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Ash formation during pulverized coal combustion
Ash formation combustion
during
pulverized
2. The significance deposits
of crystalline
anorthite
coal in boiler
John F. Unsworth, David J. Barratt, David Park and Keith Shell Research Ltd, Thornton Research Centre, PO Box I, Chester, CHl (Received
12 March
1987;
revised
14 September
J. Titchener 3SH,
UK
7987)
The chemical structure of pulverized coal-fired hoilcr deposits has been compared, using X-ray diffraction and scanning electron microscopy techniques, with the mineral matter in the parent coal. An unexpectedly high degree of crystallinity is observed in these deposits, a major constituent being anorthite (CaAl,Si,O,), a calcium mineral not present in coals. The formation of anorthite requires the mixing of separate calcium and aluminosilicate mineral domains. Thus, as calcium aluminosilicates are not found in fly ashes, their presence in boiler deposits has been used to probe the mechanism of deposit formation. Studies of a range of deposits, having different temperature histories, together with complementary investigations on mineral mixtures and coal ashes, have demonstrated that anorthite is formed via solid-state reactions and not by recrystallization from a homogeneous melt. Gehlenite (Ca,Al,SiO,) is an intermediate in this reaction and is detected in deposits colfected from the cooler regions of the combustor. Deposits generated by this mechanism are weakly bonded materials containing crystalline phases (predominantly anorthite) and a phase concentrated in elements other than calcium, aluminium and silicon. Concentration of these more abundant elements as stoichiometric phases makes the composition and melting characteristics of the complementary phase much more sensitive to any minor fluxing elements present. (Keywords: combustion of coal; ash: scanning electron microscopy)
Deposit formation within pulverized coal-fired utility boilers lowers operational efficiency by reducing the rate of heat transfer to the boiler tubes and by providing sites for corrosion. Removal of weakly-bonded deposits can be achieved during boiler operation by soot blowing, however, the extensive accumulation of tenacious deposits affects boiler availability. Apart from a rudimentary elemental relationship, the chemical composition of a deposit is markedly different from that of the parent coal ash. Furnace deposits are porous, heterogeneous assemblies (either sintered or fused) of the products from decomposition of coal minerals and their subsequent reactions. Mineral matter in bituminous coals occurs in a plethora of diverse forms, which are distributed heterogeneously in different sized domains within the organic matrix. Thus slag properties. e.g. strength and viscosity, cannot be adequately represented without due regard being given to the influence of mineralogical form, size and degree of interto elemental disproportionation and association, mechanisms within the boiler’. For practical reasons it has been necessary in the past to neglect these factors. so slagging and fouling tendencies have usually been predicted from the elemental composition. and fusion characteristics of an average ash sample2.3. Recent developments in analytical techniques, e.g. electron microscopy and X-ray diffraction, which can now provide rapid quantitative measurements, should allow deposit characterization to be improved sufficiently to elucidate the mechanisms of deposit formation and their influence on deposit strength. 0016 2361:88/050632 lO$?.OO ‘(‘8108.8Huttcrworth & Co. (Puhlishcrs) Ltd. 632
FUEL, 1988, Vol 67, May
The role of iron during slag deposition has been well studied4,5, however, much less attention has been paid to calcium, except in the case of the use of lignites. The relevance of calcium is recognized by the need for different fouling and slagging indices for bituminous and lignitic coal ashes2. It is not uncommon for bituminous coals to have ashes, termed lignitic as a result of high contents of calcium and magnesium minerals in these particular coals. In lignites, though, calcium is present predominantly in a different form, i.e. directly bonded to the organic coal matrix’. The objective of the present work was to compare the mineral structure of deposits with that of the parent coal, and to highlight mechanistic aspects capable of influencing the extent of deposit accumulation and/or strength. Studies were concentrated on the fate of calcium minerals during combustion of a range of internationally traded bituminous steam coals in a pilot scale pulverized coal combustor6. EXPERIMENTAL Several low-rank bituminous coals, covering a range of and maceral geographical and geological origin composition (nblr l), were fired in a 160 kW pilot scale pulverized coal combustorh. Four of these coals had lignitic-type ashes and three coals had bituminous-type ashes. The main combustion parameters (i.e. peak flame temperature 1500-16OO”C, mean residence time 2.0-2.5 s, and heat flux 200-300 kW mV2) were kept relatively constant for each coal, and these are comparable with
Ash formation Table I
Properties
coal combustion.
2: J. F. Unsworth
et al.
of coals A
Coal code Dry basis (wt ‘I,,) Ash Total sulphur Organic sulphur Pyritic sulphur Chlorine Carbon dioxide Fluorine (ppm) Phosphorus (ppm)
15.6 0.7 0.3 0.3 <0.02 1.30 195 707
Dry, ash-free basis (wt ‘I,,) Volatile matter Carbon Hydrogen Maceral group analysis Vitrinite Exinite Inertinite Vitrinite mean random reflectance
(vol”,,)
Ash fusion temperature (Reducing atmosphere) Initial deformation Hemisphere Fusion
( C)
Oxidizing atmosphere Initial deformation Hemisphere Fusion Ash composition analysis (wt “,, in ash) SiO, AlLO., Fe&), TiO, CaO MgO Na,O K>O P*O, SD, Ash type
during pulverized
(“,,)
B
13.4 0.7 0.3 0.4 0.02 0.86 544 2880
C
D
E
F
G
13.8 1.o 0.4 0.6 0.04 I .24 124 400
7.3 0.6 0.2 0.4 0.05 0.64 48 150
10.3 I .o 0.8 0.1 0.04 0.25 83 450
12.4 1.o 0.6 0.4 0.04 0.78 143 750
3.1 0.9 0.8 0.24 0.83 0.45 I7 120
30.3 84. I 4.3
30.6 85.1 4.8
30.4 84.4 4.8
32.5 83.1 4.7
31.8 85.7 5.0
38.3 82.6 5.3
39.9 82.0 5.5
27 4 69
24 2 74
28 2 70
38 1 61
42
43
56
52
65 IO 25
0.70
0.77
0.68
0.66
0.74
0.53
0.63
1350 1360 > 1400
1380 1400 > 1400
1380 1390 > I400
I loo 1170 1390
1250 1355 > 1400
1370 > 1400 > 1400
1070 1150 II70
1350 1400 > 1400
1360 1380 > 1400
1400 > 1400 > 1400
1270 1380 > 1400
1315 > 1400 > 1400
1380 > 1400 > 1400
1290 > 1400 > 1400
46. I 27.7 3.6 I .4 8.8 1.9 0.3 0.5 1.o 6.6 Lignitic
39.0 29.0 3.5 I.5 10.6 1.5 0.3 0.4 4.9 4.4 Ligmtic
conditions within a full-scale utility boiler. After cooling of the combustor, a series of ashes and deposits were collected from various parts of the boiler (Figurr I). These ranged in appearance from fused, to weakly sintered, to powdery. For some deposits, fused material was separated from unfused by sieving at 2 mm. Where necessary samples were crushed and representative subsamples prepared for further analysis. Scanning electron microscopy (SEM) studies were carried out using an electron probe microanalyser, an energy dispersive X-ray detector and a SEM image processing system. Samples were mounted in polished resin blocks and the concentrations of the following elements: Si, Al, Fe, Mn, Ti, Ca, Mg, Na, K, P, S and Cl, were measured by applying a ZAF procedure, which corrects for differences in the intensity, absorption and fluorescence of emitted X-rays. Low temperature ashes (LTA) were prepared from pulverized coal samples with a plasma asher. The SEM procedure selected and sized LTA particles into the following size classes: l-2,24,4-8,8-16, 16-32 and 3264 /lrn diameter. For 300 particles (50 from each size) the electron beam was centred at the middle of each particle and X-ray spectra accumulated for an area of
36.8 30.6 5.6 I.8 10.3 1.5 0.2 0.4 0.8 7.1 _ignitic
53.2 16.7 18.9 0.7 3.1 1.1 0.3 0.6 0.5 1.9 Bituminous
53.9 22.6 10.9 I.3 4.0 1.5 0.2 0.4 I .o 3.7
47.6 28.3 8.6 2.0 6.5 0.7 0.4 0.1 1.5 6.0
Bituminous
Bituminous
25.9 19.0 13.1 I.0 19.1 2.3 7.0 0.2 0.9 19.2 Lignitic
approximately l-2 iirn. From its elemental composition, each area was allocated to a specific mineral type (7?rh/r 2~). Corrections were made for density differences between minerals and for reduced back-scattered electron intensity from smaller particles. The particle size distribution of the LTA was obtained by a combination of sieve analysis and a light scattering instrument. By linking the particle size distribution with elemental and mineral type analysis, an average composition for LTA particles > 1 pm was calculated. X-ray diffraction (XRD) was performed at ambient temperatures and for qualitative studies, the samples ( = 20 mg) were supported on a low-background support of a single silicon crystal. Before quantitative analysis the ashes were mixed 1:l with fluorite as an internal standard. High temperature XRD spectra were obtained under vacuum with a high temperature camera attachment, the sample resting on a thin sheet of platinum foil wrapped around a resistively heated tantalum bar. For one LTA sample (coal A) a step-wise vacuum heat treatment from 400-1500°C was carried out. Each temperature step, typically spanning 2OO“C, was achieved in a few seconds, and then held at each new temperature level for 15 min before analysis.
