Energy & Fuels 1993, 7, 532-541
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Effect of Mineral Matter Particle Size on Ash Particle Size Distribution during 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 8, 1992. Revised Manuscript Received April 22, 1993
The objective of this work was to investigate the effect of the particle size distribution (PSD) of mineral matter in coal on the particle size distribution of ash produced during firing of two coals in pulverized coal and coal-water slurry forms. The coals used in this work were Beulah (North Dakota) lignite and Elk Creek (West Virginia) high volatile A bituminous coal. Combustion experiments were performed in a pilot-scale 316 MJ/h down-fired unit with 20% excess air. The dominant mechanism of ash formation in the Beulah pulverized coal was fragmentation of mineral particles, specifically pyrite, resulting in a finer ash particle size distribution than that of the origiml ' mineral matter (62% reduction in dw of mineral matter). By contrast, the main mechanism for determining the ash particle size in the Beulah coal-water slurry fuel (CWSF) was coalescence and agglomeration of the inorganic portion of the fuel (225% increase in dm of mineral matter). The size distribution and occurrence of inorganic matter in the fuels were the most important factors in determining ash size. Differences in pyrite PSD and occurrence between the two fuels were significant in determining the dominant mechanism for aiilh formation. The CWSF preparation process resulted in a significant reduction in the pyrite PSD and removal of organically bound sodium from the CWSF. The reduction in sodium in the CWSF did not significantly reduce the coalescence of ash particles during combustion. The PSDs of ashes from both pulverized and slurried Elk Creek coal are coarser than the original mineral matter, due to coalescence of inherent aluminosilicates and silicates during combustion. The particle size of the Elk Creek coal-water slurry fuel ash is slightly coarser than that from pulverized coal, due to the larger agglomerate formed upon atomization of the Elk Creek slurry. Atomization quality was the most important factor in determining the particle size of the ash. Subsequent papers will discuss the chemical interactions among the inorganic components during combustion of these fuels.
Introduction The combustion of coal in any form inevitably produces ash as a byproduct. An evaluation of the behavior of the inorganic components of coal in the processes of ash formation requires knowledge of the association of the inorganics with one another and with the burning carbon, the thermal behavior of the inorganic components, and the chemical processes of transformation to ash.l Factors affecting the particle size distribution (PSD) of an ash, and its composition, include the composition and size of the mineral matter, the composition and size of the coal particles, the morphology of the resulting char, the nature of the atmosphere surrounding the mineral particles, and the phase transformations of the inorganics during combustion. Changes to one or more of these factors could potentially change the ash PSD and composition. Both the preparation and combustion of a coal-water slurry fuel (CWSF) may change several or all of these factors. Combustion conditions which affect the manner in which ash forms include oxygen level, flame temperature, and residence time. In pulverized coal, mineral matter can be associated within the coal particle-so-called inherent mineral matter-or as free particles not associated with the carbonaceous portion of the fuel-extraneous mineral (1) Bryers,R. W.Symp. SlaggingFouling Steam Generators 1987,63.
matter. Inherent mineral matter is defined as that which is intimately mixed with the coal such that its thermal history during combustion is linked to the coal particle combustion. Combustion engineers classify all of the organically bound inorganics and the finely disseminated minerals not removed by pulverizing coal to 70% less than 74pm (200 mesh) as inherent mineral matter. Extraneous mineral matter is defined as not being "tied" to the coal and is generally separated upon pulverizing the coal. For purposes of this discussion, inherent mineral matter will refer to mineral matter intimately associated with the carbonaceous portion of the fuel. Extraneous mineral matter will refer to mineral matter that is entirely free of the carbonaceous portion of the fuel. The difference in inherent or extraneous mineral matter is significant regarding the temperature history of the particle (i.e.,the particle heating rate and peak temperature), the proximity to other mineral particles, and the local oxidizing or reducing environment experienced by the particle. In general, inherent mineral matter will reach peak temperatures in excess of the surrounding gas temperature, due to the proximity of the burning char during combustion. Inherent minerals and organically bound inorganics (in low-rank coals) are also in close proximity with one another and react quickly. In CWSFs, an agglomerate formed during atomization consists of coal particles containing both inherent and extraneous mineral matter and organ-
