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Ash Liberation from Included Minerals during Combustion of Pulverized Coal: The Relationship with Char Structure and Burnout Hongwei Wu,* Terry Wall, Guisu Liu, and Gary Bryant CRC for Black Coal Utilization and Department of Chemical Engineering, The University of Newcastle Callaghan, NSW 2308, Australia Received May 4, 1999
In this study, the float fraction (2 µm) are formed by mechanisms such as coal or char fragmentation and the structural disintegration of char,1-6 coalescence of ash on the surface of a char particle or within a char particle,4,7,8 fragmentation of minerals due to inorganic reaction,4,8,9 shedding of ash particles from the surface of chars during combustion,3,7,10,11 and cenosphere formation.8 The small size mode, size less than 2 µm, is referred to as fine ash particles (including fume) and results from mechanisms such as ash species vaporization, condensation, and aggregation,7,8,10,12-14 * Corresponding author. E-mail:
[email protected]. (1) Sarofim, A. F.; Howard, J. B.; Padia, A. S. Comb. Sci. Technol. 1977, 16, 187-104. (2) Helble, J. J.; Neville, M.; Sarofim, A. F. 21st Symposium (International) on Combustion; The Combustion Institute, 1988, p 411. (3) Helble, J. J.; Sarofim, A. F. Combust. Flame 1989, 16, 267-279. (4) Baxter, L. L.; Mitchell, R. E. Combust. Flame 1992, 88, 1-14. (5) Baxter, L. L. Combust. Flame 1992, 90, 261-266. (6) Mitchell, R. E.; Akanetuk, A. E. J. 26th Symposium (International) on Combustion; The Combustion Institute, 1996, pp 3137-3144. (7) Quann, R. J.; Sarofim, A. F. 19th Symposium (International) on Combustion; Pittsburgh, The Combustion Institute, 1982, pp 411-417. (8) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere: New York, 1985. (9) Helble, J. J.; Srinivasachar, S.; Katz, C. B.; Boni, A. A. Am. Chem. Soc. Div., Fuel. Chem. 1991, 34, 383. (10) Quann, R. J.; Neville, M.; Janghorbani, M.; Mims, C. A.; Sarofim, A. F. Environ. Sci. Technol. 1982, 16, 776-781. (11) Allen, R. M.; Mitchell, R. M. Proceedings of the 1985 International Conference on Coal Science, The International Energy Agency, 1985. (12) Neville, M.; Sarafim, A. F. 18th Symposium (International) on Combustion; Pittsburgh, The Combustion Institute, 1981, pp 12671274. (13) Helble, J. J.; Srinivasachar, S.; Boni, A. A. Prog. Energy Combust. Sci. 1990, 16, 267-279.
chemical reaction of mineral grains to form fume particles,4,15 convective transport of organically bound and possibly small-grained inorganic material away from the coal particle during coal devolatilization,16 thermal shock of coal particle or excluded minerals,8,17,18 rapid evolution of gases during mineral decomposition,8 char secondary fragmentation,19 and busting of cenosphere to produce fine particles.8,20 Char fragmentation and included mineral matter coalescence are the key mechanisms for the transformation of included minerals during pulverized coal combustion. One ash particle formed from one coal particle is expected if the char particle does not fragment. When fragmentation occurs, one char particle could produce 3 to 5 ash particles above 10 µm in diameter,1 and 200500 ash particles between 1 and 10 µm in diameter.2 For synthetic char combustion, macroporosity of the char, as measured by mercury porosimetry, was identified as a key parameter in determining the char fragmentation behavior under diffusion-controlled conditions.3 An increase in the macroporosity results in increases in char fragmentation and a decrease in the resultant ash particle size. Combustion of macroporous (14) Kauppinen, E. I.; Lind, T. M.; Valmari, T.; Ylatalo, S.; Jokiniemi, J. K. Applications of Advanced Technology to Ash-Related Problems in Boilers, Plenum Press: New York, 1995; pp 471-484. (15) Baxter, L. L. Prog. Energy Combust. Sci. 1990, 16, 261-266. (16) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H. Combust. Flame 1997, 107, 494-502. (17) Wall, T. F.; Lowe, A.; Stewart, I. M. Prog. Energy Combust. Sci. 1979, 5, 1. (18) Raask, E. J. Inst. Energy 1984, 57, 231-239. (19) Ten Brink, H. M.; Eenkhoorn, S.; Hamburg, G. J. Aerosol Sci. 1995, 26 Suppl., s673-s674. (20) Wibberley, L. J.; Wall, T. F. Combust. Sci. Technol. 1986, 48, 177-190. (21) Kang, S. G.; Sarofim, A. F.; Beer, J. M. 24th Symposium (International) on Combustion; The Combustion Institute, 1992, pp 1153-1159.