FUEL,
1988,
Vol 67, May
633
Ash formation
during
pulverized
coal combustion.
2: J. F. Unsworth
et al. PHOTO-TUBE (water-cooled)
_(
FURNACE
I I lcI I
, It----L, I I I I I I
THROAT-
I I
I
SUPERHEATER TUBES
e---I
ASH
DISTANCE
AIR-COOLEDSLAG
1 ‘*O”
=SURFACE
Table 2(a)
Diagram
of pilot-scale
TEMPERATURE
“This class represents KAI,(AISi,O,,)(OH),
combustor’
illustrating
1
temperature
distribution
Si
Al
Fe
> 19 > 10 > 10 < 10 <5 < 10 < 10 < 10 < 10 <5
<5 > 10 > 10 < 10 15 < IO < 10 < 10 < 10 <5
<5 <5 <5 <2 <5 <2 >l >25 >25 <5
of aluminosilicates
intermediate
FUEL, 1988, Vol67,
May
Ti
of deposits
Ca (wt ‘I’;,in particle) <5 <5 <5 > 15 > 15 > 15 > 15 _ _ <5
_ _ _ _ _
> 10
in composition
The crystalline products from high temperature reactions between the major calcium and aluminosilicate minerals found in these coals were determined qualitatively by XRD. Pairs of minerals (i.e. kaolinite with each of calcite, dolomite, ankerite, fluorapatite and anhydrite, and illite with calcite) were blended in the appropriate proportions to generate a 2:l atomic aluminium/calcium ratio, and crushed to an average particle size of 2-3 pm. These mineral mixtures were held at various elevated temperatures between 900 and 1300°C for 2 h in air using a tube furnace, before allowing to cool. The XRD spectra of the resultant materials were determined at ambient temperature. Although anhydrite itself is not present in these coals, it was included in the studies because of its rapid formation in the boiler by reaction of sulphur oxides with lime from decomposed calcite. Ashes were also prepared from pulverized samples of coals A and F by burning in an entrained flow reactor at 1450”C7 and by plasma ashing of flame pyrolysed chars prepared from these coal?.
634
and location
types by SEM
CO,
the range
m
(‘Cl
(‘Cl
Rules used to allocate
Quartz-SiO, Kaolinite-Al,Si,0,.2H,O Illite” Calcite-CaCO, Fluorapatite-3Ca,(PO.),.CaF, Dolomite-CaCO,.MgCO, Ankerite-CaCO,(Mg,Fe,Mn) Siderite-FeCO, Pyrite-FeS, Rutile-TiO,
BURNER,
CYCL .ONE
PANEL
yes%%I:ONTO”RS
Figure 1
FROM
-c
between
_ _ <2 <5 >2 >5 _ _ _
K
P
<2 <0.5 r0.5 _
_ _ _ _ > 10
of minerul
S
_ _ _ _ _ _ < 10 > 10 _
_ _ <2
montmorillonite-AI,Si,O,,(OH),
MINERALOGICAL Distribution
Mg
COMPOSITION
and
muscovite
OF COALS
types
Although a vast range of minerals have been detected in coals, in practice only lo-15 types are normally found at concentrations above 1 wt %. No single analytical technique has yet been devised which adequately measures the mineral composition. XRD is the most definitive method for crystalline materials but even here it is only semi-quantitative, being limited by variations in the crystallinity of each mineral, by matrix effects, and by the inherent complexity of coal mineral mixtures. Tab/e 2b lists the major minerals detected by XRD in the low temperature ashes (LTA) of the seven coals studied. The SEM method used measures element compositions for areas of l-2 pm diameter from different LTA particles. Hence it is restricted to particles above 1 pm diameter. This is a severe limitation as a substantial proportion (in some cases 50 wt %) of the LTA sample consists of particles below this size.
Ash formation Table 2(b)
Mineral
composition
Coal sample
during pulverized
coal combustion.
2: J. F. Unsworth
et al.
of LTAs
A
B
C
D
E
F
Cd
+(I3
+(4) + (23) + + +
- (0)
+ (20)
+ (38) + _
+ Trace
+ _
+ (22) + _ _ _ +
+ + + + _ _
+ + + _ _ +
+
Trace +
+ -
_ _
21 32 11 3
22 46 7 4 3 _~
20 47 7 11 1 Trace 1 8 4 1 Trace _
8 44 4 15
By XRD a-Quartz Kaolinite Calcite Fluorapatite Dolomite Ankerite Siderite Pyrite
+ +
By SEM wt “” + 1 Ltrn material Quartz-type Kaolinite-type Illite-type Calcite-type Fluorapatite-type Dolomite-type Ankerite-type Siderite-type Pyrite-type Rutile-type Gypsum-type Other?
12 48 9 11 4 8 3 Trace 4 Trace
+ (29) +”
_
_b
+
6 46 6 12 11
1 57 6 20 Trace‘ 6 1 Trace 8 1
/ Trace
; Trace
Trace 2 14 6 1
5 5 7 1 _
‘Signifies identified components (numbers in parentheses are wt q, f 57,) “Signifies not identifed ‘Trace = < 0.5 ‘,, ‘In LTA from coal G a significant number of particles could not be assigned concentrated in the volatile elements sulphur and sodium
E” .J”
to a specific mineral-type-these
were predominantly
2 10 2 10 0 Trace 4
< 4 pm in size and
1 TIO LINE hPOSlTlON
40
JNITE
30 z
PURE KAOLINITE
5 2 k! Y
20
10
“B
0
1 0.0
I
2.0
1.0 % IMPURITIES
(a) IMPURITY
Figure 2
3.0
4.0
I
.
i
<
5.0
6.0
15
(Ca + Fe + Mg + K + Na + Ti + S + Cl + P)
CONTENT
Element concentrations
10
LTA particles
Nevertheless, this method generates much valuable information concerning the association of elements within mineral domains. With the exception of illites, which have a variable composition, almost all the analyses resulted in element compositions corresponding closely to the stoichiometry of a single mineral type. This allows a set of rules (Tuble 2~) to be used: these allocate each point to a specific mineral-type, thus providing a quantitative composition for the + 1 pm particles in terms of such mineral-types. In all of these coals the major mineral is clearly kaolinite. The high degree of purity and homogeneity of the LTA particles allocated by SEM to the kaolinite type
25
30
% ALUMINIUM (b) SILICON
of 156 Kaolinite-type
20
AND ALUMINIUM
CONTENTS
from coal A
is demonstrated
for coal A by:
1. the very low concentration of element impurities in nearly all such particles; 2. aluminium and silicon contents, plus a constant Si/Al ratio, corresponding closely with the composition of kaolinite (Figure 2b). For the other mineral types, similar element distributions which closely resembled the appropriate stoichiometry were observed. For all seven coals studied, most (i.e. over 90%) of the mineral domains > 1 pm in size consist of single mineral types and not mixtures of minerals. Although plasma ashing techniques used to prepare
FUEL, 1988, Vol 67, May
635
Ash formation
during pulverized
coal combustion.
2: J. F. Unsworth
LTAs could have caused aggregates of minerals to disperse as separate particles, this is considered unlikely to be of any significance. Other SEM studies on bituminous coal particles classified over 90% of the mineral domains into the major individual mineral types’. Form of culcium in coals Calcium occurs predominantly as calcite, dolomite, ankerite and fluorapatite. SEM does not measure fluorine, but fluorapatite has been detected in these LTAs by XRD. Furthermore, a linear relationship between phosphorus and fluorine contents (Tuhle I) exists for these coals, and this is approximately the same ratio as in fluorapatite. Figure 3 illustrates the way in which calcium is distributed between the main mineral types for each of the seven coals being studied. The main form of calcium is usually calcite, but substantial amounts of fluorapatite (coals A and B), dolomite (coals A, B and C), and ankerite (coals E and G), were found in these samples. The amount of calcium found in association with kaolinite, illite and quartz was negligible, with no evidence for the presence of calcium aluminosilicates, such as anorthite (CaAl,Si,O,) or gehlenite (Ca,Al,SiO,). COMPOSITION ASHES
OF TEST
DEPOSITS
AND
Boiler deposits and ashes from these seven coals, which have widely different ash elemental compositions, Tub/e 1, were investigated. Their fireside behaviour varied between: the production of tenacious slag deposits (co;,1 D); easily-removed slag deposits (coals B, E and F); very weak deposits (coal A and C); and extensive fouling deposits (coal G). Despite the known heterogeneity of boiler deposits and fly asheslO,ll, characterization of these materials is often restricted to an average elemental composition. Much less attention has been paid to the quantative identification of the mineral species present. This may be partly because such deposits are often glassy (i.e. noncrystalline)’ ’ and XRD cannot identify amorphous minerals. Nevertheless, an unexpectedly high degree of crystallinity has been observed within some of the deposits examined in this study, with > 70 wt “; of the material accounted for by XRD in several cases. Details of the high temperature minerals detected by XRD for a
Figure 3
636
Distribution
FUEL,
1988,
of calcium
within mineral
Vol 67, May
types
et al.
wide range of boiler deposits and ashes are given in Tuble 3, together with quantitative data for coals A and B. The following trends were observed. 1. Anorthite (CaAl,Si,O,) occurs in nearly all the deposits in which some fusion of ash particles has clearly occurred; it is absent from powdery deposits and cyclone ashes (i.e. fly ashes). For throat and furnace deposits from A and B it contributes approximately 50 7; of the material, accounting for all the calcium present. In cooler boiler locations (i.e. phototube and superheaters), less anorthite is formed. (Ca,Al,SiO,) is found in deposits at the 2. Gehlenite cooler boiler locations for coals A, B and G. It is not present in cyclone ashes or well sintered deposits. is also present in cyclone ashes and some 3. Calcium unfused superheater deposits as anhydrite (CaSO,), lime (CaO) and undecomposed calcite. 4. Mullite (Al,Si,O,,) is detected in most deposits and ashes. It is formed in the thermal cyclone decomposition of kaolinite and other clay minerals, together with cristobalite (SiO,) which is also fairly ubiquitous. (Fe,O,) and magnetite (Fe,O,), formed 5. Haematite during thermal decomposition of pyrite and siderite, occur frequently. The elemental heterogeneity of a throat deposit was examined by SEM. This deposit had grown with a fingerlike structure into the gas stream during the combustion of coal B; Figure 4 shows the honeycomb structure of the deposit with both large and small pores (ca. 1 mm and 25 Ltrn diameter, respectively). Two distinct solid phases could be discerned : one rich in calcium and the other rich in phosphorus; both phases contained aluminium. Quantitative X-ray element analysis demonstrated that the calcium-rich phase contained predominantly only calcium, aluminium and silicon in the proportions appropriate to anorthite (Tuhle 4). The other phase was enriched in all other elements present in the ash with no regular stoichiometry.
