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Computer-Controlled Scanning Electron Microscopy (CCSEM) Investigation on the Heterogeneous Nature of Mineral Matter in Six Typical Chinese Coals† Dunxi Yu,‡ Minghou Xu,*,‡ Lian Zhang,*,§ Hong Yao,‡ Qunying Wang,| and Yoshihiko Ninomiya| State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology (HUST), Wuhan 430074, China, Energy Technology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 16-1, Onogawa, Tsukuba 305-8569, Japan and Department of Applied Chemistry, Chubu UniVersity, 1200 Matsumoto-cho, Kasugai 487-8501, Japan ReceiVed August 20, 2006. ReVised Manuscript ReceiVed NoVember 12, 2006
The heterogeneous nature of mineral matter in six typical Chinese pulverized coals has been quantitatively characterized using computer-controlled scanning electron microscopy (CCSEM). The results show that clay minerals, quartz, pyrite, and calcite form the bulk of the mineral matter. Minor minerals, such as dolomite, ankerite, rutile, nonstoichiometric iron sulfides, and gypsum, are also detected in some samples. The particlesize distribution of the included minerals is generally finer than that of the excluded ones in a certain coal. As a consequence, the coal rich in included minerals has more small mineral particles. Regarding the association of individual mineral species, the proportion of included to excluded kaolinite varies with coal greatly. Other minerals are, however, principally excluded in nature. With regard to the modes of occurrence of major inorganic elements, it is found that Si mostly occurs as quartz and clay minerals, while Al mostly occurs as silicate minerals. Fe is primarily present as iron sulfides, iron oxide, and Fe-Al-silicate. S is partitioned into iron sulfides and gypsum. Most Ca occurs as carbonates and gypsum, with a minor fraction associated with clay minerals. Mg is mainly present as dolomite and clay minerals, with a minor fraction present as ankerite. The majority of alkali elements are associated with aluminosilicates. P is mostly associated with kaolinite and/or present as more complex compounds containing Al, Si, and other elements. Apatite is the major form of P in only one of the coals studied. Ti is mainly present as rutile and kaolinite.
Introduction Because of the rapid growth of its economy, China has an increasing demand for energy. Currently, about 70% of its energy consumption is covered by coal, and it is expected that coal will continue to be the major contributor to power generation in the foreseeable future.1 Additionally, the other coal utilization technologies, such as gasification and liquefaction, are also being commercialized in China to ensure its security of energy supply.2-4 All of these highlight the importance of the efficient use of coal, mitigation of ash-related problems,5-8 and significant reduction of the pollutant emissions including harmful gases (COx, SOx, and NOx),9-13 as well as particulate † Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * To whom correspondence should be addressed. Telephone: 86-2787544779-8309. Fax: 86-27-87545526. E-mail:
[email protected] (M.X.); Telephone: 81-029-861-8437. Fax: 81-029-861-8408. Email:
[email protected] (L.Z.). ‡ Huazhong University of Science and Technology (HUST). § National Institute of Advanced Industrial Science and Technology (AIST). | Chubu University. (1) Zhou, F.; Zhou, D. China Planning Press, 1999, Beijing, China. (2) Crompton, P.; Wu, Y. Energy Econ. 2005, 27, 195. (3) Weidou, N.; Johansson, T. B. Energy Policy 2004, 32, 1225. (4) Nolan, P.; Shipman, A.; Rui, H. Eur. Manage. J. 2004, 22, 150. (5) Benson, S. A.; Sondreal, E. A.; Hurley, J. P. Fuel Process. Technol. 1995, 44, 1. (6) Bryers, R. W. Prog. Energy Combust. Sci. 1996, 22, 29. (7) Gupta, R. P.; Wall, T. F.; Kajigaya, I.; Miyamae, S.; Tsumita, Y. Prog. Energy Combust. Sci. 1998, 24, 523. (8) Huang, L. Y.; Norman, J. S.; Pourkashanian, M.; Williams, A. Fuel 1996, 75, 271.