o s s ~ - o s ~ ~ t ~ ~ i ~ ~ oo ~ @- 1993 o ~ ~American ~ $ o ~ Chemical . o ~ t Society
Effect of Mineral Matter Particle Size
ically bound cations. The formation of such an agglomerate during CWSF combustion can change the proximity, temperature history, and local atmosphere experienced by the mineral matter in the agglomerate relative to the conditions experienced by an individual coal particle during pulverized coal combustion. The literature on the formation, composition, and PSDs of ash in pulverized coal combustion is extensive. It is important to understand whether such knowledge can be directly applied to CWSF combustion. Since changes in the inorganic composition, mineral matter PSD, proximity of mineral matter to the inorganic and carbonaceous portions of the fuel, and the char PSD may alter the PSD of ash produced from CWSF, it is necessary to understand why and how the ash PSDs of a CWSF and pulverized coal fuel obtained from the same parent coal are similar or different. In addition, the preparation of a CWSF may alter the size or amount of inorganic phases present in the fuel. The coalescence and agglomeration of inherent mineral matter is important in determining the composition and PSD of ash. During char burnout, the inherent inorganic matter appears as molten particles on the receding char surface. As burnout progresses, the inorganics may also appear as a lacy network inside the char particlem2 Agglomeration and coalescence is generally favored by high inherent mineral matter content, especially during the late stage of burnout when the number of mineral matter particles released is large. This condition is observed in chars that do not easily fragment during burnout. Fragmentation of char particles during the early stage of combustion inhibits coalescenceof inherent mineral matter particles. Aluminosilicate clay minerals are the most abundant of the mineral species in coal and generally occur as dispersed inherent mineral particles. Clays and quartz account for 60-90 % of the mineral matter in coal, depending on rank.2 Illite in char melts rapidly and coalesces with other inclusions within the char.3 Kaolinite is slower to coalesce than illite.4 Clays are the structural precursor for the formation of low-melting point aluminosilicates. These aluminosilicates are important since they provide the bulk of material in ash and slag deposits. Extraneous quartz is fairly inert within the gas stream in the absence of volatilized alkalis and alkaline earth elements, but it can be highly reactive in the flame in the presence of these materials. According to Raask, quartz particles do not fragment to submicron particles on rapid heating;2 however, we will show in this paper that large extraneous silicate particles do fragment due to thermal shock. Silica vaporization is also important in the formation of submicron ash. Direct vaporization of Si02 is only significant at temperatures 22373 (ref 5). Volatilization of extraneous quartz should be minimal in pulverized coal combustion since the concentrations in the gas stream of CO and H2, necessary for the reduction of Si02 to SiO, are lows2Hence S i 0 is only formed when finely disseminated Si02 is in contact with carbon. Carbonates and sulfides exhibit fragmentation due to evolution of gas during rapid heating as they are introduced (2) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere Publishing Corp: Washington, DC, 1985. (3) Srinivaeachar, S.; Helble, J. J.; Ham, D. 0.;Domazetis. G. h o g . Energy Combust. Sci. 1990,16, 303. (4) Helbe, J. J.; Srinivasachar, S.; Katz, C. B.; Boni, A. A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,34, 383. (5) Raask, E.; Williams, D. M. J.Zmt. Fuel 1965, 38, 255.
Energy & Fuels, Vol. 7, No. 4, 1993 633
