Energy &Fuels 1994,8, 1197-1207
1197
Effect of the Occurrence and Composition of Silicate and Aluminosilicate Compounds on Ash Formation in Pilot-Scale Combustion of Pulverized Coal and Coal-Water Slurry Fuels Sharon Falcone Miller? and Harold H. Schobert* Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received September 13, 1993. Revised Manuscript Received July 19, 1994@
Beulah (North Dakota) lignite and Elk Creek (West Virginia) high-volatile A bituminous coal were burned in both pulverized coal and coal-water slurry fuel forms to study the effect of the modes of occurrence and composition of aluminosilicates and silicates on the particle size distribution and composition of ash. The dominant mechanism for ash formation in the Beulah pulverized coal was fragmentation of mineral particles, such as quartz and pyrite, during combustion. By contrast, the main mechanism for determining the Beulah CWSF ash particle size distribution was coalescence and agglomeration of inherent aluminosilicates and silicates during combustion. The particle size distribution of the inorganic phases formed during combustion of the Elk Creek fuels is slightly coarser than the original mineral matter, due to coalescence of inherent aluminosilicates and silicates during combustion. The slurry ash is slightly coarser than the pulverized coal ash as a result of the larger agglomerate formed on atomization of the Elk Creek slurry. The larger slurry agglomerate increases the number of mineral particles in proximity to one another and increases the time required for char burnout. In turn, the increased char burnout time increases the time interval during which mineral particles can coalesce, as evident by changes in the particle size distribution and composition of silicates and aluminosilicates. The result is enhanced coalescence and agglomeration of the mineral particles in the Elk Creek slurry compared to the pulverized coal. The results emphasize the importance of determining the size distribution and occurrence of inorganics in a fuel and the effect of changing either of these two parameters for a particular mineral group as a result of fuel form.
Introduction The formation of ash during combustion of coal is a complex process affected by many characteristics of the combustion system and of the coal itself. The present authors have investigated the effect that the form in which a fuel is fired-that is, pulverized coal or coalwater slurry-has on the mechanisms responsible for the size and composition of the char and ash produced during combustion and responsible for establishing the final particle size distribution (PSD) of the ash. In previous papers we have presented results addressing some aspects of this problem: the relationship of the fuel particle or droplet size distribution t o the PSD of the char at various locations within a combustor,l the effect of particle size distribution of the mineral matter in the coal on the PSD of ash and char,2 and the effect of the occurrence and composition of iron compounds on ash formation, composition, and PSD.3 The investigation has focused on a lignite and hvA bituminous coal burned in both pulverized and slurry forms in a pilotscale (316 MJk)down-fired combustor. In the present + Present address: Energy and Fuels Research Center, The Pennsylvania State University, University Park, PA 16802. Abstract published in Advance ACS Abstracts, September1, 1994. (1) Miller, S. Falcone; Schobert, H. H. Energy Fuels 1993, 7,520. (2) Miller, S. Falcone; Schobert, H. H. Energy Fuels 1993, 7,532. (3) Miller, S. Falcone; Schobert, H. H. Energy Fuels 1993, 7,1030. @
0887-0624/94/2508-1197$04.50/0
paper we extend our previous work to report on the role of the silicate and aluminosilicate minerals in ash formation. The importance of silicate and aluminosilicate minerals is that they act as the structural framework for many of the mixed-silicate species found in ash and slag deposits in industrial and utility-scale combustors. Silicate and aluminosilicate minerals tend to have higher melting points than other mineral phases, and, once molten, generally have higher viscosities and surface tensions than other molten inorganic phases. However, the physical characteristics are altered when the silicates or aluminosilicates undergo reaction during combustion;in many cases, the melting point, viscosity, and surface tension of an aluminosilicate are reduced when it incorporates alkalis, alkaline earth elements, or i r ~ n . ~When - ~ this occurs, the mixed-silicate phase is likely to act as a cement, bridging or coalescing with other mineral particles. This process contributes to the increased size of the inorganic phases in the ash as compared to the original coal mineral matter. Aluminosilicate clay minerals are the most abundant mineral species in coal. Clays and quartz account for 60-90% of the mineral matter in coal, depending on (4) Watt, J. D. BCURA Literature Survey; National Coal Board: Leatherhead, UK, 1969. (5) Falcone, S. K. US.Department of Energy Report, DE-FC2183FE60181, 1986. (6) Falcone, S. K. US. Department of Energy Report, DE-FC2185MC10637, 1987.
