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Sodium ash reactions during combustion of pulverised coal
Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 1313-1321
SODIUM ASH REACTIONS D U R I N G C O M B U S T I O N OF PULVERISED
COAL
ERIC R. LINDNER The Electricity Trust of South Australia AND
TERRY F. WALL Department of Chemical Engineering Unit~ersity of Newcastle, Australia
Reactions of sodium with mineral silicates contribute to the formation of fireside deposits during the combustion of some high sodium coals. A systematic experimental study of the reactions between sodium with inherent silica and kaolin during pulverised coal combustion was undertaken in order to examine the mechanisms of the reactions. The experiments were conducted in a drop tube furnace with coal samples which had been specially prepared to control the concentrations, the size and the distribution of the inorganic constituents, which included silica, kaolin, sodium chloride, sodium acetate, and gaseous SOz to simulate organic sulfur. The rate of formation of sodium silicate was found to increase with temperature but the extent of formation was limited by the progressive agglomeration and coalescence of the ash which reduced the surface area available for reaction. The extent of interaction between the sodimn and silica/silicates was reduced by the presence of chlorine and sulphur and this effect was greater with kaolin than with quartz. At gas temperatures of 1200-1400 ~ C, sodium was volatilised from the coal before significant reactions occurred with the silica. At 1000~ C the sodium was not fully released prior to the commencement of the silicate formation reaction. Introduction Sodium in coal is commonly associated with the tormation of fireside deposits in boilers. Experimental evidence has shown that s~xlium species can react with silica and aluminium silicates under conditions which, to va~'ing degrees, approach those that occur in pulveriscd coal combustion systems. (Brinstead and Kear, ! Boow, "z Neville and Sarofim, :~ Ounsted and Schocn4 and Wibberley and Wall. 5 The sodium silicates and alumininm silicates formed by these reactions have markedly lower melting temperatures than the original silica and silicate minerals. The sodium silicates and to lesser extent the sodium aluminium silicates tend to torm glasses which can supercool during deposition. By supercooling, the low viscosity of the glass phases on fly ash are prohmged, which enables impacting p6articles to stick to the tube/deposit. Wibbcrlcy, for example, has calculated that only a thin (0.01-0.1 IX) glaze of low viscosity sodium silicate is required to retain fly ash particles impinging onto a boiler tube. The presence of low melting temperature glassy compounds can also facilitate sintering within de-
posits on walls. In addition to forming sodium silicates, the reactions of sodium with silica and aluminium silicates also influence the degree of fouling severi~' by reducing the sodium that remains in the vapour state in the flame and the hot furnace gases. This vapourised sodium can condense as a liquid on fly ash or on and within deposits thus contributing to deposit formation and consolidation. Previous investigations into the reactions of sodium with silica and aluminium silicates have generally been performed by crucible experiments where conditions are significantly different to those in pc fired boilers. Wibberley and Walls studied the reactions of sodium chloride vapour and sodium acetate vapour with silica particles under rapid heating conditions but these reactions were independent of coal combustion. In some high sodium coMs (NazO/ash > 0.03) fine silica and kaolin is found within the coal particles (when thev are called inherent). In response to the above considerations the present study examined: 9 the reactions and reaction mechanisms of sodium with inherent silica and kaolin during coal com-
1313
1314
MINERAL MATI'ER AND ASH
bustion and the subsequent cooling of the combustion gases. 9 the dependence of these reactions on the form of the sodium in the coal, the presence of sulphur oxides and the temperature of combustion.
Experimental The interpretation of the complex coal ash reactions that occur during combustion is often difficult due to the complex chemistry involved. In order to reduce this complexity the following strategies were adopted for the experimental programme: 9 s i m p l i f y the reaction system by combusting a spe-
cially prepared coal in which the concentrations, size and distribution of the inorganic constituents were controlled. 9 c o m b u s t the modified coals under controlled and well defined conditions which simulate those in p c flames. The modified coals were prepared from a Loy Yang brown coal (Victoria, Australia) sample which contained low levels of ash (1.1% dry basis) and sulphur (0.3% dry basis). The ash was reduced to 0.5% dry basis by acid leaching the coal to remove the
ion exchangeable constituents which comprised most of the sodium, potassium, calcium and magnesium and some of the aluminium. After water washing the leached coal, known quantities of inorganic constituents were added to it and then this mixture was made into a thixotropic slurry by beifig finely ground in a household blender. Sodium was added as a solution of sodium chloride or sodium acetate (CHaCOONa). Silica and kaolin was added as discrete size fractions both having a mass mean size of 6.5 p,m, with 90% < 14 Ixm, 10% < 2.5 Ixm. A fine silica having a mass mean size of 2.5 Izm (90% < 6 Ixm, 10% < 1 p,m) was also used in a few experiments. The slurry was dried, forming lumps which were then pulverised and sieved to obtain the 63-90 Ixm size fraction used in the experiments. Table I gives the ash analyses of the modified coals. Electronmicrographs of the sectioned modified coals revealed that the minerals were dispersed uniformaly within the resultant particles, thereby approximating inherent ash, and were not ground to a smaller size during crushing of the lumps. The micrographs revealed that the particles are porous with macropores several microns in width leading from their external sections to the inner minerals. The temperature attained by a burning coal particle depends on its size, experiments using
TABLE I Ash analysis of coals, Loy Yang brown coal and comparison coals. The designation refers to the following additives (quoted as % of dry coal): NC--2.5% NaCl: 1/2 NC--1.2% NaCI: ON--3.5% CHzCOONa: 1/2 ON--1.7% CHzCOONa: Si--10% SiO2: K--10% Kaolin Modified coal designation
SiOz %
A1203 %
Na20 %
NCSi 1/2 NCSi ONSi 1/20NSi ONSi/f a Si NCSi/ab NCK Loy Yang~ Lochiel a North Dakota~ Morwell
~.1 87.3 82.0 ~.4 82.9 95.3 ~.3 45.6 25.0 21.6 27.1 8.2
2.7 3.2 2.2 2.4 2.1 2.8 0.7 36.6 42.9 4.2 11.6 0.5
12.3 9.0 13.5 6.1 12.5 <0.1 13.2 14.0 4.9 13.1 9.3 0.8
Ash analysis K20 % 0.1 0.1 0.2 0.2 0.2 0.1 0.1 0.6 0.3 0.4 0.8 0.3
MgO %
CaO %
Fe203 %
0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.6 6.9 10.7 6.2 19.6
<0.1 0.3 <0.1 <0.1 <0.1 <0.1 <0.1 0.5 2.7 11.6 21.6 21.2
0.5 0.6 0.6 0.6 0.3 0.6 0.6 1.6 6.6 4.8 5.1 5.1
Notes: a"Fine" quartz added instead of "regular" sized quartz used in the other coals. bprepared coal had a lower alumina content than the other coals achieved by hot hydrochloric leaching of the raw coal. CLoy Yang brown coal (Victoria) that was used for the preparation of the synthetic coals. aSouth Australian brown coal--(Bosio et. al.9) eCoal sample GH-S from Glenharold Mine--(Honeal~ fMorwell (Vietoria)--(Higgins and Morley ll)
