Energy & Fuels 2007, 21, 435-440
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Fine Ash Formation during Pulverized Coal CombustionsA Comparison of O2/CO2 Combustion versus Air Combustion† Changdong Sheng,*,‡ Yuhong Lu,‡ Xiangpeng Gao,§ and Hong Yao§ School of Energy and EnVironment, Southeast UniVersity, Nanjing 210096, People’s Republic of China, and State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology, Wuhan 430074, People’s Republic of China ReceiVed August 20, 2006. ReVised Manuscript ReceiVed October 23, 2006
The present paper was addressed toward the impact of O2/CO2 combustion on mineral transformation and fine ash formation. A high-aluminum coal was burned with an O2/CO2 mixture in a drop tube furnace. The collected ash samples were characterized in details to study the ash formation behaviors, and the comparison was made between O2/CO2 combustion and air combustion. It was found that boehmite transformed to θ-Al2O3 and then to R-Al2O3 and the extent of the transformation depended upon the residence time and more significantly upon the particle combustion temperature. In comparison to air combustion, O2/CO2 combustion did not affect the species of mineral phases formed in the ashes of the coal studied but did affect the relative amounts of the phases. O2/CO2 combustion had an impact on the coal particle combustion temperature and consequently on the ash mineral composition. O2/CO2 combustion decreased the yields of the fine ash particles in both the submicrometer fume region and fine fragmentation region as compared to air combustion with the same O2 concentration because of the decrease in the particle combustion temperature, while an increasing O2 concentration enhanced the formation of both region particles. The mode size of submicrometer particles formed in O2/CO2 combustion was found shifting to a smaller size when compared to that in air combustion.
1. Introduction Oxy-fuel combustion of pulverized coal, using oxygen and recycled flue gas (O2/RFG) to replace air for combustion, can achieve a CO2 concentration of more than 95% in flue gas, which enables a easy recovery of CO2. Its cost for CO2 capture was estimated to be less expensive than that of air firing combustion with conventional amine-based CO2 separation.1,2 Moreover, it was demonstrated on a pilot-scale combustor that oxy-fuel combustion reduces NOx, Hg emissions, as well as unburned carbon in ash and also reduces the cost for wet flue gas desulfurization.3 Therefore, oxy-fuel combustion has been recognized as one of the most promising technologies for pulverized coal-fired power plants, particularly for retrofitting the existing ones, to control CO2 emissions, which has motivated extensive research efforts around the world.4 Fundamentally, because of the variation in the oxidant and consequently in the † Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * To whom correspondence should be addressed: School of Energy and Environment, Southeast University, Si Pai Lou number 2, Nanjing 210096, People’s Republic of China. Telephone: +86-25-83790317. Fax: +8625-57714489. E-mail:
[email protected]. ‡ Southeast University. § Huazhong University of Science and Technology. (1) Nsakala, N. Y.; Marion, J.; Bozzuto, C.; Liljedahl, G.; Palkes, M.; Vogel, D.; et al. Engineering feasibility of CO2 capture on existing U.S. coal-fired power plant. First National Conference on Carbon Sequestration, Washington, D.C., May 15-17, 2001. (2) Singh, D.; Croiset, E.; Douglas, P. L.; Douglas, M. A. Energy ConVers. Manage. 2003, 44, 3073-3091. (3) Chaˆtel-Pe´lage, F.; Marin, O.; Perrin, N.; Carty, R.; Philo, G. R.; Farzan, H.; et al. A pilot-scale demonstration of oxy-combustion with flue gas recirculation in a pulverized coal-fired boiler. 28th International Conference on Coal Utilization and Fuel Systems, Clearwater, FL, March 2003. (4) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31, 283-307.