0 1994 American Chemical Society
1198 Energy & Fuels, Vol. 8, No. 6,1994
Miller and Schobert
structure rather than vaporizing. As evidence of this, rank.7 Most clays form an amorphous or glassy phase the concentration of sodium is greatest in small particles at 1223-1403 K. In the temperature range 1223-1323 in conditions where sodium vaporization is important, K kaolinite forms mullite (Al6Si3015), which in turn but the concentration of sodium is greatest in larger melts at 2071 K. If other elements, e.g., calcium, particles if volatilization of sodium is precluded by sodium, or iron, are incorporated, aluminosilicates havincorporation of sodium cations into inherent silicate ing melting points lower than mullite are formed. particles.16 The incorporation of sodium reduces the Pulverized-coal flame temperatures range from 1700 to melting point of the original quartz particle. With the 1850 K, therefore, free clay particles are mostly molten. formation of lower melting point phases, the mixed Inherent clay minerals (i.e., mineral matter intimately silicate is more likely to contribute to coalescence or mixed with the coal such that its thermal history during agglomeration of mineral particles within the char combustion is determined by the combustion behavior during devolatilization and char burnout. of the coal particle) also pass into a molten stage during char burnout and form spherical droplets on the char Extraneous quartz is fairly inert within the gas surface. stream in the absence of volatilized alkalis and alkaline The clay minerals in the coal provide the raw material earth elements. However, it can be highly reactive in for the formation of the silicate and aluminosilicate the flame in the presence of these volatilized species. mineral species which contribute to fouling and slagging In the presence of other mineral matter and carbon, the deposits. In addition, the structure and composition of temperature at which S i 0 begins to volatilize is the clays often dictate the type of mineral transformareduced.17 The reduction in the temperature at which tions which occur under combustion conditions. For Si0 begins to volatilize is greatest in the presence of example, illite in char melts rapidly and coalesces with pyrite or metallic iron. The degree of volatilization of other inclusions within the char.8 During melting, extraneous quartz should be minimal in pulverized coal potassium is retained in the melt phase of the clay. combustion since the concentrations of carbon monoxide Kaolinite is slower to coalesce than potassium-containand hydrogen, necessary for the reduction of Si02 to ing illite.g Melts having compositions similar to kaolinSiO, in the gas stream are ite tend to maintain higher fluid viscosities than illiteThe sequence of events leading to the combustion of derived melt phases. a coal-water slurry fuel (CWSF) differs from the The clays are the structural precursor for the formacombustion of a pulverized coal mainly in the stages up tion of the feldspathoids, low-melting-temperature t o the formation of char. The obvious difference is the aluminosilicates which are silica-deficient.1° MInerals presence of water, about 30-50%, by which the CWSF in the feldspathoid group which have been identified in is introduced into the combustor. Atomization, formacoal ash include nepheline and the melilite g r ~ u p . ' l - ~ ~ tion of coal-water droplets, and evaporation affect the These highly alkaline silicate mineral phases are found ignition and char size and morphology. The latter mostly in the ash of low-rank coals. Upon heating, these factors determine the burnout rate, efficiency of carbon new phases are formed from the interstitial infilling of conversion, and ash particle size and composition. the collapsed, and then expanded, clay structure by Formation of a CWSF droplet containing several alkalis and alkaline earth elements.ll The most comindividual coal particles may alter the nature of the mon calcium and sodium aluminosilicates are the mineral matter relative to the parent coal. Once char melilites. The melilite solid solution series have the is formed and char burnout is initiated, the processes general formula (Ca,Na,K)~(Mg,Fe+~,Fe+~,A1)(Si,Al)z07. which follow should, in principle, be similar for pulverFinely disseminated quartz