into the combustor. The overall rate of decomposition depends on the temperature, the particle size, the partial pressure of gases (oxidesof carbon or sulfur), and the extent of diffusion of decomposition gases through the particle.' In pulverized coal boilers, siderite and ankerite particles fragment into 0.1-1.0-pm FeO particles. Pyrite produces primarily iron oxide particles in bituminous coal ashes. In drop-tube furnace combustion (1650 K gas temperature, 20% excess 02)iron oxide particles are physically agglomerated with aluminosilicate ash particles? Reduction of pyritic iron and subsequent vaporization forms ironcontaining species in the submicron size range.' During devolatilization of the coal, an inherent pyrite particle is heated under reducing conditions, forming pyrrhotite. Once the coal volatiles have been consumed, 02 reaches the surface of the char and of the pyrrhotite in the char particle. The resulting oxidation of pyrrhotite to iron oxide is accompanied by particle disintegration, releasing iron oxide as small solid particles. Large extraneous pyrite particles experience rapid fragmentation when introduced into the hot combustion environment in the presence of oxygen.218 This behavior was observed in Beulah lignite in this study and by other research groups."ll Fragmentation is due to the large release of gaseous sulfur during decomposition of pyrite to pyrrhotite. The pressure exerted by the sulfur vapor within the particle is enough to fragment the pyrite grain into numerous 0.1-0.5-pm particles.8 The fume portion of the ash is generally due to reactions of volatilized species in the coal flame. Volatilized species originate from the release of organically bound cations during decarboxylation and vaporized mineral species. The factors that affect vaporization include the nature of the char matrix, the burning temperature of the particle, the 02 concentration a t the particle surface, and the nature and distribution of inorganics in the original c ~ a l . ' ~AJ ~ second source of submicron particles is homogeneous nucleation of refractory oxides after vaporization in the reducing zone of the char parti~le.'~ Volatilized organically bound cations are also important in reacting with mineral phases forming new inorganic species. During coal combustion, the inorganic portion of the coal can undergo fusion, agglomeration, shedding, fragmentation, vaporization, and condensation.'J5 These processes may be sequential or simultaneous and are determined by the spatial relation of mineral matter to the coal, the initial composition of the mineral matter, the temperature history of the coal and ash particle, and the combustion conditions. Coalescence and agglomeration (6) Helble, J. J.; Srinivasachar, S.; Boni, A. A.; Bool, L. E.; Gallagher, N.B.;Peterson,T.W.;Wendt,J.O.L.;Huggine,F.E.;Shah,N.;Huffman,
G.P.;Graham,K.A.;Sarofii,A.F.;Beer,J.M.InZnorg.Traneformatione and Ash Deposition During Combustion;Beneon,S . A., Ed.;Engineering Foundation: New York, 1991. (7) Baxter,L.L.;Mitchell,R.E.Roc. SirthZnt.A'tteburghCoa1 Conf. 1989,1,64. (8) Raask, E. J. Inst. Energy 1984,57, 231. (9) Helble, J. J.; Srinivasachar,S.;Katz, C. B.; Boni,A. Roc. Biennial Low-Rank Fuels Symp. 1989,15, (10) Helble, J. J.; Srinivasachar, S.;Katz, C. B.; Boni, A. h o g . Energy Combust. Sci. 1990, 16, 267. (11) Zygarlicke,D. L.;Toman, D. L.; Benson, S. A. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1990,35,621. (12) Neville, M.; Quann, R. J.; Haynes, B. S.; Sarofim, A. F. Roc. Symp. (Int.) Combust. 1981,18,1267. (13) Quann, R. J.; Sarofii, A. F. Roc. Symp. (Int.) Combust. 1982, 19, 1429. (14) Flagan, R. C. Roc. Symp. (Int.) Combust. 1979,17,97. (15) Sarofim,A. F.; Howard, J. B.; Padia, A. S. Combust Sci. Technol. 1977, 16, 187.
634 Energy & Fuels, Vol. 7,No. 4, 1993 increase the final ash PSD; coalescenceand agglomeration of mineral particles result in formation of ash in the 1-100pm size range.'s Shedding and fragmentation decrease the overall ash PSD.* Theoretically, shedding alone would produce a final ash PSD similar to the fuel mineral matter PSD. Vaporization and condensation contribute to the formation of the submicron portion of the ash or form coatings on some of the larger particles. Only about 1% of the mineral matter v01atilizes.l~However, volatilization of the organically bound inorganics occurs quite rapidly and contributes to the formation of submicron particles. Numerical modeling of the relationship of mineral matter PSD to the ash PSD shows that if the mineral content is varied among coal particles than both char fragmentation and the mineral matter PSD are important in determining the final ash PSD.18 The mineral matter PSDs are also important when the coal PSD is monodispersed and the mineral matter PSD is polydispersed and when both coal and mineral PSDs are polydispersed and the mineral matter varies among coal particles. Recent work has relied heavily on computer-controlled scanning electron microscopy (CCSEM) to improve modof eling of ash formation p r o c e s ~ e s . ~The Q~~ technique ~ automated or computer-controlled scanning electron microscopy has enabled the identification and sizing of coal mineral matter in situ. It is also used to determine the inherent or extraneous nature of mineral matter in coal.2' These techniques represent the current state of the art for characterizing the inorganics in coal and coal ash. The direct analysis of mineral matter in coal, size determinations, and observation