Article pubs.acs.org/EF
Computer-Controlled Scanning Electron Microscopy Investigation on Ash Formation Characteristics of a Calcium-Rich Coal under O2/CO2 Environments Tai Zhang, Zhaohui Liu,* Xiaohong Huang, Qing Sun, Chao Liu, Junjie Li, and Chuguang Zheng State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: The present study aimed to address the influence of oxy-fuel combustion on ash formation characteristics on a particle-by-particle basis. A Chinese sub-bituminous coal rich in calcium was burned under air and O2/CO2 environments in a high-temperature drop-tube furnace (HDTF). Computer-controlled scanning electron microscopy (CCSEM) and CaO−Al2O3− SiO2 ternary diagram analysis of the collected ashes were executed thereafter. The results showed that, although the change of the combustion environment has a slight effect on the bulk composition of the ashes, a significant effect on the statistics of individual ash composition and particle size distribution was observed. Distribution of Ca-related minerals for a 20% O2/CO2 atmosphere is relatively dispersed in comparison to that in other combustion environments, and more calcium is enriched in small particles that are assimilated into aluminosilicate to form sticking ash particles with an increase of the O2 concentration. The higher CO2 concentration also leads to an increasing yield of carbonate minerals. O2/CO2 combustion has insignificant effects on the coalescence of small particles (10 μm), which causes the reduced generation of medium particles (>2.5 and A max )
3. RESULT AND DISCUSSION 3.1. Mineral Composition of the Bulk Ashes. Figure 4 shows the type and content of ash components under air-firing and oxy-fuel combustion. On the basis of the melting temperature of mineral, the minerals were divided into three types:31 high melting temperature (HMT, >1773 K), moderate melting temperature (MMT, 1573−1773 K), and low melting temperature (LMT, 10 or 80 wt % are selected from the EDS data of particles tested by CCSEM and the components of the selected particles were normalized to 100% by the summation of oxides CaO, Al2O3, and SiO2 and plotted on the CaO− Al2O3−SiO2 ternary diagram, as shown in Figure 9. The liquidus surface and sub-solidus equilibria in the CaO−Al2O3− SiO2 system under 1673 K44 calculated by FACT is represented by solid lines for reference. To make the picture clear, the names of minerals are simplified as follows: L, slag liquid; C, CaO; A, Al2O3; S, SiO2; and CxAySz, (CaO)x·(Al2O3)y·(SiO2)z [e.g., C2AS, (CaO)2·(Al2O3)·(SiO2); CS, (CaO)·(SiO2); etc.]. The abbreviation of mineral with subscript s represents that the phase is of mineral solid phase at 1673 K. Figure 9a shows that the majority of the minerals in the raw coal lie close to the SiO2 apex and the Al2O3−SiO2 axis, while a few lie close to the CaO apex. Panels b−e of Figure 9 indicate different degrees of coalescence between aluminosilicate and decomposition products of calcite. At 20% O2/CO2 conditions, the Ca-related mineral particles were more dispersed. With the increase of the O2 concentration, more particles were in the 10−40 mol % Al2O3 area and the distribution of the particles moved from the (slag liquid + CS(s) + S(s)) area that had a high melting temperature, passed through the slag liquid area, to the (slag liquid + C2AS(s)) area. The change in the distribution of the particles indicated that the change in the melting temperature decreased and, thereafter, increased when the O2 concentration increased. When the O2 concentration increased to 50%, the distribution of the particles was similar to that in 20% O2/N2 conditions. In combination with the mass distribution of Carelated mineral particles, as shown in Figure 9, more Ca was assimilated into aluminosilicate to form sticking ash particles with the increase of the O2 concentration, which was intended to reduce the viscosity of the slags and increase the trend of ash deposition. Russell et al.25 suggested that the compositions of the particles with 5−40 wt % CaO resulted in more sticky ash particles. The mass fractions of the analyzed particles are presented in Figure 10 to illustrate the effect of combustion environments on the formation of the sticky ash particles, obtaining SiO2 + Al2O3 as one end member and incremental steps of 5 wt % to pure CaO. The solid bars show that the fraction of the particles resulted in the sticky ash particles. The mass distributions of the raw coal and ash samples for 20% O2/ CO2 and 30% O2/CO2 conditions showed a W-shaped pattern, whereas those for 20% O2/N2 and 50% O2/CO2 conditions showed a N-shaped pattern. The statistics on the total content of the compositions of the particle with 5−40 wt % CaO are also shown in Figure 10. The statistics indicated that, with the increasing O2 concentration under O2/CO2 conditions, the mass distribution of the particles with 5−40 wt % CaO increased, which means that a larger amount of Ca interacted
Figure 10. Mass fractions of analyzed points with incremental steps of 5 wt % CaO.