particles may interact ized coal and CWSF. with other mineral matter particles and organically However, differences in the gaseous environment and bound cations, forming silicon-rich mineral phases. relative durations of reducing and oxidizing environWith increasing temperature, quartz transforms into a ments can affect the inorganic transformations which series of high-temperature polymorphs. These transcould occur, particularly if agglomerated coal particles formations are displacive polymorphic reactions in form during atomization of the CWSF. Additionally, the which slight displacement of atoms and adjustment of way in which mineral matter is incorporated, inherent bond angles occur.15 The higher temperature polyor extraneous, can be different for a CWSF char and morphs have a more open crystal structure, making it its original parent coal. Agglomerates formed during easier to incorporate alkalis and alkaline earth elements atomization may incorporate individual coal particles within the structure. Cations may diffuse into the containing inherent mineral matter as well as particles of extraneous mineral matter. Hence the once-extrane(7) Raask, E. Mineral Impurities i n Coal Combustion; Hemisphere: ous mineral matter is now in a different relationship t o Washington, DC, 1985. (8) Srinivasachar. S.: Helble, J . J.: Ham, D. 0.; Domazetis, G. Prog. the coal particles than it was in the parent fuel. The Energy Combust. Sci. 1990,16,303. thermal history of the mineral matter and the local (9) Helble, J. J.; Srinvasachar, S.; Katz, C. B.; Boni, A. A. P r e p . oxidizing and reducing atmospheres surrounding the Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,34,383. (10) Grim, R. E., Clay Mineralogy; McGraw-Hill: New York, 1968; mineral particles may be different for CWSF and Chapter 9. pulverized coal combustion. Greater proximity of the (11)Falcone, S. K.; Schobert, H. H. In Mineral Matter i n A s h and Coal; Vorres, K. S., Ed.; American Chemical Society: Washington, DC, inorganics within the CWSF agglomerate increases the 1986; Chapter 9. probability that mineral particles will agglomerate or (12) Tangsathitkulchai, M. Ph.D. Dissertation, The Pennsylvania coalesce during char burnout. Thus it is important to State University, University Park, PA, 1986. (13) Benson, S. A. Ph.D. Dissertation, The Pennsylvania State investigate whether there will be composition differUniversity, University Park, PA, 1987. (14) Rindt, D. K.; Selle, S. J.; Beckerinp, W. A S M E Paper 79-WAl CD-5, 1979. (15)Hurlbut, C. S., Jr.; Klein, C. Manual ofMineralogy; Wiley: New York, 1977.
(16) Neville, M.; Sarofim, A. F. Fuel 1986,64,384. (17)Bryers, R. W. Symposium on Slugging and Fouling i n Steam Generators; Brigham Young Univ.: Provo, UT, 1985.
Combustion of Coal and Coal-Water Slurry
Energy &Fuels, Vol. 8, No. 6, 1994 1199
ences in ash formed by burning the same coal in pulverized and CWSF forms. We have previously shown that during the combustion of Beulah lignite slurry ash formation is dominated by coalescence and agglomeration facilitated by the fluxing action of iron incorporated into aluminosilicate^.^ The iron incorporation was in turn facilitated by a significant reduction in the particle size of the pyrite, caused by the slurry preparation process. This behavior would not have been predicted simply from the elemental analyses of the ash of the pulverized coal and its slurry. In the present paper we focus on the behavior of the silicate and aluminosilicate minerals.
Table 1. Oxide Composition (wt %: of Test Fuel Ash and Ash Collected at Port 10 of the D o d r e d Combustor Beulah Elk Creek PC CWSF PC CWSF compositionb fuelu ashC fuel ash fuel ash fuel ash Si02 23.9 26.1 26.7 22.3 55.4 54.1 55.4 54.0 A203 13.2 13.3 12.5 13.4 30.9 31.8 30.6 29.9 Ti02 0.6 0.8 0.6 0.6 1.7 1.7 1.7 1.8 Fez03 13.2 12.7 15.3 14.2 7.2 6.6 7.5 6.2 8.0 8.1 8.0 9.6 0.9 1.1 0.9 1.0 Mgo CaO 27.7 28.9 27.6 33.3 1.3 1.5 1.2 2.1 MnO 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0 NazO 12.7 9.9 8.5 6.3 0.7 1.2 0.7 2.7 KzO 0.6 0.3 0.6 0.3 2.0 2.1 2.1 2.3 so3 21.5 6.1 17.6 5.4 0.9 0.5 0.5 0.1
Experimental Section
Laboratory-preparedash. SO3 is reported as normalized to 100%. All remaining oxides are reported on a SO3-freebase. Ash collected at the exit of the radiant section of the combustor, Le., port 10.