in-furnace gas environment as compared to the conventional air firing combustion, oxy-fuel combustion has impacts on the combustion processes of pulverized coal as well as the related processes such as heat transfer. In the past decades, numerous studies were performed that covered many scientific and engineering fundamental issues, including heat transfer,5 ignition,6 combustion characteristics,7 char combustion characteristics,8 and pollutant formation,9,10 as reviewed by Buhre et al.4 The findings provide us in-depth understandings of the phenomena and fundamentals for further research and development of this technology. Basically, mineral matter coexists with organic matter in coal. After pulverization, the minerals can be classified as included or excluded according to their association with the coal matrix. During combustion, transformations of mineral matter as well as the vaporization of inorganic matter are believed to be significantly dependent upon the temperature and gas atmosphere as well.11 The included minerals will experience very high temperatures, which are mostly the consequence of the organic matter combustion, depending upon the coal particle properties and the gas atmosphere.12 The excluded minerals will not reach the very high temperatures of included minerals and thus will (5) Payne, R.; Chen, S. L.; Wolsky, A. M.; Richter, W. F. Combust. Sci. Technol. 1989, 67, 1-16. (6) Kiga, T.; Takano, S.; Kimura, N.; Omata, K.; Okawa, M.; Mori, T.; et al. Energy ConVers. Manage. 1997, 38, S129-S134. (7) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84, 833-840. (8) Murphy, J. J.; Shaddix, C. R. Combust. Flame 2006, 144, 710-729. (9) Liu, H.; Katagiri, S.; Okasaski, K. Energy Fuels 2001, 15, 403412. (10) Croiset, E.; Thambimuthu, K. V. Fuel 2001, 80, 2117-2121. (11) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere Publishing Corporation: New York, 1985; pp 61-101. (12) Timothy, L. D.; Sarofim, A. F.; Beer, J. M. Int. Symp. Combust. 1982, 19, 1123-1130.
10.1021/ef060420v CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2006
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behave differently. However, the gas environment does have effects on the transformations of some excluded minerals, e.g., iron-bearing minerals.13 Therefore, the changes of the combustion gas atmosphere between oxy-fuel combustion and air-fired combustion may affect the ash formation behaviors of the mineral matter. However, thus far, only a few investigations have been performed on the ash-related topics of oxy-fuel combustion. Krishnamoorthy and Veranth14 used a detailed char particle combustion model to study the effect of bulk gas composition (e.g., CO2 concentration) on the CO/CO2 ratio inside a burning char particle. They indicated that increasing CO2 in the bulk gas significantly changed the CO/CO2 ratio in the particle, which could affect the vaporization of refractory oxides, and consequently had an impact on the formation of fine ash particles. Zheng and Furimsky15 applied an equilibrium calculation to assess coal combustion in an O2 and CO2 mixture. The results suggested that the combustion medium had little effect on the chemical composition of ash as well as the amount and types of trace-element-containing emissions in the vapor phase as compared to air combustion, but the effects of the carbonation and/or sulfuration reactions of basic ash components on ash composition could not be excluded for some coals (low sulfur and chlorine coals containing a high alkaline mineral matter). It should be noted that the thermodynamic simulation can only provide the possible effects of oxy-fuel combustion on the ash behaviors rather than accurate predictions because the combustion processes are transport-limited and the inorganic behaviors are so complex. However, up to now, there is still a lack of experimental study specially concerning the mineral matter behaviors and ash formation in oxy-fuel combustion. Therefore, fundamental research is needed to clarify the changes of ash-related behaviors between oxy-fuel combustion and air firing combustion. The present work was addressed toward the ash formation, particularly the fine particle formation during O2/CO2 combustion of pulverized coal. In this study, a Chinese low-rank coal was burned with O2/CO2 and O2/N2 mixtures in a drop tube furnace (DTF). The residue ashes were sampled and analyzed in details with multimethods to study the mineral transformation and fine ash formation. The comparison was made between O2/ CO2 combustion and air combustion. 2. Experimental Section 2.1. Coal Sample and Characterization. The raw coal sample used in this study was a Chinese low-rank coal obtained from a coal mine. The sample was crushed and pulverized in the laboratory using a bench-scale hammer mill and passed through a sieve of 150 µm. The pulverized coal was subjected to proximate analysis, ultimate analysis, as well as ash composition analysis with an ARL9800 X-ray fluorescence spectrometer (XRF). The analysis data were summarized in Table 1. It can be found in Table 1 that the Al2O3 content of the coal ash is very high, nearly 80%. The mineral matter in the coal was analyzed semiquantitatively with a SHIMADZU XD-3A X-ray diffractometer (XRD) together with the results of XRF analysis. It was found that the major mineral matter is boehmite (see the XRD spectrum in Figure 1a), whose chemical formula is AlO(OH) or Al2O3‚H2O. It accounts for 7.8% of the coal mass and about 85% of the coal mineral matter. The rest of the minerals detected are quartz, kaolinite, etc., with minor amounts. The high content of boehmite gave us an advantage to study the (13) McLennan, A. R.; Bryant, G. W.; Stanmore, B. R.; Wall, T. F. Energy Fuels 2000, 14, 150-159. (14) Krishnamoorthy, G.; Veranth, J. M. Energy Fuels 2003, 17, 13671371. (15) Zheng, L.; Furimsky, E. Fuel Process. Technol. 2003, 81, 23-34.