MECHANISM
OF ANORTHITE
FORMATION
Most crystalline species detected in these ashes and deposits (e.g. mullite, lime, haematite, magnetite) are formed by thermal decomposition of each separate mineral type (i.e. kaolinite, calcite, pyrite/sideritejankerite, respectively) l2 . This is exclusively the case for cyclone ashes. Anorthite and gehlenite, on the other hand, are formed from reactions between different minerals, i.e. mixing of calcium and of aluminosilicate domains. Significantly, these two calcium aluminosilicates were detected in deposits, but not in cyclone ashes. Consequently, the mechanism of formation of anorthite has been considered in some detail, to establish if such compositional differences can help elucidate ash formation mechanisms in the p.f. boiler. RecrJstullizution from u homogeneous melt Anorthite has been observed in other combustor Its formation can be explained by slags 3.13.14 recrystallization from a homogeneous melt. The CaOAl,O,-SiO, phase diagram (Figure 5)’ 5 predicts that, for bulk ash compositions similar to those from six of these coals, mullite crystallizes first at temperatures below
1350/1450 1100/1300 100/1200 100/1200 < 600/ < 800 1350/1450 <6OC;<800 1350/1450 1lOO/1250 1100/1250 1100/1250 6OO/lOOO 600/1000 <600/<800 450/1450 600/1000 600/1ooO 1350/1500 1350/1450 1 loo/1250 I 100/1250 1100/1250 1100/1250 600/1000 600/1000 < 600/ < 800 1350/1450 1350/1450 1100/1300 I 800/1200 2 800/1050 3 W/950 5 SOOj850 < 600/c 800
1350/1450 1100/1250 1100/1250 1100/1250 I lOO/l250 600/1000 600/1000 <600/c 800
( C)
Metal/gas temperature
Fused Sintered Brown sinter Grey sinter Powder Agglomerated Powder Powder Powder Fused Fused Sintered Powder Powder Fused Powder Fused Fused Sintered Powder Agglomerated Powder Powder Fused Agglomerated Powder Fused Fused Sintered Brown sinter Grey sinter Powder Sintered Powder Powder Sintered Powder Powder Agglomerated Agglomerated Agglomerated Agglomerated Powder P
+ +
+
+
+ + +
+(42) + (60) +(35)
+(58) + (53) + (58) +W) +(3l) +(12)
Anorthite Ca,AI,Si,O,
and species detected
Appearance
and ashes-appearance
p, signifies possible component; +, signifies identified component; numbers “M =Microline (KAISi,O,), I = Iron (Fe), H = Halite (NaCI), N =Nepheline *PFA is pulverized fuel ash from a full-scale boiler trial
G
F
E
D
C
B
deposits
Furnace Throatiphototube Throat/phototube Throat/phototube Throat/phototube Superheater Superheater Cyclone PFA-full scaleb Furnace Throat Phototube Phototube Cyclone Furnace Cyclone Furnace Throat/phototube Throat/phototube Throat/phototube Superheater Superheater Cyclone Furnace/slagpanel Superheater Superheater Furnace/quarl Furnace/bottom Throat/phototube Throat/phototube Throat/phototube Throat/phototube Superheater Superheater Cyclone Furnace Furnace Throat Superheater/row Superheater/row Superheater/row Superheater/row Cyclone
Location
Coal code
A
Combustor
Table 3
+ + + + + + + + + + + + + +
P + + +
+ +
l t(13) +(15) f(t8) +
+(12)
+
+(15) +(18) +(l2) +(19) +(21) +(l5) +(2l) +
Mullite AI,Si,O, 3
+ +
+
+ +
+ + + + + + +
+ + + + + + + + + +
+(I) + (4) +
+ (4) +(8) + (6) +(6) +(lO) + +
+ (5)
Quartz SiO,
in parentheses are wt ‘<, f 5”,, (NaAISiO,), T=Tridymite (SiO,)
P + + + 4
P + P
Gehlenite Ca,Al,SiO,
by XRD
+ +
+ +
+ +
+ + +
Cristobalite SiO,
+ +
+ + + +
+
P
P
P
P
Anhydrite CaSO,
P
+
P
P +
Lime CaO
P
Calcite CaCO,
+
+
+ + + + + + + + + + + +
Haematite Fe,O,
Magnetite Fe@,
N(P) N(P) N(P),H(P) N(P) N(P) N(P) N(P) N(P)>H(P)
T(P)J(P) UP)
M(f)
M(P)
Others“
Ash formation
during
pulverized
coal combustion.
2: J. F. Unsworth
Figure 4 SEM/E.P.M.A. photomicrographs of a throat deposit from coal B, illustrating the presence oftwo distinct solid phases; one calcium rich, the other phosphorous rich. White signifies high element concentrations: a, back-scattered digitized electron map; b, aluminium X-ray map; c, phosphorus X-ray map; d, calcium X-ray map Table4 Element compositions” from coal B
of two phases in a fused throat deposit
Slag ashb (Wt %) 50, Al,O, Fe,O, CaO MgO TiO PzO25
Coal ash’
Phase
42 32 4 12 2 2 5
41 29 8 7 4 3 8
1
Phase 2
Anorthite (CaAI,Si,O,)
41 36 2 17 I 1 1
43 37 0 20 0 0 0
“expressed as oxides ‘by SEM averages of 6-point analyses ‘by X-ray fluorescence spectroscopy
16OO”C, then anorthite and mullite co-crystallize in the range 1500-135O”C, and finally, anorthite, mullite and silica form a eutectic, which crystallizes at 1345°C. To test the relevance of this mechanism, the high temperature ash from coal A was completely fused by heating at 1450°C for 2 h, and then allowed to cool in air at ambient temperature. The resulting glassy solid contained no crystalline species detectable by XRD. In addition, there are some indications that less anorthite is found in the most fused of the combustor deposits (‘Kzhte 3). If the combustor temperatures, which are generally too low, and the heterogeneity of deposits are also considered, then it seems most unlikely that anorthite (and hence any of these combustor deposits) is formed in this way. Solid-state
reuctions
species detected for five temperatures between 900 and 1300°C. These results demonstrate anorthite formation by reaction of kaolinte, not only with calcite but also with dolomite, ankerite, fluorapatite and anhydrite (dehydrated gypsum), and also by reaction of illite with calcite. These reactions commence at 900-1000°C with the first signs of agglomeration. Except in the case of fluorapatite, anorthite formation is preceded by the production of gehlenite, which is an intermediate. Conversion to anorthite, which, when pure, has a melting point of 1553°C is almost completed by 1200°C when the mixture is still only weakly sintered and easily broken. Thus, combination of the elements,calcium plus aluminium and silicon, can be achieved via a solid-state reaction between loosely compacted particles, which proceeds rapidly and produces crystalline products at temperatures as low as 900°C. The rate of conversion that will depend on diffusion of calcium into aluminoscilicate particles, and can be expected to be strongly influenced by the extent of inter-particle contact (i.e. mixing efficiently and particle size) as well as the temperature. Solitl-state
reactions
in coal ashes and cleposits
High temperature XRD techniques were used to demonstrate that anorthite can be formed by such a reaction sequence in the LTA from another sample of Coal A. Figure 6 illustrates the XRD spectra obtained at temperatures between 400 and 1550°C as the ash is heated up. At 800°C calcite, fluorapatite and kaolinite start to decompose, and the first formation of gehlenite is noted. From lOOO-14OO”C, gehlenite is gradually replaced by anorthite, which is dissolved in a melt by 1550°C. The composition and appearance of the deposits described earlier is clearly consistent with this solid-state mechanism for anorthite formation. Loosely agglomerated ashes from the superheater and phototube (that have associated gas and metal temperatures of approximately 850-1250°C and 100@6OO”C, respectively) contained gehlenite. Anorthite was largely founu in sintered deposits from the hotter sections of the boiler (gas and metal temperatures of approximately 1250145O’C and 1 lOO-135O”C, respectively). Ash formation
mechunisms
The absence of both anorthite and gehlenite in all fly ashes (including a power station pulverized fuel ash) suggests that element mixing of calcium with aluminium
between selected minerals
Anorthite formation has been reported at lower temperatures via a series of solid-state reactions between Confirmation has been calcite and kaolinite10,‘6. obtained by monitoring this reaction using XRD. Total conversion of all the calcium present into anorthite was observed in some deposits, thus five other mineral mixtures were studied. Tuble 5 details the appearance and
638
et al.