SODIUM ASH REACTIONS a size fraction rather than a single range therefore simplify the interpretation of observed phenomena such as ash sintering which are influenced by particle temperature. Examination of the size fraction with an optical microscope indicated the particles were discrete rather than agglomerates. The prepared coals were fed at a controlled rate into a drop tube furnace and combusted in 20% excess air. The 45 mm internal diameter, ceramic core tube of the furnace was heated externally by a cylindrical graphite element. A 150 mm hot zone of the core tube was maintained at a relatively uniform temperature. Coal heating rates in the furnace were estimated at 104-105 ~ with residence times of about 1 second, The solid ash products were extracted with a probe which rapidly quenched and diluted the products with air (with a volume of twenty times the furnace flow) to about 200 ~ C. The supra-micron ash particles were collected in a cyclone and the sub-micron particles were retained in a downstream fine filter, as shown on Fig. 1. The parametric programme of experiments undertaken is given in Table II. Experiments were conducted at furnace wall temperatures of 1000~ C, 1200~ C and 1400~ C. For every change to an experimental parameter other than temperature, a group of 3 experiments were performed at the 3 nominated furnace temperatures. The experimental Groups 1-8 (Table II) were conducted to explore the effect of the chemistry of the system, i.e. the form and concentration of the sodium, the presence of sulphur dioxide and the level of aluminium with the silica. The remaining experimental groups were deCombustion
Balance air COLLECTION PROBE O,uench air
--
FILTER ,
.~lt"q
--Suction
1315
signed to examine the reaction mechanism and the rate controlling step or steps. In Group 9, the effect of the silica particle size was investigated while in Group 10 the influence of residence time was examined by firing with a reduced coal rate and by collecting the ash at the base of the furnace. The two final experimental groups of Table II, called the separated experiments, were to determine the extent that the sodium reacted as it evaporated from the coal. For all other groups, the sodium salt and silica additives were in the same particle so that some reaction could occur before or as the sodium evaporated. For the separated experiments the additives were in different particle fractions, so that reaction could only occur after the sodium had evaporated from one fraction and then diffused through the gas phase to the particle from the other fraction which contained silica. The coarse cyclone collected ash was digested in demineralised water at room temperature for an initial period of half an hour to remove condensed salts. The residue was then digested at 120" C for fiu'ther successive periods of 4, 16, 44 and 92 hours. The leachate from each digestion as analysed separately for Si (molybdate blue method), AI, Na (AA spectrophotometry) and SO4 and CI (ion chromatography). After the final digestion the residue was analysed for the ash constituents Fe203, A1203, SiO2, MgO, Ca*, K20 and Na20. The sodium combined in the silicates was calculated as follows: Na~O (silicate = Na20 (total leachate) + Na20 (residue from final leaching) - NazO (in non-silicate forms in leachate, i.e. the molar equivalent of Na corresponding to CI-, SO4--OH- and CO3--but expressed as Na20). The submicron ash collected in the filter was predominantly condensed sodium salts. This ultra fine ash was dissolved in water and analysed for Na, K, Ca, Mg (AA), CI-, SO4--(ion chromatography), OH- and CO3--(pH and ion balance). Considering the mass balance for the experiments reported here, an average of 81% of the silica (or kaolin) was collected (standard deviation 9%) and 68% of the sodium (standard deviation 10%). Some fine ash and condensibles (NaC1 and Na2SO4) were observed to collect on the furnace walls and the water cooled feeder probe, probably by a thermophoresis mechanism which is an inevitable consequence of the presence of coal surfaces. Repeat experiments gave measured silicate formation levels within 10%.
Fine ash collection
',
Coarse ash collection
FIc. 1. The arrangement for ash collection.
The Extent of Silicate Formation The extent of silicate formation was estimated from the analysed amounts of NazO as silicate normal-
1316
MINERAL MATrER AND ASH TABLE II Details of the combustion experiments I
I
l
I
Concentration of added inorganics in coal (dry basis) Experiment group no.
1
2 3 4 5 6 7 8 9 1O 11~ 12e
Modified coal designation
Sodium as chloride %
NCSi NCSi/a NCSi 1/2 NCSi ONSi ONSi 1/20NSi NCK NCSi/f NCSi NC(50%) Si(50%)
1 1 1 0.5
Sodium as acetate %
Quartz %
1 1 0.5
10 10 10 10 10 10 10
1 1 0.5b
ON(5O%) Si(50%)
0.5
10 10 5
S~ %
--
3
--
3
10 1
Notes: a% S given is equivalent to the sulphur added bFor experiments, Groups 11 and 12, the coal CThe quench probe was inserted 200 and 180 1400~ C; for Group 10, the probe was located at
Kaolin %
Coal feed rate g/h 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 2.7 5.5 5.5
as sulphur dioxide. used was a 50:50 mixture of the two coals designated. mm for furnace temperatures of 1000~ C, 1200~ C and the exit of the furnace to increase the residence time.
ised with respect to SiO2 and Al2Oa. These values are given on Fig. 2 for the experimental groups. The effect of the form of sodium added (as chloride or acetate) was marked, with the mass of silicate formed being from three to five times greater where sodium acetate was used.
The addition of sulphur (as S02 to a level equivalent of 3% of S in the dry coal) generally decreased silicate formation but only to a small extent (reduction <19%). A reduction in the level of sodium as acetate decreased silicate formation approximately in proportion with the sodium reduction. A reduction in the sodium chloride content of coal produced a less than proportionate reduction in the silicate formation. The effect of the silica particle size was not significant in the range investigated. Although the finer silica used had a surface area approximately 3 times greater than the silica used in the other experiments there was little change in the degree of silicate formation (compare Groups 5 and 9). The effect of the addition of aluminium into the system, with sodium as sodium chloride, was dramatic. The experiments involving kaolin with NaC1, Group 8, resulted in the greatest extent of silicate formation. In these experiments the level of reacted sodium was five to eight times that found in the experiments with the same level of sodium chloride but silica instead of kaolin (Group 1).
The variation in the residence time of ash particles, at temperatures where silicates were favoured to form, did not cause a significant change in the degree of silicate formation. The residence time of the ash particles, above 1000~C and subsequent to combustion, were estimated to be between 1.7 and 3 times longer in the Group 10 experiments than those in Group 1 experiments but there was insignificant difference in the degree of reaction. It is evident that negligible reaction between sodium and silica proceeded after combustion was completed. This effect was probably due to the marked reduction in the silica/silicate surface area by the coalescence and agglomeration of the ash as combustion proceeded (see Section 4). The effect of adding the sodium and silica in separate particles (i.e. the "separated" experiments) is established by comparing the results of experimental Groups 1, 4 and 11 for the NaC1 additive and Groups 5, 7 and 12 experiments (for the CH3COONa additive). The loading of sodium and silica in the individual coals mixed for the "separated" experiments was the same as the other experiments, both the level of sodium and the sodium:silica ratio of the mixed coals did not reproduce together those of the other experiments. This makes comparison difficult. The higher chlorine/sodium ratios in experimental Groups 1 and 4 than in the Group 11 experiments (due to the retention of some chlorine
SODIUM ASH REACTIONS F EXperimentat group number 12
u
SOdium as NaCI
11 ~
;
F
a
d
d
i
t
~ i
Kaolin e
v
. ?