Sheng et al. Table 1. Properties of Pulverized Coal proximate analysis (wt %) moisture, air-dried basis ash, dry basis volatile matter, dry ash free basis ultimate analysis (wt %), air-dried basis carbon hydrogen nitrogen sulfur oxygen (by difference) ash analysis (wt %) SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 SO3
8.14 8.81 38.29 65.17 4.25 1.14 0.51 12.70 6.46 79.35 1.20 1.92 0.34 0.67 0.15 2.39 5.51
ash formation behaviors of this aluminum-containing mineral, while a complex mineral enrichment procedure was not needed. In addition, the high aluminum content also enabled us to directly observe the contribution of aluminum in fine ash formation. 2.2. DTF. The combustion experiments were carried out in a DTF located at the State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, which was designed to simulate the reaction conditions that pulverized coal and ash particles experience in large-scale utility boilers. The descriptions of the reactor and its systems were detailed elsewhere.16,17 The reactor has a 2 m long alumina tube electrically heated by three independently controlled furnaces. The nominal temperature of the reactor is 1500 °C. The pulverized coal was introduced into the top of the reactor by a Sankyo Piotech (Model MFEV-10) microfeeder at a feeding rate of about 0.2 g/min. The coal and ash particles passed through the tube, and the residence time of the particles in the reactor was estimated at about 1.5 s. The combustion products were isokinetically collected at the bottom of the reactor using a water-cooled N2-quenched sampling probe. Between the probe and the vacuum pump, a glass fiber filter with a pore size of 0.3 µm was employed to collect the whole residue ash, which was subjected to the XRD analysis for studying the transformation of main minerals. To probe the fine ash formation during O2/CO2 combustion, a cyclone and low-pressure impactor were used instead of the filter to collect the ash particles. The sampling procedure was similar to that in the previous works.18-20 The gas-particle flow first passed through a Dekati cyclone with a size cut of 10 µm to separate the coarse particles and then through a Dekati low-pressure impactor (DLPI) to size-segregate the fine particles. The DLPI consists of 13 stages having 50% aerodynamic cutoff diameters of 0.0281, 0.0565, 0.0944, 0.154, 0.258, 0.377, 0.605, 0.936, 1.58, 2.36, 3.95, 6.6, and 9.8 µm, respectively. Each stage is composed of a filter above a substrate and a substrate holder. Apiezon greased aluminum foils were used as the substrates to collect the particle samples. The sampling time was set to 15 min to ensure enough particles loaded for weighing but to avoid the particle overload or particle bounce off from the stages. The substrates were weighed before and after each experiment with a Sartorius M2P microbalance (with a readability of 1 µg) to obtain the mass distribution of the DLPI particles. The microbalance was carefully calibrated before each set of the measurements and tared before each measurement. During the experiments, the furnace temperature, coal feeding rate, gas flow rate, and the inlet and outlet (16) Yu, D.; Xu, M. Yu, Y.; Liu, X. Energy Fuels 2005, 19, 24882494. (17) Yu, D.; Xu, M.; Yao, H.; Sui, J.; Liu, X.; Yu, Y.; Cao, Q. Proc. Combust. Inst. 2006, 31, in press. (18) Bool, L., III; Helble, J. J. Energy Fuels 1995, 9, 880-887. (19) Hirsch, M. E.; Sterling, R. O.; Huggins, F. E.; Helble, J. J. EnViron. Eng. Sci. 2000, 17, 315-327. (20) Zhang, L.; Ninomiya, Y. Fuel 2006, 85, 194-203.