FUEL, 1988, Vol67,
May
Figure 5 Ca04l,O,SiO, these 7 coal ashes. Predicted
phase diagram illustrating composition sequence for coal ash A
of
Powder Some agglomeration Agglomerated Sintered Sintered and swollen
Powder Agglomerated Some sintering Sintered Sintered
Some agglomeration Agglomerated Some sintering Sintered and swollen Fused
900 1000 1100 1200 1300
900 loo0 1100 1200 1300
900 1000 1100 1200 1300
Fluoropatite and kaolinite
Anhydrite and kaolinite
Calcite and illite
component
Some agglomeration Some agglomeration Agglomerated Sintered Sintered
900 loo0 1100 1200 1300
Ankerite and kaolinite
+ signifies identified
Some agglomeration Agglomerated Agglomerated Sintered Sintered and swollen
900 1000 1100 1200 1300
Dolomite and kaolinite
Some agglomeration Agglomerated Sintered Sintered Sintered
Appearance
mixturesappearance
900 loo0 1100 1200 1300
Temperature (“C)
Mixture
mineral
Calcite and kaolinite
High temperature
Table 5
+ + + +
+ + + +
+ + + +
+ + + + +
+ + + + +
+ + + + +
Anorthite Ca,Al,Si,O,
+ +
+ +
+ +
+
Gehlenite Ca,Al,SiO,
and species identified
+
+ + +
+
Al,Si,O,
Mullite
by XRD
3
+
+
Quartz SiO,
+ +
Cristobalite SiO,
Anhydrite CaSO,
+ + + + +
Galaxite MnAl,O,
+ + + +
Fe,O,
Haematite
+ +
Fluorapatite 3Ca,(PO,),CaF,
Ash formation
during pulverized
K =
KAOLINITE
A
C
-
CALCITE
G =
GEHLENITE
F
=
FLUORAPATITE
M =
MULLITE
P
=
PYRITE
=
coal combustion.
ANORTHITE
Injluence K
K
Q =
OUARTZ
B -
BASSANITE
2: J. F. Unsworth
_._---.L
1--
1
40
38
36
-1
1
34
32
1
30
I_
28
26
High temperature
,550OC
2
24
DEGREES,
Figure 6
~-~~
----I_I-_11.-.1.--~
22
20
18
16
14
12
10
20
XRD spectra
of LTA from coal A
and silicon occurs within the deposit as it is growing. Confirmation that anorthite is not formed prior to deposition was obtained by preparing first, the LTAs of the flame pyrolysed chars, and second, ashes burnt out in an entrained flow reactor at 1450°C from coals A and F’. The XRD spectra of the resultant powders were similar to those obtained for the appropriate cyclone ash; only high temperature products of individual coal minerals were detected. This has implications concerning the mechanism by which char particles burn to form ash particles. The particles in cyclone ashes and those prepared in the entrained flow reactor have experienced temperatures in excess of 1450°C. The residence time is short ( 2 2 s), but nevertheless sufficient for fusion of quartz to occur. There is evidencei to suggest that mineral domains within a char particle coalesce to produce a single ash particle. However, hot stage microscopy studies” have indicated that, in high inertinite and higher rank coals, which may experience lower peak temperatures in the boiler than high vitrinite lower rank coals, minerals can be ejected singly during burn-out. If coalescence of calcium with aluminium and silicon domains has already occurred before deposition, then it is surprising that anorthite is only detected in deposits, especially as anorthite is known to crystallize rapidly. These results, which are mainly for high inertinite coals, are more consistent with the idea that calcium and aluminosilicate minerals can be ejected singly. Mixing, followed by gehlenite and anorthite formation, then occurs only after deposition. To be more definitive, it is necessary to establish if the temperature history of fly ashes is adequate to form anorthite.
640
FUEL, 1988, Vol67,
May
et al. on deposit strength
The properties of boiler deposits derived by aggregation of ash particles will inevitably depend on the extent of inter-particle bonding. Hence, the mechanism of formation of minerals within a deposit can be as important, if not more so, than its actual composition; recrystallization from a homogeneous melt should generate a denser and much stronger material than one derived from a solid-state reaction and may also have better heat transfer properties. The volumetric changes occurring during the calcitekaolinite reaction have been examined in detail”. Densification occurs rapidly at 600-950°C as gehlenite is formed. Transformation to anorthite does not contribute any further densification in the temperature range lOOO1200°C. However, above 1250°C further shrinkage occurs as the system begins to melt and vitrify. Clearly boiler temperatures are crucial in determining the physical structure of any deposit. Slagging problems experienced with a South African coal, of a similar ash composition to coal A, were alleviated by reducing the flame temperature l9 . The deposits then aquired a loose dry consistency and were more easily removed. The physical form of deposits from high calcium bituminous coals will depend largely on the extent of the reaction of the calcite-kaolinite minerals, (the most abundant calcium and aluminosilicate minerals). This will be determined by the boiler wall and tube temperatures, the size and degree of contact of ash particles, and by the influence of impurities, in particular iron. The existence of separate phases, enriched in elements other than calcium, aluminium and silicon, is highly significant. These components, enriched in fluxing elements other than calcium, can either act as a binder for crystalline anorthite, or promote its melting.
CONCLUSIONS The unexpectedly high degree of crystallinity within a range of boiler deposits is indicative of their formation via solid state reactions. In this way, crystalline calcium aluminosilicate phases are formed on the boiler walls after deposition of ash particles, generating weakly bonded materials. The mechanism of anorthite formation from the high temperature products of individual clay and calcium minerals, via gehlenite as an intermediate, is consistent with the composition, physical form and temperature history of the range of deposits investigated. The presence of the more abundant elements (calcium, aluminium and silicon) in the predominant stoichiometric crystalline phase within the deposit makes the composition characteristics and melting of the complementary phases, sensitive to minor elements which can act as fluxes.
ACKNOWLEDGEMENTS The authors wish to thank A. Broomfield, K. Hurst and P. A. Morgan for experimental assistance, T. G. Cunningham and T. Rayment, Chemistry Department, Cambridge University, for high-temperature XRD results, and R. B. Jones for developing the SEM method for coal LTA particles.
Ash formation
during pulverized
REFERENCES 10 Unsworth, J. F., Cunliffe, F., Graham, S. C. and Morgan, P. A. Flrel 1987. 66, 1672 Winegartner, E. C., ‘Coal Fouling and Slagging Parameters’, ASME special publication, 1974 Kalmanovitch, D. P., Sanyal. A. and Williamson, J. J. Inst. Energy 1986, 59, 20 Impurities in Coal Combustion’, Raask, E., ‘Mineral Hemisphere, Washington DC, 1985 Mraw, S. C., de Neufville, J. P., Freund, H., Baset, Z., Gorbaty, M. L. and Wright, F. J. in ‘Coal Science’ (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, London, 1983, Vol. 2, Chap. 1 McDonald, C. R., Spink, C. D., Barratt, D. J. and Cowley, L. T. Third Engineering Foundation Conference on slagging and fouling due to impurities in combustion gases. Copper Mountain, Colorado, USA, 29 July-3 August 1984 Morley, C. and Jones, R. B. 21st International Symposium on Combustion, Munich, 1986 Jones. R. B., McCourt, C. B.. Morley, C. and King, K. Fuel 1985. 64, 1460 Huggins, F. E., Kosmack, D. A., Huffman, G. P. and Lee, R. J.
11
12 13 14
19
coal combustion.
2: J. F. Unsworth
et al.