5E
EP
cate was progressively dissolved as the leaching period increased. Illustrative results are given on Fig. 3 for experiments involving NaC1 and CH3COONa with silica and NaCI with kaolin. The volumetric fraction of the silicate in the ash was calculated from the total sodium and silicon analysed and assuming a composition of NazO" xSiO2. The same densities for silica and silicate were also assumed. 50
o
5
R
:~E
5odium a~ C~jCOONa
50
x
40
~
1317
- ._
~" "~ ~R
--
40
--
30
--
20
0
u 0
+
.~
x
+
30
o x
20
0J
r,~oup 8:
10
11 i 1000
, 1200
kamlin/NeEl
--
I
It~00
I
FURNACE [EM PERATURE "C
FIG. 2. The extent of sodium reaction to silicates, from the analysis of the cyclone collected ash. Full lines--sodium in synthetic coal as NaC1; dash lines-sodium as CHzCOONa. F - - f i n e silica in synthetic coal S--sulphur added to experiment as SO~ SEP--sodium and silica added in separate particles R--reduced level of Na salt. from the hydrochloric leaching of the coals) would have caused some depression of silicate formation. However, the measured extent of sodium combined as silicate in the "separated" experiments does suggest that at 1200~ C and 1400~ C, the introduction of sodium and silica in separate coal particles did not impede the sodium-silica reactions. This supports a mechanism involving the evaporation of the sodium salts and the subsequent reaction with the remaining SiO2, with little reaction as the sodium is evaporated. The reduction in the extent of silicate formation at 1000~ C relative to that at 1200~ C was greater for the "separated" experiments than for the other experiments. This suggests that all of the sodium may not have been released from the coal prior to the commencement of the silicate formation reaction at 1000~ C.
Silicate Composition, Volumetric Fraction of Silicate, Silicate Thickness and Rate of Silicate Formation
The composition of the silicate was determined by water leaching experiments, in which the sili-
,I-i0 0
i
,
2
3 + 1000 "C x 1200"C 01~00"C
70 A
*/o Na.0
f83=A x
_
70
O
60-
60 ..--:L
o
- Na20 5i02
+
50
--
50
--
40 .~
--
30
--
20
+
4O - 0
x
x + o
o
30 -
50 e
",
x
0
Ne20 2Si 02
20 I lO
0
x
6mup 5 : SiO2/CH3COONa I
I
I
2
+
Na20 SiO2
I
3
--
+ NazO 2SiO 2 x
30
o
+7~/
so _~
---
t0
--
30
__+__
+
x
x
o
20
~
O/o~ 0
4
x
4O
o~
0 x
o
2o
o o
O
lo
6roup 1: SiO2/NoE(
perloa
% Ha20 leached
Leach period
FIG. 3. The sodium distribution in the cyclone collected ash residue as revealed by progressive hot aqueous leaching. Results at furnace temperatures of 1000~ C, +: 1200~ C, X: and 1400~ C, 0. The leach periods were 1 : 0 - 4 hr 2 : 4 - 2 0 hr 3 : 2 0 - 6 4 hr 4:64-156 hr
1318
MINERAL MATrER AND ASH
The thickness of the silicate layer was estimated by dividing the silicate volume by the initial surface area of the coal additives. The surface area was derived from the measured size distributions. The thickness calculation assumes the particles to be spherical, the silicate layer was uniform around each particle and the reaction rate per unit surface area was constant for 'all the additive particles. The values of the volumetric fraction of silicate and silicate thickness so derived are presented in Table III. The reaction rates for the formation of silicates were calculated as the mass of Na20 reacted per unit area of initial silica (or kaolin) per unit of time the silica (or kaolin) was exposed at the surface of the char during combustion. This approach assumes that reaction only proceeds while combustion is occurring as indicated by the experiments which examined the effect of residence time. The assumption that the reaction only occurred with. silica exposed at the char's surface is in accord with theoretical predictions (Lindner). 7 Coalescence of the silica/silicate particles was neglected. If all the silica was available to react throughout the burnout period, the rates would have been 40% lower that those in Table III. The rate of formation of sodium silicate for the modified coal containing 10% kaolin (NCK) is the greatest reported in Table III. Kaolin dehydrates at temperatures below 600~ C which may
lead to fragmentation and the generation of increased surface area for reaction. Electron micrographs of the ash residue and ash sizing indicated that if this fragmentation occurred, the fragments were not ejected from the coal particle, in that finer ash was not found for the experiments using the NCK coal. The rates of char combustion were estimated from a mathematical model which was based on a single particle combustion model. 7 The char reactivity used in the model was determined by conducting separate experiments with a sample which was specially prepared by removing most of the inorganic material in Loy Yang coal by hydrochloric acid leaching and then adding CaSiO3 (rather than silica and kaolin). The ash in this coal had a sharp melting point of 1400~C (as found by measurements of ash fusion temperatures) which was slightly lower than that for pure calcium silicate due to small' amounts of impurities retained after leaching the coal. These experiments involved.combusting the prepared coal in the drop tube furnace and raising the temperature of the furnace until extensive fusion of the product ash was observed. The char's reactivity was then calculated by applying a heat balance on the char particle under the defined conditions. The estimated reactivity of the char (based on the external area) was 160 kg/m z s atm Oz. This then allowed the char combustion tem-
TABLE III (continued on right) Silicate volume: ash volume, silicate thickness and rate of sodium silicate formation Concentration of inorganics in coal (dry basis) Sodium Expt. grp. no.
1 2 3 4 5 6 7 8 9 10 11 12
Coal NCSi NCSi/a NCSi 1/2 NCSi ONSi ONSi 1/20NSi NCK ONSi/f NCSi Si (50%) NC (50%) si (50%) ON (50%)
Sodium
as
as
chloride %
acetate %
1 1 1 0.5 1 1
0.5
Coal feed g/h
Ash collect. pt. mm
10 1o 5
5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 2.7 5.5
2oo/18o 200/180 200/180 200/180 200/180 200/180 200/180 200/180 200/180 Outlet 200/180
5
5.5
200/180
Quartz % 10 10 10 10 10 10 1o
Kaolin %
n
10
1 1
0.5 0.5
S %
Notes: "Silicate volume fraction = silicate volume/ash volume • 100. Where the silicate--Na~O.xSiO2--is estimated from the Na and Si analysis for the cyclone ash, assuming equal densities for silica and silicate. bSilicate thickness.