matter (PM).14-17 Coal ash is formed from the inorganic components contained in coal, which can cause critical troubles, including slagging, fouling, bed agglomeration, corrosion, erosion, etc.5,6 Coal-derived PM has been known to have adverse effects on human health and the environment and received much concern around the world.14,18-21 It also mainly comes from the mineral matter present in coal and is formed by complex physical and chemical processes, involving vaporization, condensation, melting, fragmentation, coalescence, etc.22-35 Therefore, the mineral properties are very important for a better (9) Zhang, Y.; Zhao, B. Build. EnViron. 2007, 42, 614. (10) Kasamatsu, J.; Shima, M.; Yamazaki, S.; Tamura, K.; Sun, G. Int. J. Hyg. EnViron. Health 2006, 209, 435. (11) Xie, R.; Seip, H. M.; Wibetoe, G.; Nori, S.; McLeod, C. W. Sci. Total EnViron. 2007, in press. (12) Smith, K. R.; Apte, M. G.; Yuqing, M.; Wongsekiarttirat, W.; Kulkarni, A. Energy 1994, 19, 587. (13) Zhao, Y.; Wang, S.; Aunan, K.; Martin Seip, H.; Hao, J. Sci. Total EnViron. 2006, 366, 500. (14) Bixiong, Y.; Zhihuan, Z.; Ting, M. Chemosphere 2006, 64, 525. (15) Jin, Y.; Ma, X.; Chen, X.; Cheng, Y.; Baris, E.; Ezzati, M. Soc. Sci. Med. 2006, 62, 3161. (16) Wang, X.; Mauzerall, D. L. Atmos. EnViron. 2006, 40, 1706. (17) Xu, X.; Chen, C.; Qi, H.; He, R.; You, C.; Xiang, G. Fuel Process. Technol. 2000, 62, 153. (18) Fernandez, A.; Wendt, J. O. L.; Witten, M. L. Fuel 2005, 84, 1320. (19) Garcia-Nieto, P. J. J. EnViron. Manage. 2006, 79, 372. (20) Ohlstrom, M. O.; Lehtinen, K. E. J.; Moisio, M.; Jokiniemi, J. K. Atmos. EnViron. 2000, 34, 3701. (21) Pham Van, D.; Vu Thi, T.; Pham Quang, D.; Nguyen Thanh, B. Sci. Total EnViron. 1995, 173-174, 339. (22) Baxter, L. L. Combust. Flame 1992, 90, 174. (23) Buhre, B. J. P.; Hinkley, J. T.; Gupta, R. P.; Wall, T. F.; Nelson, P. F. Fuel 2005, 84, 1206. (24) Erickson, T. A.; Ludlow, D. K.; Benson, S. A. Fuel 1992, 71, 15.
10.1021/ef060419w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/24/2006
Mineral Matter in Six Typical Chinese Coals
understanding of ash-related problems and PM formation during coal utilization. Up to now, the bulk properties of mineral matter in coal have been studied extensively. For example, the elemental composition is commonly quantified by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) or mass spectroscopy (ICP-MS) and X-ray fluorescence (XRF). The mineral species are determined by X-ray diffraction (XRD), while sequential leaching is used to determine the modes of occurrence of inorganic elements, such as organically bound, water-soluble, and carbonate- and sulfide-associated.36-38 However, these techniques are usually labor-intensive and time-consuming. More importantly, they tend to ignore the heterogeneity of coal minerals because of instrumental drawbacks; e.g., only crystalline species can be detected by XRD, while amorphous compounds or those having complicated composition cannot be determined. In this viewpoint, computer-controlled scanning electron microscopy (CCSEM) is more informative than the conventional methods.7,36,38-45 The key feature of this technique is that it allows coal minerals to be analyzed on a particle-byparticle basis. The detailed physical and chemical information of individual mineral particles can be provided, such as particle size, shape factor, and elemental composition, which clearly play important roles in mineral behavior during coal utilization. In addition, CCSEM analysis can also supply information on the particle-size distribution (PSD) of mineral species as well as their association with carbonaceous matter, which are also significant for coal utilization processes. Because of these advantages, CCSEM has been widely used in fuel science7,35,39-60 (25) Helble, J. J.; Srinivasachar, S.; Boni, A. A. Prog. Energy Combust. Sci. 1990, 16, 267. (26) Lockwood, F. C.; Yousif, S. Fuel Process. Technol. 