with other minerals. However, the extent of the interaction of aluminosilicate under O2/CO2 conditions, even under 50% O2/ CO2 conditions, was lower than that under O2/N2 conditions, which differed from the distribution trend of total minerals, as shown in Figure 4. This difference is due to the interaction of Ca with other elements (such as Mg, Na, and Fe) and the interaction of other elements (such as Mg, Na, and Fe) with aluminosilicate, which were ignored in Figure 10.
4. CONCLUSION Typical Chinese sub-bituminous coal was burnt under O2/N2 and O2/CO2 conditions in a HDTF to understand the influence of oxy-fuel combustion on the transformation of minerals, especially calcium-rich minerals. The following results are obtained: (1) Enhanced coalescence and reduced fragmentation of minerals under oxy-fuel combustion conditions by increasing carbon dioxide in the environment are proven by the CCSEM study of the ashes generated in a laboratory HDTF. (2) Oxy-fuel combustion could delay/ decrease the decomposition of calcite, inhibit its fragmentation, and decrease the enrichment of calcium in the small and medium particles. Increasing the O 2 level in O 2 /CO 2 combustion can promote the enrichment of calcium in the small and medium particles and promote the interaction of Ca with other elements. (3) Although the distribution of calciumrich mineral particles is sparse in 20% O2/CO2 combustion, calcium easily assimilates into aluminosilicate to form sticky ash particles, and the particle viscosity is reduced when the O2 concentration is increased, thereby increasing the slagging tendency.
■
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-27-87542417. Fax: +86-27-87545526. E-mail:
[email protected]. ORCID
Zhaohui Liu: 0000-0001-6771-9368 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the National Natural Science Foundation of China (51506065 and 51390494), the National Key Research and Development Plan of China (S2016G9005 and 2016YFB0600801), and the Shenhua Group (GHFKJJS12-067). The authors also acknowl325
DOI: 10.1021/acs.energyfuels.6b02416 Energy Fuels 2017, 31, 319−327
Article
Energy & Fuels
formation in pilot-scale combustion of pulverized coal and coalwater slurry fuels. Energy Fuels 1994, 8 (6), 1197−1207. (21) Matsuoka, K.; Suzuki, Y.; Eylands, K. E.; Benson, S. A.; Tomita, A. CCSEM study of ash forming reactions during lignite gasification. Fuel 2006, 85, 2371−2376. (22) Yu, D.; Xu, M.; Zhang, L.; Yao, H.; Wang, Q.; Ninomiya, Y. Computer-Controlled Scanning Electron Microscopy (CCSEM) Investigation on the Heterogeneous Nature of Mineral Matter in Six Typical Chinese Coals. Energy Fuels 2007, 21 (2), 468−476. (23) Cprek, N.; Shah, N.; Huggins, F. E.; Huffman, G. P. Computercontrolled scanning electron microscopy (CCSEM) investigation of quartz in coal fly ash. Fuel Process. Technol. 2007, 88, 1017−1020. (24) Huffman, G. P.; Shah, N.; Cprek, N.; Huggins, F. E.; Casuccio, G.; Ramer, E.; Hicks, J. B. CCSEM investigation of respirable quartz in air samples collected during power plant maintenance activities. Fuel 2012, 95, 365−370. (25) Russell, N. V.; Wigley, F.; Williamson, J. The roles of lime and iron oxide on the formation of ash and deposits in PF combustion. Fuel 2002, 81 (5), 673−681. (26) Wen, C.; Xu, M. H.; Zhou, K.; Yu, D. X.; Zhan, Z. H.; Mo, X. The melting potential of various ash components generated from coal combustion: Indicated by the circularity of individual particles using CCSEM technology. Fuel Process. Technol. 