1319
SODIUM ASH REACTIONS perature to be determined for a known gas temperature and atmospheric conditions. The char burnout times predicted by the mathematical model were 0.31 s, 0.16 s and 0.105 s, at the respective gas temperatures of 1000 ~ C, 1200~ C and 1400 ~ C used in the silicate formation experiments. The corresponding char temperatures varied during combustion, the maximum values estimated at these gas temperatures were 1090 ~ C, 1396~ C and 1537~ C respectively. Table III presents the estimates of the apparent reaction rate of silicate formation. The estimated reaction rates increase substantially with temperature even though the measured extent of reactions were similar. This is due to the shorter combustion times at higher temperatures and negligible silicate formation after combustion completion. The activat-ion energy corresponding to the increase in reaction rate with temperature ranges from 50 to 100 kJ/mole. By comparing the reaction rates with those given by Wibberley, ~ the present rates are an order of magnitude greater. Wibberley's experiments used discrete silica particles (63-90 txm) whereas the present study used finer silica (of mean size 6.5 txm) dispersed within the coal. The different estimates do not appear to be due to the uncertainty in the estimate of surface area of reacting particles in the present experiments, and indicate that dif-
ferent mechanisms were controlling the overall reaction rate in the two studies. The apparent rates reported here appear to have been determined by the mass transfer of sodium containing gases through the gaseous boundary layer surrounding particles or of sodium through the silicate m e l t /
Electron Microscopy and Ash Size Distribution Examination of the product ashes reveals that for the experiments involving the silica additive, the extent of fusion increased the extent of silicate formarion. Figure 4 (from the Group 1 experiments involving NaCI and SiO2) shows the component particles of the ash linked together by fused phased while Fig. 5 (from the Group 5 experiments involving CH3COONa and SiO2) shows the final spherical ash particles formed with the greatest extent of silicate reaction. The ash particles contained voids. This is evidenced in Fig. 6 which shows a polished section of the high temperature (1400~ C) ash of Fig. 5. The voids were probably due to the entrapment of gas during agglomeration process evident in Figs. 4 and 5. The ash from the experiments involving kaolin (Group 8), showed less fusion even though the reacted sodium levels were greater. This may be due
TABLE III, cont. Silicate volume: ash volume, silicate thickness and rate of sodium silicate formation FT1000 ~ C a
FT1200 ~ C a
FT1400 ~ C a
Sil. ~ vol. frac. %
Sil. b thick ~m
React. ~ rate g(Na20)/m 2 s
Sil." vol. frac. %
Sil. b thick ~m
React. c rate g(Na~O)/m ~ s
Sil. a vol. frac. %
Sil. b thick ~m
React. c rate g(Na~O)/m 2 s
4.6 6.8 4.3 4.4 21.0 26.8 10.9 44.8 25.7 5.0 5.5
0.07 0.10 0.06 0.06 0.30 0.38 0.16 0.61 0.17 0.07 0.08
0.16 0.24 0.13 0.11 0.98 0,89 0.48 1.18 0.42 0.13 0.16
7.5 11.7 8.3 4.4 27.9 27.8 11.9 42.7 25.1 7.2 7.5
0.11 0.17 0.12 0.06 0.40 0.40 0.17 0.59 0.16 0.10 0.11
0.42 0.68 0.39 0.22 2.08 2.03 1.10 2.08 1.02 0.41 0.54
11.5 9.2 6.7 5.5 22.9 29.4 12.9 30.6 20.8 7.7 9.6
0.15 0.13 0.10 0.08 0.33 0.42 0.18 0.42 0.14 0.11 0.14
O.65 1.09 0.57 0.51 3.10 3.21 1.41 3.10 1.52 0.64 0.78
16.8
0.24
0.44
20.6
0.29
1.14
17.6
0.25
2.26
Notes, cont. : CRate of formation of sodium silicate (expressed as Na~O) per unit surface area of the original quartz or kaolin added to the coal. aThe estimated maximum particle temperatures are 1090~ C, 1396 ~ C and 1537 ~ C for the three furnace (gas) temperatures.
1320
MINERAL MATFER AND ASH
~iiiill .......
(a)
.....
~
=. . . . . . . .
..................
(b)
FIG. 4. Ash from the combustion or coal NCSi in the Group 1 experiments (1% sodium as NaC1 and 10% silica) at furnace temperatures of (a) 1200~ C, (b) 1400~ C.
Examination of the solid collected on the filter showed it to be comprised of submicron material, between 0.2 and 0.5 Ixm in size, with the predom-
inant elements being Na, CI and S expected from condensed NaCI and Na2SO4 salts. Condensed salts were also found on the ash collected in the cyclone, as evident from the analysis of CI- and SO4--of the initial wash before the higher temperature dissolution experiments. For the experiments involving NaC1, 47-66% of condensible sodium was of submicron size (average 60%) whereas a range of 15-73% was found for the CHsCOONa experiments (average 47%). The higher range found from the CHsCOONa experiments was probably due to most of the sodium reacting to form silicate leaving only a small amount of condensible sodium and therefore greater proportional error. For experiments which included sulphur, it was found that the submicron ash contained higher chlorine levels, indicating that Na2SO4 (rather than NaC1) had preferentially condensed on the ash.
(a)
(b)
to the higher viscosity of sodium aluminium silicates, as indicated by Mills.s The mean size of the resultant ash ranged from 27 Ixm to 40 I~m which may be compared to a mean ash size of 26 l~m estimated from the size distribution of the modified coals and assuming each coal particle formed one ash particle. This is due to the ash beingg incompletely fused or hollow as indicated on Figs. 4 to 6. The particle size distributions indicated that coalescence rather than fragmentation was dominant.
Condensed Sodium Salts
F1G. 5. Ash from the combustion of coal ONSi in the Group 5 experiments (1% sodium as CH3COONa and 10% silica) at furnace temperatures of (a) 1200~ C, (b) 1400~ C.
SODIUM ASH REACTIONS
1321
Alumina based additives such as kaolin, therefore, appear capable of reducing the influence of sodium on fouling during coal combustion as sodium aluminium silicates have a higher viscosity than sodium silicates and the greater extent of reaction between sodium and additive reduces the amount of condensible sodium species.
Acknowledgment The Electricity Trust of South Australia is acknowledged for its support for this research.
REFERENCES
I1#, I
FIG. 6. Backscattered image of a polished crosssection of ash from the combustion of coal ONSi in the Group 5 experiments at a furnace temperature of 1400 ~ C.
Conclusions The experimental results showed that sodium silicates were formed during the combustion of pulverised coal with the presence of chlorine and, to a much lesser extent, sulphur reducing the extent of sodium silicate formation. The silicate forming reactions proceed while combustion occurs, with the agglomeration and coalescence of the silica/silicate particles progressively reducing the surface area for the reaction (as observed in the electron mierographs). The rate of silicate formation increased with temperature but this increase is largely offset by a parallel reduction in combustion time, resulting in only small changes in the extent of formation with temperature. Sodium, as sodium chloride, was found to react far more extensively with kaolin than with silica.