Fuel Preparation and Characteristics. The characteristics and preparation of the fuels used in this study have been given elsewhere.'J* Briefly, lignite from the Beulah-Zap seam (Mercer County, North Dakota) was prepared at the University of North Dakota Energy and Environmental Research Center (UNDEERC) and converted to CWSF by hydrothermal treatment.lg Elk Creek hvAb coal (Island Creek Coal Co., Logan County, West Virginia) was prepared by Energy International. The CWSF was prepared by a proprietary process of OXCE Fuel Co. Combustion Facility. All four fuels were fired in a 3.1 m long, 40.6 cm inside diameter down-fired combustor. The fuels were fired at 316 M J h . Details of the design and operation of the combustor and ancillary sampling probes are given elsewhere.18,20*21 Sampling ports located along the combustor are numbered sequentially, from the top, from 1 to 10. Particles were sampled isokinetically and classified using a three-stage Anderson multicyclone and filter assembly. Analytical Techniques. Ash prepared in the laboratory by standard ASTM techniques was analyzed using a Spectrometrics Spectroscan 3 direct current plasma (DCP) spectrometer after lithium metaborate fusion. Elements are reported on an oxide basis normalized to 100%. so3 is reported as the original normalized value; all other oxides are given on an so3free basis and renormalized to 100%. DCP was also used for analyses of ash samples collected from the combustor. Chemical fractionation was performed to differentiate elements found as organically bound cations or as mineral phases within each fuel. This procedure has been published elsewhere.z Computer-controlled scanning electron microscopy (CCSEM) was used to identify and quantify the size and composition of discrete inorganic phases in the coals and in ash samples collected from port 10. Sample preparation and procedures have been published elsewhere.2a22The analyses were conducted at UNDEERC. The average number of particles classified in the samples was 1675, with a range of 1054-2278. Because of the large number of particles analyzed, the area percent is statistically equivalent to volume percent, as explained in detail elsewhere.ls The relative elemental concentrations are determined and the particles are classified into one of 33 specific phases or assemblages based on that composition. The standard deviation, a t a 68% confidence limit, of the data obtained by CCSEM is expressed by eqs 1 and 2 for mineral quantities and for particle size distribution, respectively, where N represents the number of particles (18) Miller, S. F. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1992. (19)Potas, T. A.; Baker, G. G.; Maas, D. J. J. Coal Qual. 1987, 6, 53. (20)Ramachandran, P. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1990. (21) Hurley, J. P. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1990. (22) Zygarlicke, C. J.; Steadman, E. N. ScanningMMicrosc. Znt. 1990, 4 , 579.
Table 2. Beulah Pulverized Coal Chemical Fractionation Data initial removed by @gig of HzO and removed by remaining dry coal) NH40Ac (%P H C I ( ~ G ) ~ (%IC silicon 7204 6 0 94 0 4516 aluminum 49 51 77 20 247 titanium 3 26 11 63 iron 5975 2 magnesium 95 3 3111 1 calcium 20 79 12717 47 3 50 46 manganese 0 1 sodium 99 6072 potassium 6 48 46 306 na nad na 251 phosphorus 0 77 23 sulfur 7062 44 1 barium 1075 55 strontium 580 na na na Water-soluble and ion-exchangeable. Acid-soluble mineral matter. Acid-insolublemineral matter. na, not available. Insufficient sample for analysis. analyzed for a particular size range.23
SD = (2.2/h7'I2
(1)
SD = (3.0/N)"2 The CCSEM system does not identify the inorganic phases based on crystallinity, therefore, inorganic phases identified are compositionallyequivalent to the referred mineral but may or may not actually indicate the presence of that mineral in the sample. Particles are classified into one of six size classifications, from 1.0-2.2 to 46.0-100.0 pm.