Scanning Electron Microscopy 1980, 1, 531 Ramsden, A. R. and Shibaoka. M. Attnospheric Enrironnwnt 1982, 16, 2191 Fessler, R. R., Skidmore, A. J., Hazard, H. R. and Dimmer, J. P., Winter meeting, 2-7 December 1979, Paper 79-WA/CD-l A.S.M.E. Mitchell, R. S. and Gluskoter, H. J. Fuel 1976, 55, 90 Watt, J. D. B.C.II.R.A. Monthly Bull. 1959. 23(2). 49 Pollmann, S. and Albrecht, W. in ‘Fouling and Slagging Resulting from Impurities in Combustion Gases’, Engineering Foundation Conference, Henniker. New Hampshire, 12-17 July 1981, (Ed. R. W. Bryers), p. 85 ‘Phase Diaerams for Ceramists’. (Ed. E. M. Levin), American Ceramic Soiiety, Ohio, 1964 Kawamura, S. and Kurokawa, R. Go?. Intl. Res. Inst. Nrrgo~u, Japan, 1980,28(6), 157 and 164 Sarofim, A. F., Howard, J. B. and Padia, A. S. Cornhusr. Sci. Technol. 1977, 16, 187 Shibaoka, M, and Ramsden, A. R. in ‘Ash deposits and corrosion due to impurities in combustion gases’, (Ed, R. W. Bryers), Hemisphere, Washington DC, USA, 1978, p, 67 Marcus, H. and Rau. H. E,leryy Derelopmenrs June 19X1. p. 14
FUEL,
1988,
Vol 67, May
641
during
pulverized
2. The significance deposits
of crystalline
anorthite
coal in boiler
John F. Unsworth, David J. Barratt, David Park and Keith Shell Research Ltd, Thornton Research Centre, PO Box I, Chester, CHl (Received
12 March
1987;
revised
14 September
J. Titchener 3SH,
UK
7987)
The chemical structure of pulverized coal-fired hoilcr deposits has been compared, using X-ray diffraction and scanning electron microscopy techniques, with the mineral matter in the parent coal. An unexpectedly high degree of crystallinity is observed in these deposits, a major constituent being anorthite (CaAl,Si,O,), a calcium mineral not present in coals. The formation of anorthite requires the mixing of separate calcium and aluminosilicate mineral domains. Thus, as calcium aluminosilicates are not found in fly ashes, their presence in boiler deposits has been used to probe the mechanism of deposit formation. Studies of a range of deposits, having different temperature histories, together with complementary investigations on mineral mixtures and coal ashes, have demonstrated that anorthite is formed via solid-state reactions and not by recrystallization from a homogeneous melt. Gehlenite (Ca,Al,SiO,) is an intermediate in this reaction and is detected in deposits colfected from the cooler regions of the combustor. Deposits generated by this mechanism are weakly bonded materials containing crystalline phases (predominantly anorthite) and a phase concentrated in elements other than calcium, aluminium and silicon. Concentration of these more abundant elements as stoichiometric phases makes the composition and melting characteristics of the complementary phase much more sensitive to any minor fluxing elements present. (Keywords: combustion of coal; ash: scanning electron microscopy)
Deposit formation within pulverized coal-fired utility boilers lowers operational efficiency by reducing the rate of heat transfer to the boiler tubes and by providing sites for corrosion. Removal of weakly-bonded deposits can be achieved during boiler operation by soot blowing, however, the extensive accumulation of tenacious deposits affects boiler availability. Apart from a rudimentary elemental relationship, the chemical composition of a deposit is markedly different from that of the parent coal ash. Furnace deposits are porous, heterogeneous assemblies (either sintered or fused) of the products from decomposition of coal minerals and their subsequent reactions. Mineral matter in bituminous coals occurs in a plethora of diverse forms, which are distributed heterogeneously in different sized domains within the organic matrix. Thus slag properties. e.g. strength and viscosity, cannot be adequately represented without due regard being given to the influence of mineralogical form, size and degree of interto elemental disproportionation and association, mechanisms within the boiler’. For practical reasons it has been necessary in the past to neglect these factors. so slagging and fouling tendencies have usually been predicted from the elemental composition. and fusion characteristics of an average ash sample2.3. Recent developments in analytical techniques, e.g. electron microscopy and X-ray diffraction, which can now provide rapid quantitative measurements, should allow deposit characterization to be improved sufficiently to elucidate the mechanisms of deposit formation and their influence on deposit strength. 0016 2361:88/050632 lO$?.OO ‘(‘8108.8Huttcrworth & Co. (Puhlishcrs) Ltd. 632
FUEL, 1988, Vol 67, May
The role of iron during slag deposition has been well studied4,5, however, much less attention has been paid to calcium, except in the case of the use of lignites. The relevance of calcium is recognized by the need for different fouling and slagging indices for bituminous and lignitic coal ashes2. It is not uncommon for bituminous coals to have ashes, termed lignitic as a result of high contents of calcium and magnesium minerals in these particular coals. In lignites, though, calcium is present predominantly in a different form, i.e. directly bonded to the organic coal matrix’. The objective of the present work was to compare the mineral structure of deposits with that of the parent coal, and to highlight mechanistic aspects capable of influencing the extent of deposit accumulation and/or strength. Studies were concentrated on the fate of calcium minerals during combustion of a range of internationally traded bituminous steam coals in a pilot scale pulverized coal combustor6. EXPERIMENTAL Several low-rank bituminous coals, covering a range of and maceral geographical and geological origin composition (nblr l), were fired in a 160 kW pilot scale pulverized coal combustorh. Four of these coals had lignitic-type ashes and three coals had bituminous-type ashes. The main combustion parameters (i.e. peak flame temperature 1500-16OO”C, mean residence time 2.0-2.5 s, and heat flux 200-300 kW mV2) were kept relatively constant for each coal, and these are comparable with
Ash formation Table I
Properties
coal combustion.
2: J. F. Unsworth
et al.
of coals A
Coal code Dry basis (wt ‘I,,) Ash Total sulphur Organic sulphur Pyritic sulphur Chlorine Carbon dioxide Fluorine (ppm) Phosphorus (ppm)
15.6 0.7 0.3 0.3 <0.02 1.30 195 707
Dry, ash-free basis (wt ‘I,,) Volatile matter Carbon Hydrogen Maceral group analysis Vitrinite Exinite Inertinite Vitrinite mean random reflectance
(vol”,,)
Ash fusion temperature (Reducing atmosphere) Initial deformation Hemisphere Fusion
( C)
Oxidizing atmosphere Initial deformation Hemisphere Fusion Ash composition analysis (wt “,, in ash) SiO, AlLO., Fe&), TiO, CaO MgO Na,O K>O P*O, SD, Ash type
during pulverized
(“,,)
B
13.4 0.7 0.3 0.4 0.02 0.86 544 2880
C
D
E
F
G
13.8 1.o 0.4 0.6 0.04 I .24 124 400
7.3 0.6 0.2 0.4 0.05 0.64 48 150
10.3 I .o 0.8 0.1 0.04 0.25 83 450
12.4 1.o 0.6 0.4 0.04 0.78 143 750
3.1 0.9 0.8 0.24 0.83 0.45 I7 120
30.3 84. I 4.3
30.6 85.1 4.8
30.4 84.4 4.8
32.5 83.1 4.7
31.8 85.7 5.0
38.3 82.6 5.3
39.9 82.0 5.5
27 4 69
24 2 74
28 2 70
38 1 61
42
43
56
52
65 IO 25
0.70
0.77
0.68
0.66
0.74
0.53
0.63
1350 1360 > 1400
1380 1400 > 1400
1380 1390 > I400
I loo 1170 1390
1250 1355 > 1400
1370 > 1400 > 1400
1070 1150 II70
1350 1400 > 1400
1360 1380 > 1400
1400 > 1400 > 1400
1270 1380 > 1400
1315 > 1400 > 1400
1380 > 1400 > 1400
1290 > 1400 > 1400
46. I 27.7 3.6 I .4 8.8 1.9 0.3 0.5 1.o 6.6 Lignitic
39.0 29.0 3.5 I.5 10.6 1.5 0.3 0.4 4.9 4.4 Ligmtic
conditions within a full-scale utility boiler. After cooling of the combustor, a series of ashes and deposits were collected from various parts of the boiler (Figurr I). These ranged in appearance from fused, to weakly sintered, to powdery. For some deposits, fused material was separated from unfused by sieving at 2 mm. Where necessary samples were crushed and representative subsamples prepared for further analysis. Scanning electron microscopy (SEM) studies were carried out using an electron probe microanalyser, an energy dispersive X-ray detector and a SEM image processing system. Samples were mounted in polished resin blocks and the concentrations of the following elements: Si, Al, Fe, Mn, Ti, Ca, Mg, Na, K, P, S and Cl, were measured by applying a ZAF procedure, which corrects for differences in the intensity, absorption and fluorescence of emitted X-rays. Low temperature ashes (LTA) were prepared from pulverized coal samples with a plasma asher. The SEM procedure selected and sized LTA particles into the following size classes: l-2,24,4-8,8-16, 16-32 and 3264 /lrn diameter. For 300 particles (50 from each size) the electron beam was centred at the middle of each particle and X-ray spectra accumulated for an area of
36.8 30.6 5.6 I.8 10.3 1.5 0.2 0.4 0.8 7.1 _ignitic
53.2 16.7 18.9 0.7 3.1 1.1 0.3 0.6 0.5 1.9 Bituminous
53.9 22.6 10.9 I.3 4.0 1.5 0.2 0.4 I .o 3.7
47.6 28.3 8.6 2.0 6.5 0.7 0.4 0.1 1.5 6.0
Bituminous
Bituminous
25.9 19.0 13.1 I.0 19.1 2.3 7.0 0.2 0.9 19.2 Lignitic
approximately l-2 iirn. From its elemental composition, each area was allocated to a specific mineral type (7?rh/r 2~). Corrections were made for density differences between minerals and for reduced back-scattered electron intensity from smaller particles. The particle size distribution of the LTA was obtained by a combination of sieve analysis and a light scattering instrument. By linking the particle size distribution with elemental and mineral type analysis, an average composition for LTA particles > 1 pm was calculated. X-ray diffraction (XRD) was performed at ambient temperatures and for qualitative studies, the samples ( = 20 mg) were supported on a low-background support of a single silicon crystal. Before quantitative analysis the ashes were mixed 1:l with fluorite as an internal standard. High temperature XRD spectra were obtained under vacuum with a high temperature camera attachment, the sample resting on a thin sheet of platinum foil wrapped around a resistively heated tantalum bar. For one LTA sample (coal A) a step-wise vacuum heat treatment from 400-1500°C was carried out. Each temperature step, typically spanning 2OO“C, was achieved in a few seconds, and then held at each new temperature level for 15 min before analysis.
FUEL,
1988,
Vol 67, May
633
Ash formation
during
pulverized
coal combustion.