O2/CO2 Combustion Versus Air Combustion
Figure 1. XRD spectra of the pulverized coal and its DTF ashes (B, boehmite; R, R-Al2O3; θ, θ-Al2O3; M, mullite; S, Ca/Al-containing silicates).
pressures of the impactor were monitored and kept constant. Under these conditions, the uncertainty of the DLPI sampling results was extensively proven within (20%.21 Short time sampling (1-2 min) was also performed, and the samples settled on the substrates were subjected to a FEI SIRION field-emission scan electron microscope (SEM) plus a GENESIS 60S energy-dispersive X-ray analysis (EDAX) for particle morphology observation. 2.3. Combustion Conditions. The pulverized coal was burned in the DTF. Before the experiments, the coal sample was dried at 90 °C for several hours to ensure a stable feeding rate. The low temperature was used to avoid the mineral reactions to minimize the effect of the drying procedure on the particle formation and composition of inorganics during combustion. In the experiments, the furnace temperature was set to 1400 °C. The O2 and CO2 mixture was used to simulate the O2/RFG, while the minor gases such as SO2 in real RFG were not included in this study. Pulverized coal combustion in two kinds of O2/CO2 mixtures with the volumetric mixing ratios of 1:4 and 2:3, respectively, were conducted. Combustion in air simulated with the O2/N2 mixture of 1:4 (hereinafter named as air combustion) was also performed for a comparison.
3. Results and Discussion 3.1. Transformation of Boehmite. Figure 1 presents the XRD spectra of the whole residue ash samples collected from three experimental cases together with the spectrum of the pulverized coal. It can be seen that, after combustion, the major aluminum-bearing phases detected in the ashes of all three cases are θ-Al2O3 and R-Al2O3 (see parts b-d of Figure 1). It means that boehmite in the coal transforms to θ-Al2O3 and R-Al2O3 during ash formation. It can also be seen that the XRD spectrum (21) Yu, Y. The investigation on char characteristics and particulate matter formation during coal combustion. Master Degree Thesis, Huazhong University of Science and Technology, 2005.
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of the ash from the O2/CO2 combustion with a mixture ratio of 1:4 is similar to that from the air combustion, while the intensities of the peaks of θ-Al2O3 relative to those of R-Al2O3 are slightly higher in the O2/CO2 combustion, indicating less θ-Al2O3 transforming to R-Al2O3. When the O2 concentration was increased to 40%, i.e., O2/CO2 ) 2:3, the intensities of the peaks of θ-Al2O3 are much lower relatively compared to those of R-Al2O3 (Figure 1d). It implies that, in this case, R-Al2O3 is the dominant aluminum-bearing phase formed and most θ-Al2O3 has transformed into R-Al2O3. Besides the Al2O3 phases, mullite and Ca- and/or Al-containing silicates were also detected in all three ashes by the XRD analyses (see parts b-d of Figure 1). The former is believed to be from kaolinite, and the latter might be formed by the minor amount of alumino-silicates or by the reaction between the included mineral species of coal particles. It is known that the transformation of boehmite during the heat treatment for alumina ceramics production is as follows: Boehmite undergoes dehydration at about 500 °C to form γ-Al2O3, which transforms to δ-Al2O3 and then to θ-Al2O3, and finally θ-Al2O3 transforms to R-Al2O3 through a nucleation and crystal growth process. The transformation from θ-Al2O3 to R-Al2O3 generally starts at about 1000 °C and ends at 1250 °C or higher (see ref 22 and references cited therein). In our experiments, the furnace temperature was 1400 °C, and the combustion temperature of coal particles is expected to be much higher than the furnace temperature. In the ashes, no γ-Al2O3 and δ-Al2O3 were detected by the XRD, which means that their transformations finished. However, θ-Al2O3 was detected in all three ashes. It implies that the transformation from θ-Al2O3 to R-Al2O3 did not complete even during the O2/CO2 combustion with 40% O2, when the coal char combustion temperature was estimated much higher than 2000 °C. The reason is that the heating rate of the particles in the experiments was much higher than that in ceramics production. The activation energies of the transformation processes from θ-Al2O3 to R-Al2O3, particularly the activation energy of the crystal growth, were reported very high.22 As a result, the residence time in the DTF was too short to finish this transformation. Note that the residence time was the same in three combustion cases. Therefore, the amount of θ-Al2O3 