Results and Discussion Beulah Pulverized Coal. The amounts of silicon and aluminum and their modes of occurrence in the two Beulah fuels are quite similar (Table 1). In both fuels the silicon and aluminum are associated primarily with mineral phases in the coal (Tables 2 and 3). The silicate and aluminosilicate minerals identified by CCSEM for the two fuels are given in Table 4. The particle size distributions (PSDs) of the total silicate and aluminosilicate phases in the Beulah pulverized coal and CWSF and their respective ashes are shown in Figure 1. The PSD of the total silicate and (23)Zygarlicke, C. J.; Erickson,T. A.; Murali Ramanathan; Toman, D. L. Seventh Annual Coal Preparation, Utilization and Environmental Control Contractors Conference, July, 1991.
Miller and Schobert
1200 Energy & Fuels, Vol. 8, No. 6, 1994 Table 3. Beulah Coal-Water Slurry Fuel Chemical Fractionation Data initial removed by bg/g of H20 and dry coal) NH40Ac (%oP silicon 8792 17 aluminum 4660 3 266 titanium 19 iron 7522 19 3394 84 magnesium calcium 13807 70 manganese 55 35 sodium 4436 97 potassium 369 46 262 phosphorus nad sulfur 6003 63 1104 barium 38 strontium 604 na
_-
removed by remaining HCl (%)b (%Y
1 30 0 35 13 29 57 0 21 na 35 17 na
82 67 81 46 3 1 8 3 33 na 2 45 na
1.0-2.2
silicate and pulverized coal coal-water slurry fuel aluminosilicate phases fuel asha fuel asha quartz 9.2 13.1 26.1 12.8 kaolinite 4.9 1.2 6.8 0.3 montmorillonite 1.0 1.6 0.4 0.0 K-AI silicate 2.5 0.0 0.4 0.0 Fe-AI silicate 0.1 0.8 0.0 0.0 Ca-AI silicate 1.1 2.5 0.8 2.4 Na-AI silicate 0.1 0.0 2.9 0.3 aluminosilicate 0.9 0.0 0.2 0.0 mixed 0.9 0.4 0.1 0.3 aluminosilicate Fe silicate 0.0 0.8 0.1 0.2 Ca silicate 0.1 0.1 4.4 5.0 Si-rich 5.4 4.5 2.0 1.1 Ca- Si-rich 0.0 0.7 0.0 2.3
a Ash
30.9 47.8
36.9 17.4
25.6 49.5
refers to sample collected at port 10.
I
100
10
Size (Microns)
Figure 1. Particle size distribution of total silicate and aluminosilicate phases identified by fuels and their respective ashes.
4.6-10.0
10.0-22.0
22.0-46.0 46.0-100.0
Figure 2. Particle size distribution of quartz in the Beulah pulverized coal and pulverized coal ash.
Table 4. Weight Percent of Silicate and Aluminosilicate Phases and Unknown Phase in the Beulah Pulverized Coal and Coal-Water Slurry Fuel and Their Respective Ashes Determined by CCSEM (Weight Percent on a Mineral Basis)
27.4 9.0
2.2-4.6
Site (Microu)
a Water-soluble and ion-exchangeable. Acid-soluble mineral matter. Acid-insoluble mineral matter. na, not available. Insufficient sample for analysis.
total unknown phase
I
CCSEM in the Beulah
aluminosilicates in the pulverized coal is coarser than in the CWSF. However, the PSD of the total silicate and aluminosilicates in the pulverized coal ash is finer than in the CWSF ash. The d50 of the total silicates and aluminosilicates in the pulverized coal ash is 6.2 pm, compared with 20.5 pm for the CWSF ash. If the
silicates and aluminosilicates were behaving in a similar manner, one would expect to see similar shifts in the PSDs of the ashes relative to the fuels. This is not the case and thus suggests that different kinds of mineral matter transformations are occumng during combustion of the two fuels. The PSD of the total silicate and aluminosilicates in the pulverized coal is somewhat coarser than that in the corresponding ash, although the decrease in d50 values from 9.0 to 6.2pm is not substantial. A greater decrease in volume percent of the larger particle sizes is observed. The dw values of the total silicate and aluminosilicates decrease from 60pm in the pulverized coal to 32 pm in the ash. The total silicates and aluminosilicates include all inorganic phases listed in Table 4. The mixed silicates include all inorganic phases in Table 4 except quartz. The corresponding d50 values are given in Table 5. The quartz particles in the pulverized coal ash are significantly smaller than those in the coal itself, the d50 decreasing by 47%, from 10.0 to 5.3 pm. Since quartz accounts for 33.6% of the total silicates and aluminosilicates in the pulverized coal, its size distribution and behavior during combustion is important. The size distributions are shown in Figure 2. Approximately 35.5%, by volume, of the quartz particles in the pulverized coal are 222 pm diameter. These particles occur as extraneous mineral matter in the coal. The finer PSD of quartz in the ash is a