10.1021/ef990081o CCC: $18.00 © 1999 American Chemical Society Published on Web 10/27/1999
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spherocarb doped with sodium silicate yielded 75 ash particles greater that 1 µm in diameter per char particle, while nonmacroporous sucrose/carbon black char doped with sodium silicate yielded only 1 ash particle per char particle. Kang21 found the structural differences between chars generated at high and low heating rates determined the particle size for the ash formed. Baxter 5 indicated that the fragmentation of char is strongly rank dependent, which in turn contributes to the differences in char structure. Bituminous coals fragment more extensively than lignite, with the extent of fragmentation exhibiting a strong particle size dependence and a weaker ash loading dependence. Large bituminous char particles produce many more fragments than small bituminous char particles. For a char particle size greater than 80 µm, over 100, or possibly even several hundred fragments were formed compared to 1-10 for a char particle size less than 20 µm. Lignite char fragmentation is not extensive and does not vary dramatically with initial char particle size. The work of Mitchell6 indicated that the fragmentation of synthetic char during burnoff is percolative in nature. Fragmentation of char particles during devolatilization is also percolative, with the extent of fragmentation increasing with coal volatile yield. Mathematical models have been proposed to describe the char fragmentation and ash coalescence behavior, including the percolative model and modified percolative model,22,23 full coalescence and no-coalescence models,24 and cenospherical char model.25 Ash formed during pulverized coal combustion at high pressure has a finer particle size distribution compared to that at atmospheric pressure, which has been linked to char structure and morphology.26,27 Macroporosity obtained by mercury porosimetry cannot adequately explain the ash formation at high pressures. A simplified classification system,28,29 which is based on twodimensional cross-sectional images of char particles, is used to describe char structure and morphology and has been linked to ash formation at high pressure.26,27,29 The system classifies the char particles into three groups, the group I type (thin-walled with high porosity), the group II type (thick-walled with medium porosity) and the group III type (dense with low porosity). A detailed description of the system can be found elsewhere.28,29 Surface characterization was employed to gain a better description of the char structure and morphology and has been linked to ash formation at high pressure.26,27,29 New mechanisms for ash formation from char particles of different structure and morphology are proposed.26,27 (22) Kerstein, A. R.; Edwards, B. F. Chem. Eng. Sci. 1987, 42 (7), 1629-1634. (23) Kang, S. G.; Helble, J. J.; Sarofim, A. F.; Beer, J. M. 22nd Symposium (International) on Combustion; The Combustion Institute, 1988, pp 231-238. (24) Wilemski, G.; Srinivasachar, S.; Sarofim, A. F. Engineering Foundation Conference: Inorganics Transformations and Ash Deposition During Combustion; New York; Engineering Foundation, 1991, p 545. (25) Wilemski, G.; Srinivasachar, S. The Impact of Ash Deposition on Coal Fired Plants; Taylor & Francis: Solihull, England, 1994; p 151. (26) Wu, H.; Bryant, G.; Wall, T. 2nd CRC annual conference of participants; ATC, Pacific Power, Black Coal CRC, 1998, Nov, 1998. (27) Wu, H.; Bryant, G.; Wall, T. Energy Fuels, submitted. (28) Benfell, K. E.; Bailey, J. G. AIE 8th Australian Coal Science Conference; Sydney, AIE, 1998; pp 157-162. (29) Wu, H.; Bryant, G.; Benfell, K.; Wall, T. Energy Fuels, submitted.
Wu et al. Table 1. Properties of Float Fraction Used in the Experimental Study proximate analysis (a.d. %) moisture ash volatile matter fixed carbon ultimate analysis (daf %) carbon hydrogen nitrogen sulfur oxygen petrographic analysis (mmf, vol %) vitrinite liptinite inertinite ash analysis (oxide wt %) SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 Mn3O4 P2O5 SO3 SrO BaO ZnO V2O5
2.90 9.2 28.1 59.8 82.8 4.68 1.48 11.02 37.7 4.6 57.8 43.37 46.05 0.64 0.18 1.00 0.20 0.25 1.17 0.01 1.13 0.30 0.26 0.62 0.04 0.02
The current study aims to provide a mechanistic understanding of the fragmentation and burnout behavior for char particles of the different group types and the included mineral matter transformations occurring during the various combustion stages. Methods Coal Selection. An Australian bituminous coal (with narrow size bin of 63-90 µm), which was used in our related studies,26,27,29 was separated by a sink/float separation at a specific gravity of 2.0. The float material obtained, which contains primarily included minerals, was used in the current investigation. The proximate, ultimate, and ash analyses for the float fraction are listed in Table 1. Drop Tube Furnace. Combustion experiments were performed in a drop tube furnace (DTF). A detailed description of the apparatus can be found elsewhere.30 The DTF employs an entrained flow coal feeder. The feeding rate for the coal samples was approximately 2 g/h. The combustion residues were collected using a cyclone with a size cut at 2 µm and a followed filter paper. Combustion experiments were conducted in an air atmosphere at a gas temperature of 1300 °C and under atmospheric pressure. Results for combustion modeling showed that the particle temperature is approximately 250 °C higher than the gas temperature and that combustion occurred in the diffusion-controlled regime. Samples representing different levels of burnoff were collected by positioning the collection probe within the drop tube. Five positions were used, representing residence times for the coal particle within the furnace hot (30) Wu, H. Pressure effect on ash formation mechanisms, Ph.D. Thesis, The University of Newcastle, Newcastle, 1999, in preparation.
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Table 2. Simplified Char Classification System group classification
types included in the classification system of Bailey et al.35 and at ICCP meeting in Chania, Crete, 1993
typical characterization
I II III
tenuisphere, tenuinetwork crassisphere, crassinetwork, mesosphere, mixed porous inertoid, solid, fusinoid, mixed dense
high porosity (>70%), thin wall medium porosity (40%-70%), thick wall low porosity (