transforming to R-Al2O3 may mainly depend upon the coal particle combustion temperature. During O2/CO2 combustion, because the heat capacity of CO2 is higher than that of N2, more heat is required from a burning particle to heat up the gases in the particle boundary layer than during air combustion. Moreover, the endothermic reaction between carbon and CO2 may play an important role in the internal reaction of a coal char particle burning at a high temperature and high CO2 concentration because the oxygen may be exhausted within the particle. This gasification reaction also leads to the particle burning at a lower temperature. Therefore, the combustion temperature of the coal char particles during O2/CO2 combustion is expected to be lower than that during O2/N2 combustion with the same oxygen concentration. This explains the slightly higher intensities of the peaks of θ-Al2O3 relative to those of R-Al2O3, i.e., less θ-Al2O3 transforming to R-Al2O3, in the ash formed during the O2/CO2 combustion with 20% O2 compared to those in the ash formed during air combustion. The higher particle temperature achieved in oxygen-enhanced combustion (e.g., O2/ CO2 combustion with 40% O2) resulted in more included θ-Al2O3 transforming into R-Al2O3. On the basis of the XRD analysis of the ashes and the above discussion, it can be found that, in comparison to the air (22) Yen, F. S.; Lo, H. S.; Wen, H. L.; Yang, R. J. J. Cryst. Growth 2003, 249, 283-293.
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Table 2. Fine Ash Particles Generated during O2/CO2 Combustion and Air Combustion particle size
1:4 O2/N2
1:4 O2/CO2
2:3 O2/CO2
MOL/Mair
MOH/Mair
PM2.5 submicrometer
0.202 0.116
0.154 0.079
0.685 0.190
0.76 0.68
3.40 1.64
combustion, O2/CO2 combustion did not change the major mineral phases formed in the ash of the pulverized coal studied because no new mineral phase was identified but it did affect the relative amounts (or the composition) of the mineral phases in the coal ashes. The combustion gas environment has an impact on the coal particle combustion temperature and consequently on the mineral transformation. 3.2. Distribution of Fine Ash Particles. The differential mass distributions of the ash particles collected in the DLPI for all three experimental cases are shown and compared in Figure 2. It can be seen that the size distributions are all dual-modal; the boundary between the two modes is around 0.5 µm. It should be noted that the size distribution of ash particles formed during pulverized coal combustion was recognized to be trimodal, 23-25 i.e., a submicrometer aerosol fume mode, a fine fragmentation mode, and a bulk ash mode. The submicrometer fume is believed to form through the vaporization of the refractory oxides in the particles and the subsequent nucleation and growth.26,27 In the experiments, the cyclone was used before the DLPI, which actually has removed the bulk ash mode. In Figure 2, it can be seen that the fine mode of ash particles formed in the air combustion is near 0.2 µm, consistent with the observations of the submicrometer fume mode in other studies.23,24 However, the size of the coarse mode is larger than that observed for the fine fragmentation regime.23-25 The reason is that the cyclone may be only partially effective; as a result, the data above 3 µm may not represent the accurate size distribution. Nevertheless, the size distribution of the particles less than 3 µm is reliable because the cyclone is believed to have a negligible effect on these fine particles. Therefore, the size distribution of the ash particles collected in the DLPI only covers the submicrometer fume region and the fine fragmentation region. When the results between the combustion in air and the combustion in the O2/CO2 mixture with a mixture ratio of 1:4 are compared, it can be seen in Figure 2 that the submicrometer fume mode shifts to a smaller size, i.e., around 0.1 µm in O2/ CO2 combustion. Additionally, it can be found that O2/CO2 combustion leads to the decreases in the yields of the DLPI particles in both regions to a certain degree as compared to the air combustion. When the O2 concentration of the O2/CO2 mixture was increased to 40%, the ash productions of both regions, particularly of the fine fragmentation region, increase significantly, while the size of the submicrometer mode is the same as that of O2/CO2 combustion with a lower oxygen concentration. To quantitatively compare the fine particle formation in different combustion conditions, the yields of the fine particles
Figure 2. Size distribution of the ash particles collected in the DLPI.