2: J. F. Unsworth
et al. PHOTO-TUBE (water-cooled)
_(
FURNACE
I I lcI I
, It----L, I I I I I I
THROAT-
I I
I
SUPERHEATER TUBES
e---I
ASH
DISTANCE
AIR-COOLEDSLAG
1 ‘*O”
=SURFACE
Table 2(a)
Diagram
of pilot-scale
TEMPERATURE
“This class represents KAI,(AISi,O,,)(OH),
combustor’
illustrating
1
temperature
distribution
Si
Al
Fe
> 19 > 10 > 10 < 10 <5 < 10 < 10 < 10 < 10 <5
<5 > 10 > 10 < 10 15 < IO < 10 < 10 < 10 <5
<5 <5 <5 <2 <5 <2 >l >25 >25 <5
of aluminosilicates
intermediate
FUEL, 1988, Vol67,
May
Ti
of deposits
Ca (wt ‘I’;,in particle) <5 <5 <5 > 15 > 15 > 15 > 15 _ _ <5
_ _ _ _ _
> 10
in composition
The crystalline products from high temperature reactions between the major calcium and aluminosilicate minerals found in these coals were determined qualitatively by XRD. Pairs of minerals (i.e. kaolinite with each of calcite, dolomite, ankerite, fluorapatite and anhydrite, and illite with calcite) were blended in the appropriate proportions to generate a 2:l atomic aluminium/calcium ratio, and crushed to an average particle size of 2-3 pm. These mineral mixtures were held at various elevated temperatures between 900 and 1300°C for 2 h in air using a tube furnace, before allowing to cool. The XRD spectra of the resultant materials were determined at ambient temperature. Although anhydrite itself is not present in these coals, it was included in the studies because of its rapid formation in the boiler by reaction of sulphur oxides with lime from decomposed calcite. Ashes were also prepared from pulverized samples of coals A and F by burning in an entrained flow reactor at 1450”C7 and by plasma ashing of flame pyrolysed chars prepared from these coal?.
634
and location
types by SEM
CO,
the range
m
(‘Cl
(‘Cl
Rules used to allocate
Quartz-SiO, Kaolinite-Al,Si,0,.2H,O Illite” Calcite-CaCO, Fluorapatite-3Ca,(PO.),.CaF, Dolomite-CaCO,.MgCO, Ankerite-CaCO,(Mg,Fe,Mn) Siderite-FeCO, Pyrite-FeS, Rutile-TiO,
BURNER,
CYCL .ONE
PANEL
yes%%I:ONTO”RS
Figure 1
FROM
-c
between
_ _ <2 <5 >2 >5 _ _ _
K
P
<2 <0.5 r0.5 _
_ _ _ _ > 10
of minerul
S
_ _ _ _ _ _ < 10 > 10 _
_ _ <2
montmorillonite-AI,Si,O,,(OH),
MINERALOGICAL Distribution
Mg
COMPOSITION
and
muscovite
OF COALS
types
Although a vast range of minerals have been detected in coals, in practice only lo-15 types are normally found at concentrations above 1 wt %. No single analytical technique has yet been devised which adequately measures the mineral composition. XRD is the most definitive method for crystalline materials but even here it is only semi-quantitative, being limited by variations in the crystallinity of each mineral, by matrix effects, and by the inherent complexity of coal mineral mixtures. Tab/e 2b lists the major minerals detected by XRD in the low temperature ashes (LTA) of the seven coals studied. The SEM method used measures element compositions for areas of l-2 pm diameter from different LTA particles. Hence it is restricted to particles above 1 pm diameter. This is a severe limitation as a substantial proportion (in some cases 50 wt %) of the LTA sample consists of particles below this size.
Ash formation Table 2(b)
Mineral
composition
Coal sample
during pulverized
coal combustion.
2: J. F. Unsworth
et al.
of LTAs
A
B
C
D
E
F
Cd
+(I3
+(4) + (23) + + +
- (0)
+ (20)
+ (38) + _
+ Trace
+ _
+ (22) + _ _ _ +
+ + + + _ _
+ + + _ _ +
+
Trace +
+ -
_ _
21 32 11 3
22 46 7 4 3 _~
20 47 7 11 1 Trace 1 8 4 1 Trace _
8 44 4 15
By XRD a-Quartz Kaolinite Calcite Fluorapatite Dolomite Ankerite Siderite Pyrite
+ +
By SEM wt “” + 1 Ltrn material Quartz-type Kaolinite-type Illite-type Calcite-type Fluorapatite-type Dolomite-type Ankerite-type Siderite-type Pyrite-type Rutile-type Gypsum-type Other?
12 48 9 11 4 8 3 Trace 4 Trace
+ (29) +”
_
_b
+
6 46 6 12 11
1 57 6 20 Trace‘ 6 1 Trace 8 1
/ Trace
; Trace
Trace 2 14 6 1
5 5 7 1 _
‘Signifies identified components (numbers in parentheses are wt q, f 57,) “Signifies not identifed ‘Trace = < 0.5 ‘,, ‘In LTA from coal G a significant number of particles could not be assigned concentrated in the volatile elements sulphur and sodium
E” .J”
to a specific mineral-type-these
were predominantly
2 10 2 10 0 Trace 4
< 4 pm in size and
1 TIO LINE hPOSlTlON
40
JNITE
30 z
PURE KAOLINITE
5 2 k! Y
20
10
“B
0
1 0.0
I
2.0
1.0 % IMPURITIES
(a) IMPURITY
Figure 2
3.0
4.0
I
.
i
<
5.0
6.0
15
(Ca + Fe + Mg + K + Na + Ti + S + Cl + P)
CONTENT
Element concentrations
10
LTA particles
Nevertheless, this method generates much valuable information concerning the association of elements within mineral domains. With the exception of illites, which have a variable composition, almost all the analyses resulted in element compositions corresponding closely to the stoichiometry of a single mineral type. This allows a set of rules (Tuble 2~) to be used: these allocate each point to a specific mineral-type, thus providing a quantitative composition for the + 1 pm particles in terms of such mineral-types. In all of these coals the major mineral is clearly kaolinite. The high degree of purity and homogeneity of the LTA particles allocated by SEM to the kaolinite type
25
30
% ALUMINIUM (b) SILICON
of 156 Kaolinite-type
20
AND ALUMINIUM
CONTENTS
from coal A
is demonstrated
for coal A by:
1. the very low concentration of element impurities in nearly all such particles; 2. aluminium and silicon contents, plus a constant Si/Al ratio, corresponding closely with the composition of kaolinite (Figure 2b). For the other mineral types, similar element distributions which closely resembled the appropriate stoichiometry were observed. For all seven coals studied, most (i.e. over 90%) of the mineral domains > 1 pm in size consist of single mineral types and not mixtures of minerals. Although plasma ashing techniques used to prepare
FUEL, 1988, Vol 67, May
635
Ash formation
during pulverized
coal combustion.
2: J. F. Unsworth
LTAs could have caused aggregates of minerals to disperse as separate particles, this is considered unlikely to be of any significance. Other SEM studies on bituminous coal particles classified over 90% of the mineral domains into the major individual mineral types’. Form of culcium in coals Calcium occurs predominantly as calcite, dolomite, ankerite and fluorapatite. SEM does not measure fluorine, but fluorapatite has been detected in these LTAs by XRD. Furthermore, a linear relationship between phosphorus and fluorine contents (Tuhle I) exists for these coals, and this is approximately the same ratio as in fluorapatite. Figure 3 illustrates the way in which calcium is distributed between the main mineral types for each of the seven coals being studied. The main form of calcium is usually calcite, but substantial amounts of fluorapatite (coals A and B), dolomite (coals A, B and C), and ankerite (coals E and G), were found in these samples. The amount of calcium found in association with kaolinite, illite and quartz was negligible, with no evidence for the presence of calcium aluminosilicates, such as anorthite (CaAl,Si,O,) or gehlenite (Ca,Al,SiO,). COMPOSITION ASHES
OF TEST
DEPOSITS
AND
Boiler deposits and ashes from these seven coals, which have widely different ash elemental compositions, Tub/e 1, were investigated. Their fireside behaviour varied between: the production of tenacious slag deposits (co;,1 D); easily-removed slag deposits (coals B, E and F); very weak deposits (coal A and C); and extensive fouling deposits (coal G). Despite the known heterogeneity of boiler deposits and fly asheslO,ll, characterization of these materials is often restricted to an average elemental composition. Much less attention has been paid to the quantative identification of the mineral species present. This may be partly because such deposits are often glassy (i.e. noncrystalline)’ ’ and XRD cannot identify amorphous minerals. Nevertheless, an unexpectedly high degree of crystallinity has been observed within some of the deposits examined in this study, with > 70 wt “; of the material accounted for by XRD in several cases. Details of the high temperature minerals detected by XRD for a
Figure 3
636
Distribution
FUEL,
1988,
of calcium
within mineral
Vol 67, May
types
et al.
wide range of boiler deposits and ashes are given in Tuble 3, together with quantitative data for coals A and B. The following trends were observed. 1. Anorthite (CaAl,Si,O,) occurs in nearly all the deposits in which some fusion of ash particles has clearly occurred; it is absent from powdery deposits and cyclone ashes (i.e. fly ashes). For throat and furnace deposits from A and B it contributes approximately 50 7; of the material, accounting for all the calcium present. In cooler boiler locations (i.e. phototube and superheaters), less anorthite is formed. (Ca,Al,SiO,) is found in deposits at the 2. Gehlenite cooler boiler locations for coals A, B and G. It is not present in cyclone ashes or well sintered deposits. is also present in cyclone ashes and some 3. Calcium unfused superheater deposits as anhydrite (CaSO,), lime (CaO) and undecomposed calcite. 4. Mullite (Al,Si,O,,) is detected in most deposits and ashes. It is formed in the thermal cyclone decomposition of kaolinite and other clay minerals, together with cristobalite (SiO,) which is also fairly ubiquitous. (Fe,O,) and magnetite (Fe,O,), formed 5. Haematite during thermal decomposition of pyrite and siderite, occur frequently. The elemental heterogeneity of a throat deposit was examined by SEM. This deposit had grown with a fingerlike structure into the gas stream during the combustion of coal B; Figure 4 shows the honeycomb structure of the deposit with both large and small pores (ca. 1 mm and 25 Ltrn diameter, respectively). Two distinct solid phases could be discerned : one rich in calcium and the other rich in phosphorus; both phases contained aluminium. Quantitative X-ray element analysis demonstrated that the calcium-rich phase contained predominantly only calcium, aluminium and silicon in the proportions appropriate to anorthite (Tuhle 4). The other phase was enriched in all other elements present in the ash with no regular stoichiometry.