2015, 133, 128−136. (27) Zhan, Z. H.; Bool, L. E.; Fry, A.; Fan, W. D.; Xu, M. H.; Yu, D. X.; Wendt, J. O. L. Novel Temperature-Controlled Ash Deposition Probe System and Its Application to Oxy-coal Combustion with 50% Inlet O2. Energy Fuels 2014, 28 (1), 146−154. (28) Huggins, F. E.; Kosmack, D.; Huffman, G. P.; Lee, R. Coal mineralogies by SEM automatic image analysis. Scanning Electron Microsc. 1980, 1, 531−540. (29) King, R. P. Determination of the distribution of size of irregularly shaped particles from measurements on sections or projected areas. Powder Technol. 1982, 32 (1), 87−100. (30) Sheng, C.; Li, Y.; Liu, X.; Yao, H.; Xu, M. Ash particle formation during O2/CO2 combustion of pulverized coals. Fuel Process. Technol. 2007, 88, 1021−1028. (31) Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T. Influence of mineral and chemical composition of coal ashes on their fusibility. Fuel Process. Technol. 1995, 45 (1), 27−51. (32) Hurley, J. P.; Schobert, H. H. Ash formation during pulverized sub-bituminous coal combustion: Inorganic transformation during middle and late stage of burnout. Energy Fuels 1993, 7, 542−553. (33) Mönckert, P.; Dhungel, B.; Kull, R.; Maier, J. Impact of combustion conditions on emission formation (SO2, NOx) and fly ash. Proceedings of the 3rd Workshop of the IEAGHG International OxyCombustion Network; Yokohama, Japan, March 5−6, 2008. (34) Bordenet, B. Influence of novel cycle concepts on the hightemperature corrosion of power plants. Mater. Corros. 2008, 59 (5), 361−366. (35) Bejarano, P. A.; Levendis, Y. A. Single-coal-particle combustion in O2/N2 and O2/CO2 environments. Combust. Flame 2008, 153 (1− 2), 270−287. (36) ten Brink, H. M.; Eenkhoorn, S.; Hamburg, G. Fragmentation of calcite in a simulated coal-flame. J. Aerosol Sci. 1995, 26 (Supplement 1), S177−S178. (37) Fernandez-Turiel, J. L.; Georgakopoulos, A.; Gimeno, D.; Papastergios, G.; Kolovos, N. Ash deposition in a pulverized coal-fired power plant after high-calcium lignite combustion. Energy Fuels 2004, 18 (5), 1512−1518. (38) Yamashita, T.; Tominaga, H.; Orimoto, M. Modeling of ash formation behavior during pulverized coal combustion. IFRF Combust. J. 2000, 20008. (39) Wang, Q. Y.; Zhang, L.; Sato, A.; Ninomiya, Y.; Yamashita, T. Interactions among Inherent Minerals during Coal Combustion and Their Impacts on the Emission of PM10. 1. Emission of MicrometerSized Particles. Energy Fuels 2007, 21 (2), 756−765. (40) Wang, Q. Y.; Zhang, L.; Sato, A.; Ninomiya, Y.; Yamashita, T. Effects of coal blending on the reduction of PM 10, during high-
edge the support of the Analytical and Testing Center of Huazhong University of Science and Technology.
■
REFERENCES
(1) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Oxy-fuel combustion technology for coal-fired power generation. Prog. Energy Combust. Sci. 2005, 31 (4), 283−307. (2) Stanger, R.; Wall, T. Sulphur impacts during pulverised coal combustion in oxy-fuel technology for carbon capture and storage. Prog. Energy Combust. Sci. 2011, 37 (1), 69−88. (3) Zheng, C. G.; Liu, Z. H.; Xiang, J.; Zhang, L. Q.; Zhang, S. H.; Luo, C.; Zhao, Y. C. Fundamental and Technical Challenges for a Compatible Design Scheme of Oxyfuel Combustion Technology. Engineering 2015, 1 (1), 139−149. (4) Toftegaard, M. B.; Brix, J.; Jensen, P. A.; Glarborg, P.; Jensen, A. D. Oxyfuel combustion of solid fuels. Prog. Energy Combust. Sci. 2010, 36 (5), 581−625. (5) Barnes, D. I. Understanding pulverised coal, biomass and waste combustionA brief overview. Appl. Therm. Eng. 2015, 74 (5), 89− 