2000, 65, 439. (27) Nettleton, M. A. Prog. Energy Combust. Sci. 1979, 5, 223. (28) Ratafia-Brown, J. A. Fuel Process. Technol. 1994, 39, 139. (29) Senior, C. L.; Bool, Iii, L. E.; Srinivasachar, S.; Pease, B. R.; Porle, K. Fuel Process. Technol. 2000, 63, 149. (30) Smith, R. D.; Campbell, J. A.; Nielson, K. K. Atmos. EnViron. 1979, 13, 607. (31) Smith, R. D.; Campbell, J. A.; Nielson, K. K. Fuel 1980, 59, 661. (32) Takuwa, T.; Mkilaha, I. S. N.; Naruse, I. Fuel 2006, 85, 671. (33) Wolski, N.; Maier, J.; Hein, K. R. G. Fuel Process. Technol. 2004, 85, 673. (34) Yan, L.; Gupta, R. P.; Wall, T. F. Fuel 2001, 80, 1333. (35) Yan, L.; Gupta, R. P.; Wall, T. F. Fuel 2002, 81, 337. (36) Huggins, F. E. Int. J. Coal Geol. 2002, 50, 169. (37) Vassilev, S. V.; Tascon, J. M. D. Energy Fuels 2003, 17, 271. (38) Ward, C. R. Int. J. Coal Geol. 2002, 50, 135. (39) Zygarlicke, C. J.; Steadman, E. N.; Benson, S. A. Prog. Energy Combust. Sci. 1990, 16, 195. (40) Zhang, L.; Sato, A.; Ninomiya, Y. Fuel 2002, 81, 1499. (41) Wigley, F.; Williamson, J. Prog. Energy Combust. Sci. 1998, 24, 337. (42) Huffman, G. P.; Huggins, F. E.; Dunmyre, G. R.; Pignocco, A. J.; Lin, M.-C. Fuel 1985, 64, 849. (43) Galbreath, K. Fuel Energy Abstr. 1996, 37, 231. (44) Charon, O.; Sarofim, A. F.; Beer, J. M. Prog. Energy Combust. Sci. 1990, 16, 319. (45) Bool, I. I. I. L. E.; Peterson, T. W.; Wendt, J. O. L. Combust. Flame 1995, 100, 262. (46) Zygarlicke, C. J.; Stomberg, A. L.; Folkedahl, B. C.; Strege, J. R. Fuel Process. Technol. 2006, 87, 855. (47) Zhang, L.; Sato, A.; Ninomiya, Y.; Sasaoka, E. Fuel 2003, 82, 255. (48) Zhang, L.; Ninomiya, Y. Fuel 2006, 85, 194. (49) Wigley, F.; Williamson, J.; Gibb, W. H. Fuel 1997, 76, 1283. (50) Wells, J. J.; Wigley, F.; Foster, D. J.; Livingston, W. R.; Gibb, W. H.; Williamson, J. Fuel Process. Technol. 2005, 86, 535. (51) ten Brink, H. M.; Eenkhoorn, S.; Hamburg, G. Fuel 1996, 75, 945. (52) Russell, N. V.; Wigley, F.; Williamson, J. Fuel 2002, 81, 673. (53) Ninomiya, Y.; Zhang, L.; Sato, A.; Dong, Z. Fuel Process. Technol. 2004, 85, 1065. (54) Lee, F. C. C.; Lockwood, F. C. Prog. Energy Combust. Sci. 1998, 25, 117. (55) Kim, D. S.; Hopke, P. K.; Casuccio, G. S.; Lee, R. J.; Miller, S. E.; Sverdrup, G. M.; Garber, R. W. Atmos. EnViron. 1989, 23, 81.
Energy & Fuels, Vol. 21, No. 2, 2007 469 Table 1. Chemical Properties of Coal Samples JZH
YZH
ZHJ
JCH
fixed carbon volatile matter ash
Proximate Analysis (wt %, db) 48.4 58.4 55.6 43.8 44.4 30.8 33.3 44.4 7.2 10.8 11.1 11.8
DT
WFG
47.1 28.6 24.3
44.6 13.3 42.1
SiO2 Al2O3 CaO MgO Fe2O3 SO3 TiO2 Na2O K2O P2O5
LTA Analysis (Normalized wt %) 24.8 34.2 38 14.8 14.3 19.6 26.4 10.2 10.6 4.8 3.2 14.2 1.4 0.6 0.7 0.7 21.5 17.6 10.1 26.2 24.1 18.6 14.5 32.7 1.1 1.2 5 0.6 0.5 0.5 0.3 0.1 1.3 1.7 1.3 0.4 0.4 1.2 0.5 0.1
46 30.7 6.2 0.6 8.3 3.6 2.7 0.4 1 0.5
46.1 31.7 5.4 0.6 6.4 4.2 1.5 0.8 2.9 0.4
since its inception. However, few studies have been conducted on the application of the CCSEM technique in the field of coal utilization in China. This paper aims to elucidate the heterogeneous nature of mineral matter in six typical Chinese pulverized coals, which are widely used in the power plants throughout China. The detailed information on the mineral species present in these coals has been determined and discussed. The PSDs of the minerals as well as their association with the organic matter are also investigated. In addition, the modes of occurrence of major inorganic elements are studied. The implications of those mineral properties on ash formation and deposition are also discussed. Such information can provide insights into the ash-related problems when these coals are used at high temperatures. Experimental Section Sample Selection and Properties. Six typical Chinese coals were selected in the present study. Two samples, namely, JZH and YZH, are from Shandong province. The DT and JCH samples are from Shanxi province. The other two samples, WFG and ZHJ, are from Anhui province. These collected samples were ground to less than 125 µm and