburgh, Pa. (1983). 13. SENIOR,C. L. and FLAGAN, R. C., Ash Vaporization and Condensation During Combustion of a Suspended Coal Particle, Aerosol Sci. Technol., 1, 371 383 (1982). 14. QUANN' R. J. and SAROFIM,A. F., Nineteenth Symposium (International) on Combustion, The Combustion Institute, pp. 1429-1439, Pittsburgh, Pa. (19831. 15. MIMS, C. A., NEVILLE, M., HOUSE, K. and QUANN, R. J.. AIChE Syrup. Ser. No. 201, 76, 188 194 ( 19801. 16. QUANN, R. J., NEVILLE, M., JANGHORBANI, M., MIMS, C. A. and SAROFIM, A. F., Envir. Sci. Technol.. 16, 776 781 (1982). 17. QUANN, R. J., NEVILLE, M. and SAROEIM,A. F., Proceedinos of the 1983 Int. Conf. on Coal Science, pp. 587 59(1 (1983). 18. NATUSCH,D. F. S., WALLACE,J. R. and EVANS,C. A, Jr., Envir. Sci. Technol., 8, 1107 (1974). 19. KEYSER,T. R., NATUSCH, D. F. S., EVANS, C. A., Jr. and LINTON, R. W., Envir. Sci. Technol., 12, 769 (19781. 20. LINTON, R. W., LOK, A., NAIUSCH, D. F. S., EVANS, C. A., Jr. and WILLIAMS, P., Surface Predominance of Trace Elements in Airborne Particles, Science, 191,852 854 (1976). 21. GOKOGLU, S. A. and ROSNER, D. E., Correlations of Thermophoretically-Modified Small Particle Diffusional Deposition Rates in Forced Convection Systems with Variable Properties, Transpiration Cooling and/or Viscous Dissipation, Int. J. Heat Mass Tram@r, in press (1983L 22. ROSNER, D. E. and FERNANDEZ DE LA MORA, J., 4SME Trans. J. Engn 9 Pwr, 104, 885-894 (1982). 23. ROSNER, D. E., PCH-J. Phys.-clin. Hydrodyn., I, 159 185 (1980). 24. FRIEDLANDER, S. K., Smoke, Dust and ttaze Fundamentals of Aerosol Behavior, J. Wiley and Sons, New York (1977).