95. (6) Liu, X. W.; Xu, M. H.; Yao, H.; Yu, D. X.; Gao, X. P.; Cao, Q.; Cai, Y. M. Effect of combustion parameters on the emission and chemical composition of particulate matter during coal combustion. Energy Fuels 2007, 21, 157−162. (7) Buhre, B. J. P.; Hinkley, J. T.; Gupta, R. P.; Wall, T. F.; Nelson, P. F. Submicron ash formation from coal combustion. Fuel 2005, 84, 1206−1214. (8) Zhang, L.; Ninomiya, Y.; Yamashita, T. Formation of submicron particulate matter (PM1) during coal combustion and influence of reaction temperature. Fuel 2006, 85, 1446−1457. (9) Song, W. J.; Tang, L. H.; Zhu, X. D.; Wu, Y. Q.; Rong, Y. Q.; Zhu, Z. B.; Koyama, S. Fusibility and flow properties of coal ash and slag. Fuel 2009, 88 (2), 297−304. (10) Yang, X.; Huang, J. J.; Fang, Y.; Wang, Y. Slagging characteristics of fly ash from anthracite gasification in fluidized bed. Journal of Fuel Chemistry and Technology 2013, 41 (1), 1−8. (11) Suriyawong, A.; Gamble, M.; Lee, M. H.; Axelbaum, R.; Biswas, P. Submicrometer particle formation and mercury speciation under oxygen−carbon dioxide coal combustion. Energy Fuels 2006, 20, 2357−2363. (12) Yu, D. X.; Morris, W. J.; Erickson, R.; Wendt, J. O. L.; Fry, A.; Senior, C. L. Iron Ash and deposit formation from oxy-coal combustion in a 100kW test furnace. Int. J. Greenhouse Gas Control 2011, 5 (Supplement1), S159−S167. (13) Fryda, L.; Sobrino, C.; Cieplik, M.; van de Kamp, W. L. Study on ash deposition under oxyfuel combustion of coal/biomass blends. Fuel 2010, 89 (8), 1889−1902. (14) Fryda, L.; Sobrino, C.; Glazer, M.; Bertrand, C.; Cieplik, M. Study of ash deposition during coal combustion under oxyfuel conditions. Fuel 2012, 92, 308−317. (15) Xu, Y. Q.; Luo, C.; Zheng, Y.; Ding, H. R.; Zhang, L. Q. Macropore Stabilized Limestone Sorbents Prepared by the Simultaneous Hydration−Impregnation Method for High-Temperature CO2 Capture. Energy Fuels 2016, 30 (4), 3219−3226. (16) Sheng, C. D.; Li, Y. Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures. Fuel 2008, 87, 1297−1305. (17) Sheng, C. D.; Lin, J.; Li, Y.; Wang, C. Transformation behaviors of excluded pyrite during O2 /CO2 combustion of pulverized coal. Asia-Pac. J. Chem. Eng. 2010, 5 (2), 304−9. (18) Bhargava, S.; Garg, A.; Subasinghe, N. In situ high-temperature phase transformation studies on pyrite. Fuel 2009, 88 (6), 988−93. (19) Gupta, R. P.; Wall, T. F.; Kajigaya, I.; Miyamae, S.; Tsumita, Y. Computer-controlled scanning electron microscopy of minerals in coalImplications for ash deposition. Prog. Energy Combust. Sci. 1998, 24 (6), 523−543. (20) Miller, S. F.; Schobert, H. H. Effect of the occurrence and xomposition of silicate and aluminosilicate compounds on ash 326
DOI: 10.1021/acs.energyfuels.6b02416 Energy Fuels 2017, 31, 319−327
Article
Energy & Fuels temperature combustion 1. Mineral transformations. Fuel 2008, 87 (13−14), 2997−3005. (41) Wang, Q. Y.; Zhang, L.; Sato, A.; Ninomiya, Y.; Yamashita, T. Effects of coal blending on the reduction of PM 10, during hightemperature combustion 2. A coalescence-fragmentation model. Fuel 2009, 88 (1), 150−157. (42) Gallagher, P. K.; Johnson, D. W. Kinetics of the thermal decomposition of CaCO3, in CO2, and some observations on the kinetic compensation effect. Thermochim. Acta 1976, 14 (3), 255−261. (43) Stevenson, A. J.; Thomas, G. R.; Evans, D. G. Modelling the ignition of brown-coal particles. Fuel 1973, 52 (4), 281−287. (44) Verein Deutscher Eisenhü t tenleute. Slag Atlas; Verlag Stahleisen: Düsseldorf, Germany, 1995; pp 106−106.
327
DOI: 10.1021/acs.energyfuels.6b02416 Energy Fuels 2017, 31, 319−327