dried prior to analysis. Their proximate properties and low-temperature ash (LTA) composition are shown in Table 1. XRF was used for LTA elemental composition quantification. The fixed carbon varies from 43.8 to 58.4%, while the volatile matter covers a wider range of ∼13.3-44.4%. The ash content also varies significantly. The JZH coal has the lowest ash content of 7.2%, while the ash content in the JCH coal is as high as 42.1%. According to the ash content, the coals are classified into three groups: low-ash (7.2-10.8%) coals, including the JZH and DT samples; medium-ash (11.1-24.3%) coals, including the WFG, YZH, and ZHJ samples; and high-ash (42.1%) coal, i.e., the JCH sample. The ash composition shows that SiO2 and Al2O3 are abundant in the coals studied. SO3 also accounts for a significant fraction in most coal ashes, indicating the presence of a large amount of S-bearing species. The contents of CaO and Fe2O3 vary greatly with coals. MgO, TiO2, Na2O, K2O, and P2O5 only account for a very small fraction in the coal ashes. CCSEM Analysis Procedure and Result Evaluation. The CCSEM system used consists of a JEOL scanning electron microscope (SEM, model JSM 5600) and an EDAX energydispersive X-ray spectrometer (EDS, model CDU-LEAP). The (56) Huffman, G. P.; Huggins, F. E.; Shah, N.; Shah, A. Prog. Energy Combust. Sci. 1990, 16, 243. (57) Huffman, G. P.; Huggins, F. E.; Levasseur, A. A.; Chow, O.; Srinivasachar, S.; Mehta, A. K. Fuel 1989, 68, 485. (58) Ghosal, S.; Ebert, J. L.; Self, S. A. Fuel Process. Technol. 1995, 44, 81. (59) Galbreath, K.; Zygarlicke, C.; Casuccio, G.; Moore, T.; Gottlieb, P.; Agron-Olshina, N.; Huffman, G.; Shah, A.; Yang, N.; Vleeskens, J.; Hamburg, G. Fuel 1996, 75, 424. (60) Chen, Y.; Shah, N.; Huggins, F. E.; Huffman, G. P.; Linak, W. P.; Miller, C. A. Fuel Process. Technol. 2004, 85, 743.
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Figure 1. Comparison of the results from CCSEM and XRF.
smallest particle size that can be determined by CCSEM is about 0.5 µm. For sample preparation, about 1.0 g of coal sample was first mixed with about 2.0 g of molten carnauba wax. After the sample was cooled to a solid, the as-formed pellet was carefully cross-sectioned, polished, and carbon-coated. Three magnifications, ×800 for particles of 0.5-4.6 µm, ×250 for 4.6-22.0 µm, and ×150 for 22.0-211.0 µm, were selected during CCSEM analysis. The working distance and voltage were 20 mm and 15 kV, respectively. More than 3000 mineral particles were analyzed for each sample, and the raw data on each particle, involving its size, shape, area, chemistry, location, etc., were stored in a computer for off-line processing. Minerals in coals were determined according to the elemental composition of individual particles analyzed. The mineral categories based on the composition criteria developed by Zygarlicke and Steadman61 were adopted. Because this classification is based on particle composition, the mineral phases determined by CCSEM
involve both the crystalline species, which are detectable by conventional XRD analysis, and the amorphous ones with a complex nonstoichiometric composition. For statistical calculation on the mineral weight percentages, the area of a certain particle was assumed to represent its volume. Its weight was then calculated by multiplying its area by the density. It should be noted that the areas of particles determined at the magnifications of ×800 or ×250 must be multiplied by a factor (F150N150)/(FN), where F150 and N150 are the frame size and the number of frames obtained at ×150, respectively, while F and N are the frame size and the number of frames obtained at ×800 or ×250. During CCSEM analysis, the back-scattered electron (BSE) images were continuously collected, on which the coal minerals are usually the brightest because of their heavy molecular weights compared with the wax and coal carbonaceous matrix. The (61) Zygarlicke, C. J.; Steadman, E. N. Scanning Microsc. 1990, 4, 579.