are summed according to the upper limit sizes and presented in Table 2. The submicrometer particles are those smaller than 0.605 µm, virtually most of those in the submicrometer region shown in Figure 2; the PM2.5 refers to the particles smaller than 2.36 µm, representing the particles collected in the DLPI whose size distribution is not affected by the cyclone. In addition, the yield of the PM2.5 is considered because the PM2.5 emission is an important environmental issue related to pulverized coal combustion. In Table 2, columns 2-4 are the yields of the fine particles expressed as the percentage normalized with the total ash production, i.e., including those collected in the DLPI stages and in the cyclone. For the sake of comparison, the ratios of the yields of the fine particles in two O2/CO2 combustion cases relative to those in air combustion are calculated and included in Table 2, in which M denotes the mass percentage and the subscripts air, OH, and OL represent the combustion in air and in O2/CO2 with the mixture ratios of 2:3 and 1:4, respectively. From Table 2, it can be found that the mass percentage of the submicrometer ash particles formed in air combustion is comparable to the values reported for similar coals by Quann and Sarofim28 and Buhre et al.29,30 It should be noted that the submicrometer ash yield in air combustion is actually at the lower limit of these literature data. One reason is that the coal studied has a low sulfur content, while according to Buhre et al.,29 low sulfur coal generally produces less submicrometer ash. Another reason, maybe the more important reason, is that the coal ash is very high in Al2O3 content, while alumina is very difficult to vaporize to generate submicrometer particles at a low combustion temperature, as indicated by Lee et al.31 The values of MOL/Mair in Table 2 indicate that, at the same O2 concentration (i.e., 20%), O2/CO2 combustion produced 32% less submicrometer particles and 24% less PM2.5 particles than air combustion. It can be attributed to the lower combustion temperature of coal/char particles achieved during O2/CO2 combustion than that during air combustion. The temperature decrease during O2/CO2 combustion not only reduces the ash vaporization and thus the formation of submicrometer particles31-33 but also reduces the particle fragmentation. When the O2 concentration is increased to 40%, the yields of the two kinds of particles increase significantly compared to the air (23) Linak, W. P.; Miller, C. A.; Wendt, J. O. L. J. Air Waste Manage. Assoc. 2000, 50, 1532-1544. (24) Linak, W. P.; Miller, C. A.; Seames, W. S.; Wendt, J. O. L.; Ishinomori, T.; Endo, Y.; Miyamae, S. Proc. Combust. Inst. 2002, 29, 441447. (25) Seames, W. S. Fuel Process. Technol. 2003, 81, 109-125. (26) Sarofim, A. F.; Howard, J. B.; Padia, A. S. Combust. Sci. Technol. 1977, 16, 187-204. (27) Flagan, R. C.; Friedlander, S. K. in Recent DeVelopments in Aerosol Science; Shaw, D. T., Ed.; Wiley: New York, 1978; pp 25-59. (28) Quann, R. J.; Neville, M.; Sarofim, A. F. Combust. Sci. Technol. 1990, 74, 245-265. (29) Buhre, B. J. P.; Hinkley, J. T.; Gupta, R. P.; Wall, T. F.; Nelson, P. F. Fuel 2005, 84, 1206-1214. (30) Buhre, B. J. P.; Hinkley, J. T.; Gupta, R. P.; Nelson, P. F.; Wall, T. F. Fuel 2006, 85, 185-193. (31) Lee, C. M.; Davis, K. D.; Heap, M. P.; Eddings, E.; Sarofim, A. F. Proc. Combust. Inst. 2000, 28, 2375-2382. (32) Quann, R. J.; Sarofim, A. F. Int. Symp. Combust. 1982, 19, 14291440. (33) Flagan, R. C.; Taylor, D. D. Int. Symp. Combust. 1981, 18, 12271237.