result of fragmentation of the larger extraneous quartz particles due to thermal shock on introduction into the hot combustion environment. The quartz particles show a decrease in the volume percent in the 22.0-100.0 pm size fraction (Table 6),with a corresponding increase in particles in the 2.2-10.0 pm range. The greatest increase is in the 2.2-4.6 pm range. Thus thermal shock would produce quartz particles approximately one-tenth of the original quartz particle size in the coal. Raask has argued that quartz particles do not fragment on rapid heating to form submicrometer particles.' We agree that submicrometer particles are not likely formed on fragmentation of large, extraneous quartz particles, but our data suggest that fragmentation of quartz on rapid heating does occur, forming particles greater than 1pm. Hurley observed a similar decrease in overall particle size of quartz during combustion of two subbituminous coals in the same combustor as we have used.21 Baxter reported a significant change in particle size distribution of silica during the early stages
Combustion of Coal and Coal-Water Slurry
Energy &Fuels, Vol. 8,No. 6, 1994 1201
Table 5. dm Values of the Size Distribution Curves for Silicate and Aluminosilicate Phases Identified by CCSEM in the Beulah Fuels and Their Respective Ashes d;n h m ) pulverized coal pulverized coal ash CWSF CWSF ash total silicates and aluminosilicates 9.0 6.2 4.3 20.5 7.0 4.1 17.0 mixed silicates and aluminosilicates 8.4 quartz 10.0 5.3 4.4 23.0 Table 6. Volume Percent Distribution of Quartz Particles by Size Fraction in the Beulah Pulverized Coal and Ash volume percent of total quartz pulverized particle pulverized size (um) coal coal ash % change 1.0-2.2 2.2-4.6 4.6-10.0 10.0-22.0 22.0-46.0 46.0-100.0
9.9 15.7 24.0 14.9 22.3 13.2
10.1 31.9 34.8 15.2 8.0 0.0
Si 0
" 100
f2.0 f103.0 +45.0 $2.0 -64.0 -100.0
of combustion of Illinois No. 6 coal in a laminar flow reactor.24 In that case, the particle size was reduced by a factor of 4-5. Baxter presents three possible mechanisms for the reduction in silica PSD: fragmentation of silica along cleavage planes due to rapid heating, the dissociation of secondary quartz microcrystalline structures under induced stress, or the fragmentation of hydrated quartz during rapid dehydration. Based on the mineralogical data in this study, the mechanism most favored is that of thermal shock of quartz particles resulting in the fragmentation along cleavage planes. In general, the concentration of A1203 and Si02 increases in larger particles with increased burnout. The shift in A1203 concentration is due to the coalescence of aluminosilicates, forming larger particles. The mineral transformations involving large amounts of silicon and aluminum are essentially complete after 0.5 s. The smaller size fraction of silicates is composed primarily of kaolinite, montmorillonite, and potassium aluminosilicate (including illite) particles. These three phases comprise 32.8% by weight of the total silicate and aluminosilicate phases in the pulverized coal, but only 5.2%of the ash. The sodium aluminosilicates and silicates, and a portion of the calcium aluminosilicates and silicates, represent newly formed phases not originally present in the coal mineral matter. Calcium aluminosilicate particles in the ash are larger than those present in the coal, and no sodium aluminosilicates were identified in the coal. The increased PSD of calcium aluminosilicates, and formation of sodium aluminosilicates, suggests that the particles are involved in coalescence or agglomeration during char combustion; however, because they represent but a small fraction of coal mineral matter and ash, their increased PSD has little effect on the final overall ash PSD. The calcium and sodium aluminosilicates formed from reaction of clays with organically bound cations released by decarboxylation. In this coal, 79%of the calcium and 99% of the sodium are present as ion-exchangeable cations (Table 2). Inherent aluminosilicates in Beulah lignite have been shown to be more efficient in capturing sodium than is extraneous k a ~ l i n i t esuggesting ,~~ that the proximity of aluminosilicates to sodium enhances reaction. Mixed silicates form as a result of reacting with vaporized calcium and sodium in the gas stream. ~~~~
(24) Baxter, L. L. Prog. Energy Combust. Sci. 1990,16, 261.