1. BRINSTEAD,K. H. AND KEAR, R. W.: Fuel, 35, 84 (1956). 2. Boow, J.: Fuel, 51, 172 (1972). 3. NEVILLE, M. AND SABOFIM,A. F.: Fuel, 64, 384 (1985). 4. OUNSTED, D. AND SCHOEN, J.: J. Inst. Fuel, 199 (1060). 5. WIBBERLEY, L. J. AND WALL, T. F.: Fuel, 61, 93 (1982). 6. WIBBERLEY,L. J.: Alkali-Ash Reactions and Deposit Formation in Pulverised-Coal-Fired Boilers, Ph.i). thesis, University of Newcastle, Australia, 1980. 7. LINDNEB, E. R.: A Study of Sodium-Ash Reactions During the Combustion of Pulverised Coal, Ph.D. thesis, University of Newcastle, Australia, 1988. 8. MILLS, K. C.: Estimation of Physicochemical Properties of Coal Slags and Ashes in Mineral Matter in Coal Ash, (Ed. K. S. Vorres), ACS Symposium Series 301, p. 195, 1986. 9. BosIo, M. AND LINDNEB, E. R., BORIO, R. W., HEIN, K. R. G.: Utilisation of Fouling/Slagging/Corrosive Low Rank Coals, 3rd Engineering Foundation Conference on Fouling and Slagging, ASME, Colorado, p. 385, 1984. 10. HONEA, F. T.: Studies of Ash Fouling Potential and Deposit Strength, Conference on Ash Deposits and Corrosion, ASME, New England College, p. 117, 1987. 11. HIGGINS, R. S. AND MORLEY, W. J.: An Evaluation of Loy Yang Coal as a Fuel for Power Station Boilers, SECV, Scientific Division, Report No. 241, 1971.
SODIUM ASH REACTIONS D U R I N G C O M B U S T I O N OF PULVERISED
COAL
ERIC R. LINDNER The Electricity Trust of South Australia AND
TERRY F. WALL Department of Chemical Engineering Unit~ersity of Newcastle, Australia
Reactions of sodium with mineral silicates contribute to the formation of fireside deposits during the combustion of some high sodium coals. A systematic experimental study of the reactions between sodium with inherent silica and kaolin during pulverised coal combustion was undertaken in order to examine the mechanisms of the reactions. The experiments were conducted in a drop tube furnace with coal samples which had been specially prepared to control the concentrations, the size and the distribution of the inorganic constituents, which included silica, kaolin, sodium chloride, sodium acetate, and gaseous SOz to simulate organic sulfur. The rate of formation of sodium silicate was found to increase with temperature but the extent of formation was limited by the progressive agglomeration and coalescence of the ash which reduced the surface area available for reaction. The extent of interaction between the sodimn and silica/silicates was reduced by the presence of chlorine and sulphur and this effect was greater with kaolin than with quartz. At gas temperatures of 1200-1400 ~ C, sodium was volatilised from the coal before significant reactions occurred with the silica. At 1000~ C the sodium was not fully released prior to the commencement of the silicate formation reaction. Introduction Sodium in coal is commonly associated with the tormation of fireside deposits in boilers. Experimental evidence has shown that s~xlium species can react with silica and aluminium silicates under conditions which, to va~'ing degrees, approach those that occur in pulveriscd coal combustion systems. (Brinstead and Kear, ! Boow, "z Neville and Sarofim, :~ Ounsted and Schocn4 and Wibberley and Wall. 5 The sodium silicates and alumininm silicates formed by these reactions have markedly lower melting temperatures than the original silica and silicate minerals. The sodium silicates and to lesser extent the sodium aluminium silicates tend to torm glasses which can supercool during deposition. By supercooling, the low viscosity of the glass phases on fly ash are prohmged, which enables impacting p6articles to stick to the tube/deposit. Wibbcrlcy, for example, has calculated that only a thin (0.01-0.1 IX) glaze of low viscosity sodium silicate is required to retain fly ash particles impinging onto a boiler tube. The presence of low melting temperature glassy compounds can also facilitate sintering within de-
posits on walls. In addition to forming sodium silicates, the reactions of sodium with silica and aluminium silicates also influence the degree of fouling severi~' by reducing the sodium that remains in the vapour state in the flame and the hot furnace gases. This vapourised sodium can condense as a liquid on fly ash or on and within deposits thus contributing to deposit formation and consolidation. Previous investigations into the reactions of sodium with silica and aluminium silicates have generally been performed by crucible experiments where conditions are significantly different to those in pc fired boilers. Wibberley and Walls studied the reactions of sodium chloride vapour and sodium acetate vapour with silica particles under rapid heating conditions but these reactions were independent of coal combustion. In some high sodium coMs (NazO/ash > 0.03) fine silica and kaolin is found within the coal particles (when thev are called inherent). In response to the above considerations the present study examined: 9 the reactions and reaction mechanisms of sodium with inherent silica and kaolin during coal com-
1313
1314
MINERAL MATI'ER AND ASH
bustion and the subsequent cooling of the combustion gases. 9 the dependence of these reactions on the form of the sodium in the coal, the presence of sulphur oxides and the temperature of combustion.
Experimental The interpretation of the complex coal ash reactions that occur during combustion is often difficult due to the complex chemistry involved. In order to reduce this complexity the following strategies were adopted for the experimental programme: 9 s i m p l i f y the reaction system by combusting a spe-
cially prepared coal in which the concentrations, size and distribution of the inorganic constituents were controlled. 9 c o m b u s t the modified coals under controlled and well defined conditions which simulate those in p c flames. The modified coals were prepared from a Loy Yang brown coal (Victoria, Australia) sample which contained low levels of ash (1.1% dry basis) and sulphur (0.3% dry basis). The ash was reduced to 0.5% dry basis by acid leaching the coal to remove the
ion exchangeable constituents which comprised most of the sodium, potassium, calcium and magnesium and some of the aluminium. After water washing the leached coal, known quantities of inorganic constituents were added to it and then this mixture was made into a thixotropic slurry by beifig finely ground in a household blender. Sodium was added as a solution of sodium chloride or sodium acetate (CHaCOONa). Silica and kaolin was added as discrete size fractions both having a mass mean size of 6.5 p,m, with 90% < 14 Ixm, 10% < 2.5 Ixm. A fine silica having a mass mean size of 2.5 Izm (90% < 6 Ixm, 10% < 1 p,m) was also used in a few experiments. The slurry was dried, forming lumps which were then pulverised and sieved to obtain the 63-90 Ixm size fraction used in the experiments. Table I gives the ash analyses of the modified coals. Electronmicrographs of the sectioned modified coals revealed that the minerals were dispersed uniformaly within the resultant particles, thereby approximating inherent ash, and were not ground to a smaller size during crushing of the lumps. The micrographs revealed that the particles are porous with macropores several microns in width leading from their external sections to the inner minerals. The temperature attained by a burning coal particle depends on its size, experiments using
TABLE I Ash analysis of coals, Loy Yang brown coal and comparison coals. The designation refers to the following additives (quoted as % of dry coal): NC--2.5% NaCl: 1/2 NC--1.2% NaCI: ON--3.5% CHzCOONa: 1/2 ON--1.7% CHzCOONa: Si--10% SiO2: K--10% Kaolin Modified coal designation