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Figure 3. Morphology of fine ash particles collected on 3.95 and 0.0944 µm stages of DLPI (the size bar is 50 µm for 3.95 µm particles and 1 µm for 0.0944 µm particles).
Figure 4. Typical DLPI 3.95 µm particles formed at the O2/CO2 combustion with the mixture ratio of 1:4.
combustion (see MOH/Mair values in Table 2). It is straightforward that increasing the oxygen concentration of the combustion gases means the increase of the coal/char combustion temperature, which enhances the vaporization of alumina to form more submicrometer ash particles because the vaporization of alumina was found more sensitive to the temperature than the vaporization of other refractory oxides, e.g., SiO2.31 On the other hand, the increase of the char combustion temperature also significantly enhances the char fragmentation to form more PM2.5 particles, while because of the high melting temperature of alumina (2015-2050 °C), the coalescence of included minerals to form coarse particles might not be significant. 3.3. Morphology of Fine Ash Particles. The morphology of fine ash particles near the peaks, i.e., particles collected on 3.95 and 0.0944 µm stages, was observed with SEM, and the images are shown in Figure 3 for a comparison. It can be seen that, in air combustion, the 3.95 µm particles include spherical/ near-spherical particles and irregular shape particles (Figure 3a). Apparently, the irregular particles have a much bigger projective size than the spherical ones. The reason is that the DLPI size segregates particles according to the aerodynamic diameters and the irregular particles have a very irregular structure but are much lower in density. The irregular particles look like fragments but with slightly internal melting observed in some particles under SEM. The formation of spherical particles and
the melting within the fragments indicate that many char particles were burned at a temperature higher than the melting temperature of alumina. While the spherical ones were from char particles burning at higher temperatures, both the spherical and irregular particles might be generated by the particle fragmentation during the char burnout process. For the O2/CO2 combustion with the same oxygen concentration (i.e., 20%), the morphology of 3.95 µm particles is similar to that at air combustion (Figure 3b). However, slightly more particles were observed with near-spherical or irregular shape, which indicates a bit lower overall combustion temperature. When the oxygen concentration of O2/CO2 combustion was increased to 40%, 3.95 µm particles were observed in a spherical or an aggregate-like shape (Figure 3c). Even the aggregate-like particles are composed of spherical particles sintering together. The large amount of spherical particles as well as aggregate-like particles formed implies that the spherical particles were released from the char fragmentation, while the aggregates might also be generated from the fragmentation of burning char particles. However, before the fragmentation, the included alumina particles had undergone a partial coalescence because they experienced a temperature much higher than the melting point. The high temperature not only promoted the fragmentation to yield small spherical particles and aggregates but also enhanced the vaporization of coal minerals and, therefore, lead to significant increases in the yields of the particles in both regions in Figure 2. For 0.0944 µm particles, the particles formed at air combustion are all spherical but with a size distribution (Figure 3d) that implies that the mechanism of submicrometer ash particle formation is the mineral vaporization and the subsequent condensation and particle growth. The rapid cooling of the condensed particles in the char particle boundary layer leads to the particles formed in spherical shape. In O2/CO2 combustion at the same oxygen concentration, although most particles formed are spherical, a small amount of aggregate-like particles were observed (Figure 3e), indicating a lower combustion temperature than that at air combustion. The aggregates should be due to the aggregation of primary particles after their condensation. For O2/CO2 combustion at a higher oxygen concentration, it can be seen in Figure 3f that a greater amount of small spherical particles was formed, resulting from the high combustion temperature enhancing the ash vaporization.