. . .... . . .
/': 100
Ca
J.'.
1
a.
'
. .... ,
.
. -. .:*
50
25
*.
*
:
. ' a
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:
. '. . .., . *
.
\ o
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I AI 15
100
Si 0
%,loo
100
Ca
Figure 3. (A, top) Calcium-silicon-aluminum ternary diagram of uncorrected X-ray counts for the Beulah pulverized coal inorganics based on CCSEM data (Ca + Si Al = 100). (B, bottom) Calcium-silicon-aluminum ternary diagram of uncorrected X-ray counts for the Beulah pulverized coal port 10 ash inorganics based on CCSEM data (Ca Si Al = 100).
+
+ +
Two sets of ternary diagrams were constructed to observe the incorporation of calcium and sodium into silicates and aluminosilicates. The normalized X-ray count percents of Si, Ca, and Al of the inorganic particles classified in the pulverized coal and its ash are plotted in Figure 3, A and B. Similarly, parts A and B of Figure 4 are the analogous plots of Si, Na, and Al. (We emphasize that these plots are not ternary phase diagrams but are used only as a convenient way of illustrating the variation of three elements on a single (25) Gallagher, N. B.; Peterson, T. W.; Wendt, J. 0. L. P r e p . Pap.-Am. Chem. SOC., Diu.Fuel Chem. 1991,36,181.
Miller and Schobert
1202 Energy & Fuels, Vol. 8, No. 6, 1994 Si
.
. . . . . .. . .. \
50
r
,f'
i
/
i
,i 100
Na
/ 0
?
.
j
'3
.
'
1
25
-
1
50
'
75
,
I
.*-',!/
-AI
0
100
Si 0,100
25
Figure 4. (A, top) Sodium-silicon-aluminum ternary diagram of uncorrected X-ray counts for the Beulah pulverized coal inorganics based on CCSEM data (Na + Si + Al = 100). (B, bottom) Sodium-silicon-aluminum ternary diagram of uncorrected X-ray counts for the Beulah pulverized coal port 10 ash inorganics based on CCSEM data (Na + Si Al = 100).
+
two-dimensional diagram.) Each point on the diagram represents analysis of an individual particle. Since the points represent normalized data, points located at, for example, the aluminum vertex do not represent particles that are 100% pure aluminum but rather are particles that contain no sodium or silicon with respect t o calcium. They may contain other elements, in addition to aluminum, that are not identified on the ternary diagram. Also, the original data were not corrected by the ZAF procedure.26 The ZAF correction is used to relate X-ray intensities back to the concentrations of the elements necessary for calculating the mole or weight percent of each oxide. Normalizing the X-ray count data incorrectly gives equal weight to the percentage of X-ray counts accounted for by all the elements regardless of atomic number, absorption, and fluorescence making it dificult to calculate the mole or weight (26) Russ, J.C. Fundamentals of Energy Dispersive X-Ray Analysis; Butterworths: London, 1984; Chapter 8.
percent of the elements on an oxide basis. Therefore, no attempt is made to present the data in the traditional ternary phase format to avoid misrepresentation of the data. In general, there is a significant increase in the number of particles in the pulverized coal ash, relative to the coal, corresponding to calcium silicates such as CazSiOs, CasSiOs, and pseudowollastonite; silicates such as cristobalite and tridymite; and calcium aluminosilicates such as gehlenite.27 The shift in composition suggests that calcium is actively involved in reacting with aluminosilicates and silicates. Similar shifts have been observed by Huffman and co-workers.28 In the sodium systems, there is a slight shift in particles toward the sodium end member, with an increase in the number of particles having compositions similar t o albite or nephelineaZ7In a companion paper, data are presented that sodium is associated with sulfur forming sodium sulfates. The incorporation of alkalis and alkaline earth elements into aluminosilicates increases the likelihood of their coalescence, due to the lower viscosity and surface tension of the melt phase. Silica-rich phases, those having