SiOz %
A1203 %
Na20 %
NCSi 1/2 NCSi ONSi 1/20NSi ONSi/f a Si NCSi/ab NCK Loy Yang~ Lochiel a North Dakota~ Morwell
~.1 87.3 82.0 ~.4 82.9 95.3 ~.3 45.6 25.0 21.6 27.1 8.2
2.7 3.2 2.2 2.4 2.1 2.8 0.7 36.6 42.9 4.2 11.6 0.5
12.3 9.0 13.5 6.1 12.5 <0.1 13.2 14.0 4.9 13.1 9.3 0.8
Ash analysis K20 % 0.1 0.1 0.2 0.2 0.2 0.1 0.1 0.6 0.3 0.4 0.8 0.3
MgO %
CaO %
Fe203 %
0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.6 6.9 10.7 6.2 19.6
<0.1 0.3 <0.1 <0.1 <0.1 <0.1 <0.1 0.5 2.7 11.6 21.6 21.2
0.5 0.6 0.6 0.6 0.3 0.6 0.6 1.6 6.6 4.8 5.1 5.1
Notes: a"Fine" quartz added instead of "regular" sized quartz used in the other coals. bprepared coal had a lower alumina content than the other coals achieved by hot hydrochloric leaching of the raw coal. CLoy Yang brown coal (Victoria) that was used for the preparation of the synthetic coals. aSouth Australian brown coal--(Bosio et. al.9) eCoal sample GH-S from Glenharold Mine--(Honeal~ fMorwell (Vietoria)--(Higgins and Morley ll)
SODIUM ASH REACTIONS a size fraction rather than a single range therefore simplify the interpretation of observed phenomena such as ash sintering which are influenced by particle temperature. Examination of the size fraction with an optical microscope indicated the particles were discrete rather than agglomerates. The prepared coals were fed at a controlled rate into a drop tube furnace and combusted in 20% excess air. The 45 mm internal diameter, ceramic core tube of the furnace was heated externally by a cylindrical graphite element. A 150 mm hot zone of the core tube was maintained at a relatively uniform temperature. Coal heating rates in the furnace were estimated at 104-105 ~ with residence times of about 1 second, The solid ash products were extracted with a probe which rapidly quenched and diluted the products with air (with a volume of twenty times the furnace flow) to about 200 ~ C. The supra-micron ash particles were collected in a cyclone and the sub-micron particles were retained in a downstream fine filter, as shown on Fig. 1. The parametric programme of experiments undertaken is given in Table II. Experiments were conducted at furnace wall temperatures of 1000~ C, 1200~ C and 1400~ C. For every change to an experimental parameter other than temperature, a group of 3 experiments were performed at the 3 nominated furnace temperatures. The experimental Groups 1-8 (Table II) were conducted to explore the effect of the chemistry of the system, i.e. the form and concentration of the sodium, the presence of sulphur dioxide and the level of aluminium with the silica. The remaining experimental groups were deCombustion
Balance air COLLECTION PROBE O,uench air
--
FILTER ,
.~lt"q
--Suction
1315
signed to examine the reaction mechanism and the rate controlling step or steps. In Group 9, the effect of the silica particle size was investigated while in Group 10 the influence of residence time was examined by firing with a reduced coal rate and by collecting the ash at the base of the furnace. The two final experimental groups of Table II, called the separated experiments, were to determine the extent that the sodium reacted as it evaporated from the coal. For all other groups, the sodium salt and silica additives were in the same particle so that some reaction could occur before or as the sodium evaporated. For the separated experiments the additives were in different particle fractions, so that reaction could only occur after the sodium had evaporated from one fraction and then diffused through the gas phase to the particle from the other fraction which contained silica. The coarse cyclone collected ash was digested in demineralised water at room temperature for an initial period of half an hour to remove condensed salts. The residue was then digested at 120" C for fiu'ther successive periods of 4, 16, 44 and 92 hours. The leachate from each digestion as analysed separately for Si (molybdate blue method), AI, Na (AA spectrophotometry) and SO4 and CI (ion chromatography). After the final digestion the residue was analysed for the ash constituents Fe203, A1203, SiO2, MgO, Ca*, K20 and Na20. The sodium combined in the silicates was calculated as follows: Na~O (silicate = Na20 (total leachate) + Na20 (residue from final leaching) - NazO (in non-silicate forms in leachate, i.e. the molar equivalent of Na corresponding to CI-, SO4--OH- and CO3--but expressed as Na20). The submicron ash collected in the filter was predominantly condensed sodium salts. This ultra fine ash was dissolved in water and analysed for Na, K, Ca, Mg (AA), CI-, SO4--(ion chromatography), OH- and CO3--(pH and ion balance). Considering the mass balance for the experiments reported here, an average of 81% of the silica (or kaolin) was collected (standard deviation 9%) and 68% of the sodium (standard deviation 10%). Some fine ash and condensibles (NaC1 and Na2SO4) were observed to collect on the furnace walls and the water cooled feeder probe, probably by a thermophoresis mechanism which is an inevitable consequence of the presence of coal surfaces. Repeat experiments gave measured silicate formation levels within 10%.
Fine ash collection
',
Coarse ash collection
FIc. 1. The arrangement for ash collection.
The Extent of Silicate Formation The extent of silicate formation was estimated from the analysed amounts of NazO as silicate normal-
1316
MINERAL MATrER AND ASH TABLE II Details of the combustion experiments I
I
l
I
Concentration of added inorganics in coal (dry basis) Experiment group no.
1
2 3 4 5 6 7 8 9 1O 11~ 12e
Modified coal designation
Sodium as chloride %
NCSi NCSi/a NCSi 1/2 NCSi ONSi ONSi 1/20NSi NCK NCSi/f NCSi NC(50%) Si(50%)
1 1 1 0.5
Sodium as acetate %
Quartz %
1 1 0.5
10 10 10 10 10 10 10
1 1 0.5b
ON(5O%) Si(50%)
0.5
10 10 5
S~ %
--
3
--
3
10 1
Notes: a% S given is equivalent to the sulphur added bFor experiments, Groups 11 and 12, the coal CThe quench probe was inserted 200 and 180 1400~ C; for Group 10, the probe was located at
Kaolin %
Coal feed rate g/h 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 2.7 5.5 5.5
as sulphur dioxide. used was a 50:50 mixture of the two coals designated. mm for furnace temperatures of 1000~ C, 1200~ C and the exit of the furnace to increase the residence time.
ised with respect to SiO2 and Al2Oa. These values are given on Fig. 2 for the experimental groups. The effect of the form of sodium added (as chloride or acetate) was marked, with the mass of silicate formed being from three to five times greater where sodium acetate was used.
The addition of sulphur (as S02 to a level equivalent of 3% of S in the dry coal) generally decreased silicate formation but only to a small extent (reduction <19%). A reduction in the level of sodium as acetate decreased silicate formation approximately in proportion with the sodium reduction. A reduction in the sodium chloride content of coal produced a less than proportionate reduction in the silicate formation. The effect of the silica particle size was not significant in the range investigated. Although the finer silica used had a surface area approximately 3 times greater than the silica used in the other experiments there was little change in the degree of silicate formation (compare Groups 5 and 9). The effect of the addition of aluminium into the system, with sodium as sodium chloride, was dramatic. The experiments involving kaolin with NaC1, Group 8, resulted in the greatest extent of silicate formation. In these experiments the level of reacted sodium was five to eight times that found in the experiments with the same level of sodium chloride but silica instead of kaolin (Group 1).