MECHANISM
OF ANORTHITE
FORMATION
Most crystalline species detected in these ashes and deposits (e.g. mullite, lime, haematite, magnetite) are formed by thermal decomposition of each separate mineral type (i.e. kaolinite, calcite, pyrite/sideritejankerite, respectively) l2 . This is exclusively the case for cyclone ashes. Anorthite and gehlenite, on the other hand, are formed from reactions between different minerals, i.e. mixing of calcium and of aluminosilicate domains. Significantly, these two calcium aluminosilicates were detected in deposits, but not in cyclone ashes. Consequently, the mechanism of formation of anorthite has been considered in some detail, to establish if such compositional differences can help elucidate ash formation mechanisms in the p.f. boiler. RecrJstullizution from u homogeneous melt Anorthite has been observed in other combustor Its formation can be explained by slags 3.13.14 recrystallization from a homogeneous melt. The CaOAl,O,-SiO, phase diagram (Figure 5)’ 5 predicts that, for bulk ash compositions similar to those from six of these coals, mullite crystallizes first at temperatures below
1350/1450 1100/1300 100/1200 100/1200 < 600/ < 800 1350/1450 <6OC;<800 1350/1450 1lOO/1250 1100/1250 1100/1250 6OO/lOOO 600/1000 <600/<800 450/1450 600/1000 600/1ooO 1350/1500 1350/1450 1 loo/1250 I 100/1250 1100/1250 1100/1250 600/1000 600/1000 < 600/ < 800 1350/1450 1350/1450 1100/1300 I 800/1200 2 800/1050 3 W/950 5 SOOj850 < 600/c 800
1350/1450 1100/1250 1100/1250 1100/1250 I lOO/l250 600/1000 600/1000 <600/c 800
( C)
Metal/gas temperature
Fused Sintered Brown sinter Grey sinter Powder Agglomerated Powder Powder Powder Fused Fused Sintered Powder Powder Fused Powder Fused Fused Sintered Powder Agglomerated Powder Powder Fused Agglomerated Powder Fused Fused Sintered Brown sinter Grey sinter Powder Sintered Powder Powder Sintered Powder Powder Agglomerated Agglomerated Agglomerated Agglomerated Powder P
+ +
+
+
+ + +
+(42) + (60) +(35)
+(58) + (53) + (58) +W) +(3l) +(12)
Anorthite Ca,AI,Si,O,
and species detected
Appearance
and ashes-appearance
p, signifies possible component; +, signifies identified component; numbers “M =Microline (KAISi,O,), I = Iron (Fe), H = Halite (NaCI), N =Nepheline *PFA is pulverized fuel ash from a full-scale boiler trial
G
F
E
D
C
B
deposits
Furnace Throatiphototube Throat/phototube Throat/phototube Throat/phototube Superheater Superheater Cyclone PFA-full scaleb Furnace Throat Phototube Phototube Cyclone Furnace Cyclone Furnace Throat/phototube Throat/phototube Throat/phototube Superheater Superheater Cyclone Furnace/slagpanel Superheater Superheater Furnace/quarl Furnace/bottom Throat/phototube Throat/phototube Throat/phototube Throat/phototube Superheater Superheater Cyclone Furnace Furnace Throat Superheater/row Superheater/row Superheater/row Superheater/row Cyclone
Location
Coal code
A
Combustor
Table 3
+ + + + + + + + + + + + + +
P + + +
+ +
l t(13) +(15) f(t8) +
+(12)
+
+(15) +(18) +(l2) +(19) +(21) +(l5) +(2l) +
Mullite AI,Si,O, 3
+ +
+
+ +
+ + + + + + +
+ + + + + + + + + +
+(I) + (4) +
+ (4) +(8) + (6) +(6) +(lO) + +
+ (5)
Quartz SiO,
in parentheses are wt ‘<, f 5”,, (NaAISiO,), T=Tridymite (SiO,)
P + + + 4
P + P
Gehlenite Ca,Al,SiO,
by XRD
+ +
+ +
+ +
+ + +
Cristobalite SiO,
+ +
+ + + +
+
P
P
P
P
Anhydrite CaSO,
P
+
P
P +
Lime CaO
P
Calcite CaCO,
+
+
+ + + + + + + + + + + +
Haematite Fe,O,
Magnetite Fe@,
N(P) N(P) N(P),H(P) N(P) N(P) N(P) N(P) N(P)>H(P)
T(P)J(P) UP)
M(f)
M(P)
Others“
Ash formation
during
pulverized
coal combustion.
2: J. F. Unsworth
Figure 4 SEM/E.P.M.A. photomicrographs of a throat deposit from coal B, illustrating the presence oftwo distinct solid phases; one calcium rich, the other phosphorous rich. White signifies high element concentrations: a, back-scattered digitized electron map; b, aluminium X-ray map; c, phosphorus X-ray map; d, calcium X-ray map Table4 Element compositions” from coal B
of two phases in a fused throat deposit
Slag ashb (Wt %) 50, Al,O, Fe,O, CaO MgO TiO PzO25
Coal ash’
Phase
42 32 4 12 2 2 5
41 29 8 7 4 3 8
1
Phase 2
Anorthite (CaAI,Si,O,)
41 36 2 17 I 1 1
43 37 0 20 0 0 0
“expressed as oxides ‘by SEM averages of 6-point analyses ‘by X-ray fluorescence spectroscopy
16OO”C, then anorthite and mullite co-crystallize in the range 1500-135O”C, and finally, anorthite, mullite and silica form a eutectic, which crystallizes at 1345°C. To test the relevance of this mechanism, the high temperature ash from coal A was completely fused by heating at 1450°C for 2 h, and then allowed to cool in air at ambient temperature. The resulting glassy solid contained no crystalline species detectable by XRD. In addition, there are some indications that less anorthite is found in the most fused of the combustor deposits (‘Kzhte 3). If the combustor temperatures, which are generally too low, and the heterogeneity of deposits are also considered, then it seems most unlikely that anorthite (and hence any of these combustor deposits) is formed in this way. Solid-state
reuctions
species detected for five temperatures between 900 and 1300°C. These results demonstrate anorthite formation by reaction of kaolinte, not only with calcite but also with dolomite, ankerite, fluorapatite and anhydrite (dehydrated gypsum), and also by reaction of illite with calcite. These reactions commence at 900-1000°C with the first signs of agglomeration. Except in the case of fluorapatite, anorthite formation is preceded by the production of gehlenite, which is an intermediate. Conversion to anorthite, which, when pure, has a melting point of 1553°C is almost completed by 1200°C when the mixture is still only weakly sintered and easily broken. Thus, combination of the elements,calcium plus aluminium and silicon, can be achieved via a solid-state reaction between loosely compacted particles, which proceeds rapidly and produces crystalline products at temperatures as low as 900°C. The rate of conversion that will depend on diffusion of calcium into aluminoscilicate particles, and can be expected to be strongly influenced by the extent of inter-particle contact (i.e. mixing efficiently and particle size) as well as the temperature. Solitl-state
reactions
in coal ashes and cleposits
High temperature XRD techniques were used to demonstrate that anorthite can be formed by such a reaction sequence in the LTA from another sample of Coal A. Figure 6 illustrates the XRD spectra obtained at temperatures between 400 and 1550°C as the ash is heated up. At 800°C calcite, fluorapatite and kaolinite start to decompose, and the first formation of gehlenite is noted. From lOOO-14OO”C, gehlenite is gradually replaced by anorthite, which is dissolved in a melt by 1550°C. The composition and appearance of the deposits described earlier is clearly consistent with this solid-state mechanism for anorthite formation. Loosely agglomerated ashes from the superheater and phototube (that have associated gas and metal temperatures of approximately 850-1250°C and 100@6OO”C, respectively) contained gehlenite. Anorthite was largely founu in sintered deposits from the hotter sections of the boiler (gas and metal temperatures of approximately 1250145O’C and 1 lOO-135O”C, respectively). Ash formation
mechunisms
The absence of both anorthite and gehlenite in all fly ashes (including a power station pulverized fuel ash) suggests that element mixing of calcium with aluminium
between selected minerals
Anorthite formation has been reported at lower temperatures via a series of solid-state reactions between Confirmation has been calcite and kaolinite10,‘6. obtained by monitoring this reaction using XRD. Total conversion of all the calcium present into anorthite was observed in some deposits, thus five other mineral mixtures were studied. Tuble 5 details the appearance and
638
et al.