of the association of mineral matter with coal particles are essential to determine the validity of ash formation models and the behavior of mineral matter during combustion of pulverized coal and CWSF. The sequence of events leading to the combustion of a CWSF differs from those leading to the combustion of pulverized coal mainly in the stages leading to the formation of the char. Differences in the morphology, size, and strength of the char may result in different characteristics of the ashes produced from pulverized coal and CWSF combustion, even though the kindsof processes for ash formation are the same. For example, extraneous mineral matter may be incorporated into the coal agglomerate, formed during atomization of a CWSF, such that it becomes intimately related to the coal and hence subjected to a different thermal history and to different oxidizing or reducing conditions than if it had retained its original extraneous character. In a previous paper we have discussed the effect of the fuel particle size and droplet size distribution on the PSDs of char and ash during pilot-scale combustion of pulverized coal and C W S F S . ~That ~ work showed that in the combustion of a high-volatile A bituminous coal the PSDs (16)Raaak, E. h o g . Energy Combust. Sci. 1968,8,261. (17)Helble,J.J.;Neville, M.; Sarofii,A. F. Boc. Symp. (Int.)Combust. 1986,21,411. Sarofim, A. F.; Beer, J. M. h o c . Sixth (18)Kang, S.G.;Charon, 0.; Int. Pittsburgh Coal Conf. 1989,1 , 74. (19)Wilemki, G.;Srinivasachar, S.; Sarofim, A. F. In Inorg. Transformations and Ash Deposition During Combustion;Benson, 5. A., Ed.; Engineering Foundation: New York, 1991. (20)Zygarlicke, C. J.; Ramanathan, M.; Erickson, T. A. In Inorg. Tramformations and Ash Deposition During Combustion; Benson, S. A., Ed.; Engineering Foundation: New York, 1991. (21)Straszheim,W. E.;Markuezewski,R. In CoalScienceII;Schobert, H. H., Bartle, K. D., Lynch, L. J., Eds.;American Chemical Society: Washington, DC, 1991;Chapter 4. (22)Miller, S.F.; Schobert, H. H. Energy Fuels, companion paper in this issue.
Miller and Schobert Table I. Selected Properties of Test Fuels Beulah Elk Creek PC CWSF PC CWSF proximate analysis, wt % (dry basis) moisture (30.0) (50.0) (0.9) (32.4) 44.3 volatile matter 42.2 30.5 32.2 fixed carbon 41.1 49.5 63.4 61.8 ash 8.6 8.3 6.1 6.0 ultimate analysis, wt % (dry basis) carbon 65.3 66.6 81.0 80.4 hydrogen 4.3 4.2 4.9 5.0 1.0 nitrogen 1.1 1.6 1.5 Sulfur 0.9 1.0 0.7 0.7 oxygen 19.9 18.8 5.8 6.4 ash 8.6 8.3 6.0 6.0 higher heating value, 25.2 25.0 33.6 33.5 MJ/kg (dry basis) 0 6 free swelling index laboratory prepared ash composition0 (wt %) Si02 23.87 26.73 55.35 55.40 13.21 12.51 30.92 30.56 &os Ti02 0.64 0.63 1.68 1.66 15.26 7.16 7.46 Fez03 13.21 8.02 8.03 0.93 0.88 MgO CaO 27.71 27.61 1.27 1.21 MnO 0.09 0.10 0.02 0.02 Nap0 12.67 8.52 0.69 0.74 0.57 0.63 1.99 2.07 K2O 21.45 17.57 0.93 0.51 so3 SO3 is reported as normalized to 100% All remaining oxides are reported on a SO8 free basis. I
of ashes were determined largely by the original pulverized coal and CWSF droplet size distributions. For a lignite, however,the differencein ash PSDs between the pulverized coal and CWSF appears to be related to differences in the PSDs and composition of the inorganic portion of the two fuels and not char size. In the present paper we extend our previous work to examine the relationship of mineral matter PSDs to ash PSDs during firing of the same coals in pulverized and CWSF forms.
Experimental Section Fuel Preparation and Characteristics. The characteristics of the four fuels used in this study are summarized in Table I. Details have been given elsewhere.2219 Briefly, the Beulah lignite was obtained from the Beulah-Zap seam, Mercer County, North Dakota. The Beulah lignite was ground to 80% 1200 mesh (74 rm) at the University of North Dakota Energy and Environmental Research Center (UNDEERC) and converted to slurry by a hotwater drying (HWD) process at 603 K and 15.2 MPa steam pressure for 5 min." Elk Creek hvAb coal was obtained from the Island Creek Coal Company, Logan County, West Virginia. It was pulverized to 88% 1200 mesh by the OXCE Fuel Company, and prepared in slurry form by their proprietary process. Down-Fired Combustor Facility. The combustor has been described in a companion paper and in other publications from this laboratory.22.z Extensive descriptions of the design, construction, and operating procedures are documented.ag~~~n The (23)Miller, Sharon Falcone, Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1992. (24)Potas, T. A.; Baker, G. G.; Maas, D. J. J . Coal Qual. 1987,6(2), (26)Hurley, J. P.;Schobert, H. H. Energy Fuels 1992,6,47. (26)Ramachandran, Prakash. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1990. (27)Hurley, John P. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1990.