The variation in the residence time of ash particles, at temperatures where silicates were favoured to form, did not cause a significant change in the degree of silicate formation. The residence time of the ash particles, above 1000~C and subsequent to combustion, were estimated to be between 1.7 and 3 times longer in the Group 10 experiments than those in Group 1 experiments but there was insignificant difference in the degree of reaction. It is evident that negligible reaction between sodium and silica proceeded after combustion was completed. This effect was probably due to the marked reduction in the silica/silicate surface area by the coalescence and agglomeration of the ash as combustion proceeded (see Section 4). The effect of adding the sodium and silica in separate particles (i.e. the "separated" experiments) is established by comparing the results of experimental Groups 1, 4 and 11 for the NaC1 additive and Groups 5, 7 and 12 experiments (for the CH3COONa additive). The loading of sodium and silica in the individual coals mixed for the "separated" experiments was the same as the other experiments, both the level of sodium and the sodium:silica ratio of the mixed coals did not reproduce together those of the other experiments. This makes comparison difficult. The higher chlorine/sodium ratios in experimental Groups 1 and 4 than in the Group 11 experiments (due to the retention of some chlorine
SODIUM ASH REACTIONS F EXperimentat group number 12
u
SOdium as NaCI
11 ~
;
F
a
d
d
i
t
~ i
Kaolin e
v
. ?
5E
EP
cate was progressively dissolved as the leaching period increased. Illustrative results are given on Fig. 3 for experiments involving NaC1 and CH3COONa with silica and NaCI with kaolin. The volumetric fraction of the silicate in the ash was calculated from the total sodium and silicon analysed and assuming a composition of NazO" xSiO2. The same densities for silica and silicate were also assumed. 50
o
5
R
:~E
5odium a~ C~jCOONa
50
x
40
~
1317
- ._
~" "~ ~R
--
40
--
30
--
20
0
u 0
+
.~
x
+
30
o x
20
0J
r,~oup 8:
10
11 i 1000
, 1200
kamlin/NeEl
--
I
It~00
I
FURNACE [EM PERATURE "C
FIG. 2. The extent of sodium reaction to silicates, from the analysis of the cyclone collected ash. Full lines--sodium in synthetic coal as NaC1; dash lines-sodium as CHzCOONa. F - - f i n e silica in synthetic coal S--sulphur added to experiment as SO~ SEP--sodium and silica added in separate particles R--reduced level of Na salt. from the hydrochloric leaching of the coals) would have caused some depression of silicate formation. However, the measured extent of sodium combined as silicate in the "separated" experiments does suggest that at 1200~ C and 1400~ C, the introduction of sodium and silica in separate coal particles did not impede the sodium-silica reactions. This supports a mechanism involving the evaporation of the sodium salts and the subsequent reaction with the remaining SiO2, with little reaction as the sodium is evaporated. The reduction in the extent of silicate formation at 1000~ C relative to that at 1200~ C was greater for the "separated" experiments than for the other experiments. This suggests that all of the sodium may not have been released from the coal prior to the commencement of the silicate formation reaction at 1000~ C.
Silicate Composition, Volumetric Fraction of Silicate, Silicate Thickness and Rate of Silicate Formation
The composition of the silicate was determined by water leaching experiments, in which the sili-
,I-i0 0
i
,
2
3 + 1000 "C x 1200"C 01~00"C
70 A
*/o Na.0
f83=A x
_
70
O
60-
60 ..--:L
o
- Na20 5i02
+
50
--
50
--
40 .~
--
30
--
20
+
4O - 0
x
x + o
o
30 -
50 e
",
x
0
Ne20 2Si 02
20 I lO
0
x
6mup 5 : SiO2/CH3COONa I
I
I
2
+
Na20 SiO2
I
3
--
+ NazO 2SiO 2 x
30
o
+7~/
so _~
---
t0
--
30
__+__
+
x
x
o
20
~
O/o~ 0
4
x
4O
o~
0 x
o
2o
o o
O
lo
6roup 1: SiO2/NoE(
perloa
% Ha20 leached
Leach period
FIG. 3. The sodium distribution in the cyclone collected ash residue as revealed by progressive hot aqueous leaching. Results at furnace temperatures of 1000~ C, +: 1200~ C, X: and 1400~ C, 0. The leach periods were 1 : 0 - 4 hr 2 : 4 - 2 0 hr 3 : 2 0 - 6 4 hr 4:64-156 hr
1318
MINERAL MATrER AND ASH
The thickness of the silicate layer was estimated by dividing the silicate volume by the initial surface area of the coal additives. The surface area was derived from the measured size distributions. The thickness calculation assumes the particles to be spherical, the silicate layer was uniform around each particle and the reaction rate per unit surface area was constant for 'all the additive particles. The values of the volumetric fraction of silicate and silicate thickness so derived are presented in Table III. The reaction rates for the formation of silicates were calculated as the mass of Na20 reacted per unit area of initial silica (or kaolin) per unit of time the silica (or kaolin) was exposed at the surface of the char during combustion. This approach assumes that reaction only proceeds while combustion is occurring as indicated by the experiments which examined the effect of residence time. The assumption that the reaction only occurred with. silica exposed at the char's surface is in accord with theoretical predictions (Lindner). 7 Coalescence of the silica/silicate particles was neglected. If all the silica was available to react throughout the burnout period, the rates would have been 40% lower that those in Table III. The rate of formation of sodium silicate for the modified coal containing 10% kaolin (NCK) is the greatest reported in Table III. Kaolin dehydrates at temperatures below 600~ C which may
lead to fragmentation and the generation of increased surface area for reaction. Electron micrographs of the ash residue and ash sizing indicated that if this fragmentation occurred, the fragments were not ejected from the coal particle, in that finer ash was not found for the experiments using the NCK coal. The rates of char combustion were estimated from a mathematical model which was based on a single particle combustion model. 7 The char reactivity used in the model was determined by conducting separate experiments with a sample which was specially prepared by removing most of the inorganic material in Loy Yang coal by hydrochloric acid leaching and then adding CaSiO3 (rather than silica and kaolin). The ash in this coal had a sharp melting point of 1400~C (as found by measurements of ash fusion temperatures) which was slightly lower than that for pure calcium silicate due to small' amounts of impurities retained after leaching the coal. These experiments involved.combusting the prepared coal in the drop tube furnace and raising the temperature of the furnace until extensive fusion of the product ash was observed. The char's reactivity was then calculated by applying a heat balance on the char particle under the defined conditions. The estimated reactivity of the char (based on the external area) was 160 kg/m z s atm Oz. This then allowed the char combustion tem-
TABLE III (continued on right) Silicate volume: ash volume, silicate thickness and rate of sodium silicate formation Concentration of inorganics in coal (dry basis) Sodium Expt. grp. no.
1 2 3 4 5 6 7 8 9 10 11 12
Coal NCSi NCSi/a NCSi 1/2 NCSi ONSi ONSi 1/20NSi NCK ONSi/f NCSi Si (50%) NC (50%) si (50%) ON (50%)
Sodium
as
as
chloride %
acetate %
1 1 1 0.5 1 1
0.5
Coal feed g/h
Ash collect. pt. mm
10 1o 5
5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 2.7 5.5
2oo/18o 200/180 200/180 200/180 200/180 200/180 200/180 200/180 200/180 Outlet 200/180
5
5.5
200/180
Quartz % 10 10 10 10 10 10 1o
Kaolin %
n
10
1 1
0.5 0.5
S %
Notes: "Silicate volume fraction = silicate volume/ash volume • 100. Where the silicate--Na~O.xSiO2--is estimated from the Na and Si analysis for the cyclone ash, assuming equal densities for silica and silicate. bSilicate thickness.