FUEL, 1988, Vol67,
May
Figure 5 Ca04l,O,SiO, these 7 coal ashes. Predicted
phase diagram illustrating composition sequence for coal ash A
of
Powder Some agglomeration Agglomerated Sintered Sintered and swollen
Powder Agglomerated Some sintering Sintered Sintered
Some agglomeration Agglomerated Some sintering Sintered and swollen Fused
900 1000 1100 1200 1300
900 loo0 1100 1200 1300
900 1000 1100 1200 1300
Fluoropatite and kaolinite
Anhydrite and kaolinite
Calcite and illite
component
Some agglomeration Some agglomeration Agglomerated Sintered Sintered
900 loo0 1100 1200 1300
Ankerite and kaolinite
+ signifies identified
Some agglomeration Agglomerated Agglomerated Sintered Sintered and swollen
900 1000 1100 1200 1300
Dolomite and kaolinite
Some agglomeration Agglomerated Sintered Sintered Sintered
Appearance
mixturesappearance
900 loo0 1100 1200 1300
Temperature (“C)
Mixture
mineral
Calcite and kaolinite
High temperature
Table 5
+ + + +
+ + + +
+ + + +
+ + + + +
+ + + + +
+ + + + +
Anorthite Ca,Al,Si,O,
+ +
+ +
+ +
+
Gehlenite Ca,Al,SiO,
and species identified
+
+ + +
+
Al,Si,O,
Mullite
by XRD
3
+
+
Quartz SiO,
+ +
Cristobalite SiO,
Anhydrite CaSO,
+ + + + +
Galaxite MnAl,O,
+ + + +
Fe,O,
Haematite
+ +
Fluorapatite 3Ca,(PO,),CaF,
Ash formation
during pulverized
K =
KAOLINITE
A
C
-
CALCITE
G =
GEHLENITE
F
=
FLUORAPATITE
M =
MULLITE
P
=
PYRITE
=
coal combustion.
ANORTHITE
Injluence K
K
Q =
OUARTZ
B -
BASSANITE
2: J. F. Unsworth
_._---.L
1--
1
40
38
36
-1
1
34
32
1
30
I_
28
26
High temperature
,550OC
2
24
DEGREES,
Figure 6
~-~~
----I_I-_11.-.1.--~
22
20
18
16
14
12
10
20
XRD spectra
of LTA from coal A
and silicon occurs within the deposit as it is growing. Confirmation that anorthite is not formed prior to deposition was obtained by preparing first, the LTAs of the flame pyrolysed chars, and second, ashes burnt out in an entrained flow reactor at 1450°C from coals A and F’. The XRD spectra of the resultant powders were similar to those obtained for the appropriate cyclone ash; only high temperature products of individual coal minerals were detected. This has implications concerning the mechanism by which char particles burn to form ash particles. The particles in cyclone ashes and those prepared in the entrained flow reactor have experienced temperatures in excess of 1450°C. The residence time is short ( 2 2 s), but nevertheless sufficient for fusion of quartz to occur. There is evidencei to suggest that mineral domains within a char particle coalesce to produce a single ash particle. However, hot stage microscopy studies” have indicated that, in high inertinite and higher rank coals, which may experience lower peak temperatures in the boiler than high vitrinite lower rank coals, minerals can be ejected singly during burn-out. If coalescence of calcium with aluminium and silicon domains has already occurred before deposition, then it is surprising that anorthite is only detected in deposits, especially as anorthite is known to crystallize rapidly. These results, which are mainly for high inertinite coals, are more consistent with the idea that calcium and aluminosilicate minerals can be ejected singly. Mixing, followed by gehlenite and anorthite formation, then occurs only after deposition. To be more definitive, it is necessary to establish if the temperature history of fly ashes is adequate to form anorthite.
640
FUEL, 1988, Vol67,
May
et al. on deposit strength
The properties of boiler deposits derived by aggregation of ash particles will inevitably depend on the extent of inter-particle bonding. Hence, the mechanism of formation of minerals within a deposit can be as important, if not more so, than its actual composition; recrystallization from a homogeneous melt should generate a denser and much stronger material than one derived from a solid-state reaction and may also have better heat transfer properties. The volumetric changes occurring during the calcitekaolinite reaction have been examined in detail”. Densification occurs rapidly at 600-950°C as gehlenite is formed. Transformation to anorthite does not contribute any further densification in the temperature range lOOO1200°C. However, above 1250°C further shrinkage occurs as the system begins to melt and vitrify. Clearly boiler temperatures are crucial in determining the physical structure of any deposit. Slagging problems experienced with a South African coal, of a similar ash composition to coal A, were alleviated by reducing the flame temperature l9 . The deposits then aquired a loose dry consistency and were more easily removed. The physical form of deposits from high calcium bituminous coals will depend largely on the extent of the reaction of the calcite-kaolinite minerals, (the most abundant calcium and aluminosilicate minerals). This will be determined by the boiler wall and tube temperatures, the size and degree of contact of ash particles, and by the influence of impurities, in particular iron. The existence of separate phases, enriched in elements other than calcium, aluminium and silicon, is highly significant. These components, enriched in fluxing elements other than calcium, can either act as a binder for crystalline anorthite, or promote its melting.
CONCLUSIONS The unexpectedly high degree of crystallinity within a range of boiler deposits is indicative of their formation via solid state reactions. In this way, crystalline calcium aluminosilicate phases are formed on the boiler walls after deposition of ash particles, generating weakly bonded materials. The mechanism of anorthite formation from the high temperature products of individual clay and calcium minerals, via gehlenite as an intermediate, is consistent with the composition, physical form and temperature history of the range of deposits investigated. The presence of the more abundant elements (calcium, aluminium and silicon) in the predominant stoichiometric crystalline phase within the deposit makes the composition characteristics and melting of the complementary phases, sensitive to minor elements which can act as fluxes.
ACKNOWLEDGEMENTS The authors wish to thank A. Broomfield, K. Hurst and P. A. Morgan for experimental assistance, T. G. Cunningham and T. Rayment, Chemistry Department, Cambridge University, for high-temperature XRD results, and R. B. Jones for developing the SEM method for coal LTA particles.
Ash formation
during pulverized
REFERENCES 10 Unsworth, J. F., Cunliffe, F., Graham, S. C. and Morgan, P. A. Flrel 1987. 66, 1672 Winegartner, E. C., ‘Coal Fouling and Slagging Parameters’, ASME special publication, 1974 Kalmanovitch, D. P., Sanyal. A. and Williamson, J. J. Inst. Energy 1986, 59, 20 Impurities in Coal Combustion’, Raask, E., ‘Mineral Hemisphere, Washington DC, 1985 Mraw, S. C., de Neufville, J. P., Freund, H., Baset, Z., Gorbaty, M. L. and Wright, F. J. in ‘Coal Science’ (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, London, 1983, Vol. 2, Chap. 1 McDonald, C. R., Spink, C. D., Barratt, D. J. and Cowley, L. T. Third Engineering Foundation Conference on slagging and fouling due to impurities in combustion gases. Copper Mountain, Colorado, USA, 29 July-3 August 1984 Morley, C. and Jones, R. B. 21st International Symposium on Combustion, Munich, 1986 Jones. R. B., McCourt, C. B.. Morley, C. and King, K. Fuel 1985. 64, 1460 Huggins, F. E., Kosmack, D. A., Huffman, G. P. and Lee, R. J.
11
12 13 14
19
coal combustion.
2: J. F. Unsworth
et al.
Scanning Electron Microscopy 1980, 1, 531 Ramsden, A. R. and Shibaoka. M. Attnospheric Enrironnwnt 1982, 16, 2191 Fessler, R. R., Skidmore, A. J., Hazard, H. R. and Dimmer, J. P., Winter meeting, 2-7 December 1979, Paper 79-WA/CD-l A.S.M.E. Mitchell, R. S. and Gluskoter, H. J. Fuel 1976, 55, 90 Watt, J. D. B.C.II.R.A. Monthly Bull. 1959. 23(2). 49 Pollmann, S. and Albrecht, W. in ‘Fouling and Slagging Resulting from Impurities in Combustion Gases’, Engineering Foundation Conference, Henniker. New Hampshire, 12-17 July 1981, (Ed. R. W. Bryers), p. 85 ‘Phase Diaerams for Ceramists’. (Ed. E. M. Levin), American Ceramic Soiiety, Ohio, 1964 Kawamura, S. and Kurokawa, R. Go?. Intl. Res. Inst. Nrrgo~u, Japan, 1980,28(6), 157 and 164 Sarofim, A. F., Howard, J. B. and Padia, A. S. Cornhusr. Sci. Technol. 1977, 16, 187 Shibaoka, M, and Ramsden, A. R. in ‘Ash deposits and corrosion due to impurities in combustion gases’, (Ed, R. W. Bryers), Hemisphere, Washington DC, USA, 1978, p, 67 Marcus, H. and Rau. H. E,leryy Derelopmenrs June 19X1. p. 14
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1988,
Vol 67, May
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