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Effect of Mineral Matter Particle Size pulverized coals and CWSFs were fired at 316 MJ/h. A series of sampling porta extends the length of the combustor, labeled numerically as porta 1 through 10,starting at the top. The work described in this paper uses samples collected at port 10,the bottom-most samplingport accessiblein the combustor. Particles were sampled isokinetically and classified using a water-cooled sampling probe and cyclones. The design and operation of the sampling probes has been discussed elsewhere.22.29.zenA threestage Anderson multicyclone and filter assembly was used to classify the particles collected with the probes. Inorganic Characterization. CCSEM was used to identify the discrete inorganic phases in the fuels and ashes. The procedures used for sample preparation and analysis have been published elsewhereem The CCSEM system is operated in conjunction with energy-dispersiveanalysis provided by a Tracor Northern Electron Probe Microanalyzer EDS system at UNDEERC. The analysis is conduced by a JEOL Model JXA-35 scanning electron microscope and electron microprobe (SEMI EMPA) operating in the backscattered electron imaging mode. The system determines the relative concentrations of Al, Si, Ca, Na, K, S. Mg, C1, Ba, Fe, Ti, and P. The composition of each particle is compared with known compositional ranges of 33 mineral species, and the computer assigns each particle to a specific mineral phase or inorganic assemblage. After the inorganic particles are identified, they are classified by average diameter into six size classifications: 1.0-2.2 pm, 2.2-4.6 pm, 4.6-10.0 pm, 10-22 pm, 22-46 pm, and 46-100 pm. CCSEM does not identify particles less than 1 pm in diameter. The limited resolutionof the CCSEM prevents the identification of submicron particles. However, chemical analysis by spectrochemical techniques ensures that the compositions of submicron particles are included in the bulk composition. The total area of particles measured is determined, as well as the area percent of each mineral species in each size category. This, together with known or calculated densities of minerals, is used to calculate the weight percent of each mineral in the different size categories. The number of particles classified by CCSEM in the fuel and ash samples was in the range 1054-2278, with an average of 1675. (Given the large number of particles analyzed, the area percent is statistically equivalent to volume percent.”) Another limitation of the CCSEM technique is the larger percentage of ash particles classified as “unknowns.” Particle analyses which do not fall into any of the prescribed categories are classified as “unknowns.” The large percentage of particles classified as “unknownnin ash is due to the great extent to which the inorganics in coal interact forming new highly mixed inorganic phases. The inorganic phases determined by CCSEM are routinely referred to as mineral phases; however, the classification is based solely on composition and not on atomic arrangement or crystal structure. Some of the inorganicphases identified in ash samples are amorphous and, strictly speaking, are not true minerals accordingto definitions given in standard mineralogy mannuals.% For convenience in discussion we refer to the solid inorganic phases as “minerals,” though the reader should understand that they do not necessarily display particular crystalline characteristics. CCSEM only identifies discrete inorganic phases. In the case of lignites, a significant amount of organically bound cations is present. In order to identify the composition and amount of organicallybound cations, each fuel was subjected to a laboratory ion-exchangetechnique. A fractionation procedure was used to differentiate elements found as cations or as mineral phases. A 25-g sample of coal was stirred with 100 mL of 1M NH4OAc for 24 h at 70 OC and then filtered. This procedure was repeated (28) Zygarlicke, C. J.; Steafman, E. N. Scanning Microsc. Znt. 1990,
4, 579.
(29) DeHoff, R. T.; Rhines, F. N. Quantitative Microscopy; McGraw-Hill: New York, 1968. (30)Hurlbut, C. S., Jr.; Klein, C. Manual ofhfineralogy; Wiley: New York, 1977.
Table 11. Particle Size Distribution of Inorganic Particles Identified by CCSEM in the Beulah and Elk Creek Fuels and Their Respective Port 10 Ashes Beulah PC fuel ash
vol % of total inorganica Beulah Elk Creek Elk Creek CWSF PC CWSF fuel ash fuel ash fuel ash
diameter (pm) 1.0-2.2 2.2-4.6 4.6-10.0 10.0-22.0 22.0-46.0 46.0-100.0
11.2 8.5 27.2 1.9 15.3 8.0 30.3 13.9 15.8 25.5 26.4 12.3 35.0 35.2 38.6 41.3 17.4 35.0 25.6 26.8 32.0 42.2 14.6 34.8 9.9 12.9 12.8 26.8 8.5 10.1 10.4 4.4 17.6 9.6 6.6 24.0 6.5 3.8 5.2 2.2 28.1 8.5 1.4 8.1 2.6 0.5 0.9 3.5