1319
SODIUM ASH REACTIONS perature to be determined for a known gas temperature and atmospheric conditions. The char burnout times predicted by the mathematical model were 0.31 s, 0.16 s and 0.105 s, at the respective gas temperatures of 1000 ~ C, 1200~ C and 1400 ~ C used in the silicate formation experiments. The corresponding char temperatures varied during combustion, the maximum values estimated at these gas temperatures were 1090 ~ C, 1396~ C and 1537~ C respectively. Table III presents the estimates of the apparent reaction rate of silicate formation. The estimated reaction rates increase substantially with temperature even though the measured extent of reactions were similar. This is due to the shorter combustion times at higher temperatures and negligible silicate formation after combustion completion. The activat-ion energy corresponding to the increase in reaction rate with temperature ranges from 50 to 100 kJ/mole. By comparing the reaction rates with those given by Wibberley, ~ the present rates are an order of magnitude greater. Wibberley's experiments used discrete silica particles (63-90 txm) whereas the present study used finer silica (of mean size 6.5 txm) dispersed within the coal. The different estimates do not appear to be due to the uncertainty in the estimate of surface area of reacting particles in the present experiments, and indicate that dif-
ferent mechanisms were controlling the overall reaction rate in the two studies. The apparent rates reported here appear to have been determined by the mass transfer of sodium containing gases through the gaseous boundary layer surrounding particles or of sodium through the silicate m e l t /
Electron Microscopy and Ash Size Distribution Examination of the product ashes reveals that for the experiments involving the silica additive, the extent of fusion increased the extent of silicate formarion. Figure 4 (from the Group 1 experiments involving NaCI and SiO2) shows the component particles of the ash linked together by fused phased while Fig. 5 (from the Group 5 experiments involving CH3COONa and SiO2) shows the final spherical ash particles formed with the greatest extent of silicate reaction. The ash particles contained voids. This is evidenced in Fig. 6 which shows a polished section of the high temperature (1400~ C) ash of Fig. 5. The voids were probably due to the entrapment of gas during agglomeration process evident in Figs. 4 and 5. The ash from the experiments involving kaolin (Group 8), showed less fusion even though the reacted sodium levels were greater. This may be due
TABLE III, cont. Silicate volume: ash volume, silicate thickness and rate of sodium silicate formation FT1000 ~ C a
FT1200 ~ C a
FT1400 ~ C a
Sil. ~ vol. frac. %
Sil. b thick ~m
React. ~ rate g(Na20)/m 2 s
Sil." vol. frac. %
Sil. b thick ~m
React. c rate g(Na~O)/m ~ s
Sil. a vol. frac. %
Sil. b thick ~m
React. c rate g(Na~O)/m 2 s
4.6 6.8 4.3 4.4 21.0 26.8 10.9 44.8 25.7 5.0 5.5
0.07 0.10 0.06 0.06 0.30 0.38 0.16 0.61 0.17 0.07 0.08
0.16 0.24 0.13 0.11 0.98 0,89 0.48 1.18 0.42 0.13 0.16
7.5 11.7 8.3 4.4 27.9 27.8 11.9 42.7 25.1 7.2 7.5
0.11 0.17 0.12 0.06 0.40 0.40 0.17 0.59 0.16 0.10 0.11
0.42 0.68 0.39 0.22 2.08 2.03 1.10 2.08 1.02 0.41 0.54
11.5 9.2 6.7 5.5 22.9 29.4 12.9 30.6 20.8 7.7 9.6
0.15 0.13 0.10 0.08 0.33 0.42 0.18 0.42 0.14 0.11 0.14
O.65 1.09 0.57 0.51 3.10 3.21 1.41 3.10 1.52 0.64 0.78
16.8
0.24
0.44
20.6
0.29
1.14
17.6
0.25
2.26
Notes, cont. : CRate of formation of sodium silicate (expressed as Na~O) per unit surface area of the original quartz or kaolin added to the coal. aThe estimated maximum particle temperatures are 1090~ C, 1396 ~ C and 1537 ~ C for the three furnace (gas) temperatures.
1320
MINERAL MATFER AND ASH
~iiiill .......
(a)
.....
~
=. . . . . . . .
..................
(b)
FIG. 4. Ash from the combustion or coal NCSi in the Group 1 experiments (1% sodium as NaC1 and 10% silica) at furnace temperatures of (a) 1200~ C, (b) 1400~ C.
Examination of the solid collected on the filter showed it to be comprised of submicron material, between 0.2 and 0.5 Ixm in size, with the predom-
inant elements being Na, CI and S expected from condensed NaCI and Na2SO4 salts. Condensed salts were also found on the ash collected in the cyclone, as evident from the analysis of CI- and SO4--of the initial wash before the higher temperature dissolution experiments. For the experiments involving NaC1, 47-66% of condensible sodium was of submicron size (average 60%) whereas a range of 15-73% was found for the CHsCOONa experiments (average 47%). The higher range found from the CHsCOONa experiments was probably due to most of the sodium reacting to form silicate leaving only a small amount of condensible sodium and therefore greater proportional error. For experiments which included sulphur, it was found that the submicron ash contained higher chlorine levels, indicating that Na2SO4 (rather than NaC1) had preferentially condensed on the ash.
(a)
(b)
to the higher viscosity of sodium aluminium silicates, as indicated by Mills.s The mean size of the resultant ash ranged from 27 Ixm to 40 I~m which may be compared to a mean ash size of 26 l~m estimated from the size distribution of the modified coals and assuming each coal particle formed one ash particle. This is due to the ash beingg incompletely fused or hollow as indicated on Figs. 4 to 6. The particle size distributions indicated that coalescence rather than fragmentation was dominant.
Condensed Sodium Salts
F1G. 5. Ash from the combustion of coal ONSi in the Group 5 experiments (1% sodium as CH3COONa and 10% silica) at furnace temperatures of (a) 1200~ C, (b) 1400~ C.
SODIUM ASH REACTIONS
1321
Alumina based additives such as kaolin, therefore, appear capable of reducing the influence of sodium on fouling during coal combustion as sodium aluminium silicates have a higher viscosity than sodium silicates and the greater extent of reaction between sodium and additive reduces the amount of condensible sodium species.
Acknowledgment The Electricity Trust of South Australia is acknowledged for its support for this research.
REFERENCES
I1#, I
FIG. 6. Backscattered image of a polished crosssection of ash from the combustion of coal ONSi in the Group 5 experiments at a furnace temperature of 1400 ~ C.
Conclusions The experimental results showed that sodium silicates were formed during the combustion of pulverised coal with the presence of chlorine and, to a much lesser extent, sulphur reducing the extent of sodium silicate formation. The silicate forming reactions proceed while combustion occurs, with the agglomeration and coalescence of the silica/silicate particles progressively reducing the surface area for the reaction (as observed in the electron mierographs). The rate of silicate formation increased with temperature but this increase is largely offset by a parallel reduction in combustion time, resulting in only small changes in the extent of formation with temperature. Sodium, as sodium chloride, was found to react far more extensively with kaolin than with silica.
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