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Table 3. Elemental Composition of Ash Particles, as Atomic Percentage (%) particle number
C
O
Na
Mg
Al
Si
P
K
Ca
Ti
Fe
1 2 3 4
30.47 26.21 28.19 30.53
36.81 45.73 47.05 42.06
0.29 3.32 0.76 0.42
0.32 0.08 0.14 0.19
26.88 17.95 21.37 23.25
1.46 6.05 1.74 2.16
0.85 0.02 0.03 0.15
nd 0.26 0.12 0.13
2.28 0.08 0.16 0.56
0.26 0.12 0.26 0.31
0.36 0.18 0.18 0.23
3.4. Particle Elemental Composition. Because the coal studied has a very high aluminum content, its contributions to the ash particle formation were studied by analyzing the elemental composition of fine ash particles using EDAX. The typical particles of 3.95 µm formed at O2/CO2 combustion with an O2 concentration of 20%, shown in Figure 4, were analyzed, and the elemental compositions are presented in Table 3. It can be seen from Figure 4 that most of the spherical particles formed are actually not perfectly spherical, which are indicated as the mineral crystal structure. The EDAX analysis of this typical particle (particle 1 in Table 3) shows that the particles are mostly composed of alumina. Besides, only a small fraction of CaO and other elements occurs, which might result from the condensation during the combustion. There are also some perfect spherical particles (particle 2). The EDAX analysis indicates that it is high in Na, Si, and Al contents, indicating alumino-silicate glass that might originate from the aluminosilicate minerals of coal. In regard to the irregular structure particles (particles 3 and 4), they are dominant in alumina content but also contain silica. It might be the particle fragments that are formed by the coalescence of the included alumina and clay minerals. It can be found that all of the particles high in alumina content showed an experienced melting or a partial melting process. Because the melting temperature of alumina is much higher than the reactor temperature, it implies that such particles were originally contained in coal particles, which reached high temperatures during the char burnout process. Therefore, a conclusion can be drawn that many boehmite particles occur inherently in pulverized coal particles, which were released to form super-micrometer particles because of the particle fragmentation during char burnout. Before the fragmentation, the alumina particles had undergone melting alone or under the help of included clay minerals. 4. Conclusions The impact of O2/CO2 combustion on mineral transformation, particularly on the fine ash particle formation, was studied by
burning a high-aluminum coal in a DTF. The comparison of the ash formation behaviors was made between O2/CO2 combustion and air combustion. The following conclusions were drawn: (a) During pulverized coal combustion, boehmite transforms to θ-Al2O3 and then to R-Al2O3. Although the pulverized coal was burned in the DTF at 1400 °C, the transformation from θ-Al2O3 to R-Al2O3 was not completed. The reason was that the residence time was not sufficient for the completion. A high particle combustion temperature promotes this transformation. (b) In comparison to air combustion, O2/CO2 combustion did not affect the species of mineral phases formed in the ash, because no new mineral phase was identified, but did affect the relative amounts of the phases in the ashes. O2/CO2 combustion had an impact on the coal particle combustion temperature and consequently on the ash mineral composition. (c) In comparison to air combustion, O2/CO2 combustion at the same O2 concentration decreased the yields of the fine particles in both submicrometer and fine fragmentation regions because it resulted in a lower particle combustion temperature, while increasing the O2 concentration of the O2/CO2 mixture enhanced the formation of the fine particles in both regions. It was found that the O2/CO2 combustion resulted in the size of the submicrometer fume mode shifting to a smaller size. It should be noted that the above conclusions were based on the combustion of a high-aluminum coal. A further experimental study with more coals is needed for obtaining more general conclusions. Acknowledgment. The authors acknowledge the financial support by the National Science Foundation of China under the project number 50576012 and the support of part of the present work by the Opening Foundation of the State Key Laboratory of Coal Combustion (number 200601) at Huazhong University of Science and Technology, Wuhan, China. EF060420V