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Table 2. Mineral Composition of the Coals Analyzed (wt %, on a Mineral Basis) quartz iron oxide periclase rutile alumina calcite dolomite ankerite kaolinite montmorillonite K-Al-silicate Fe-Al-silicate Ca-Al-silicate Na-Al-silicate aluminosilicate mixed silicate Fe silicate Ca silicate Ca aluminate pyrite pyrrhotite oxidized pyrrhotite gypsum barite apatite Ca-Al-P KCl gypsum/barite gypsum/Al-silicate Si-rich Ca-rich Ca-Si rich unknown totals
JZH
DT
WFG
YZH
ZHJ
JCH
13.7 1.5 0 0.3 0.5 0.9 0.5 0.2 22.9 3.4 7.9 4.3 0.4 0.7 0 1.1 0 0.1 0 6.3 1.7 9.2 5.6 0 0 0 0 0 0.3 1 0.1 0.1 17.3 100
11.7 0.3 0 1.2 0.1 1.3 2.3 0 27.9 3.3 16.5 1.3 0.4 0 0 0 0 0 0 10 2.3 1.1 0.7 0 1.1 0 0 0 0.1 1.4 0 0 17.0 100
2.9 1.7 0 0.6 0.3 1.4 0.6 0.4 74.6 1.6 1.1 0.3 0.3 0.2 0.1 0.3 0.2 0 0 2.6 0 0.2 0.1 0 0 0 0 0 0 0.1 0.1 0 10.3 100
1.7 3.4 0 0.1 0 1.8 0.5 1.4 71.6 2.2 3.8 1.2 0.3 0.2 0 0.7 0 0 0 2.9 0 0.3 0 0 0 0 0 0 0 0.3 0.1 0 7.5 100
2.2 0.4 0 0.1 0.1 3.2 0.4 0.5 73.5 3 1.2 3.3 2.4 0.1 0 0.1 0 0.1 0 0.6 0 0 0 0 0.1 0 0 0 0 0.3 0.8 0 7.6 100
0.6 0 0 0 0.7 1 0.2 1.5 48 2.3 22.7 4.9 4.2 0.5 0.1 0.9 0 0 0 0.2 1.1 0 0.3 0 0 0 0 0 0 0.2 0.5 0 10.1 100
properties of each mineral particle were automatically determined by EDS, which were stored for further processing. In regard to the association of mineral particles, the particles embedded in the carbonaceous matrix are defined as included minerals, while those kept separated from the carbonaceous matrix are excluded minerals. To evaluate the precision of the CCSEM results, the equivalent oxide composition of the minerals obtained by CCSEM is compared with that of the LTA by XRF in Figure 1. For all of the coal samples, the weight percentage of SiO2 by CCSEM analysis is higher than that by XRF. A rather good consistence was obtained for Al2O3, except the YZH coal. The weight percentages of the minor oxides, such as CaO, Fe2O3, and SO3 determined by CCSEM, are lower than those by XRF. The reasons for the overestimation of SiO2 by CCSEM are not well-known. CaO, Fe2O3, and SO3 may be partially present as submicrometer particles or organically bound materials, which cannot be detected by CCSEM. This may be responsible for their higher percentages in the XRF results.
Results and Discussion Mineral Composition of Studied Coals. The results of the mineralogical analysis performed on the six coal samples are summarized in Table 2. It can be seen that clay minerals (kaolinite, montmorillonite, and K-, Fe-, and Ca-bearing aluminosilicates), quartz, pyrite, and calcite form the bulk of the minerals in these coal samples. Other minor but important minerals, including dolomite, ankerite, rutile, pyrrhotite, oxidized pyrrhotite, and gypsum, have also been found in some samples. Although a similar qualitative assemblage of major and minor minerals has been detected, their amounts vary greatly from coal to coal. Kaolinite is the dominant mineral (22.9-74.6%) in all of the coals studied. The JZH and DT samples, with lower ash contents of 7.1 and 10.8%, have a smaller fraction of kaolinite (22.9 and 27.9%, respectively) than higher ash coals. The mediumash coals (11.1-24.3%), including WFG, YZH, and ZHJ, have the highest content of kaolinite (71.6-74.6%). However, the JCH sample, with the highest ash content (42.1%), has a medium
Figure 2. Mineral particle-size distribution.
proportion of kaolinite. As discussed above, the various species determined by CCSEM are based on the mineral composition rather than crystalline structures. The K-, Fe-, and Ca-bearing aluminosilicates are generally taken to represent illite or feldspars. Higher contents (7.9-16.5%) of K-bearing aluminosilicates are present in the JZH and DT coals than in the medium-ash coals (WFG, YZH, and ZHJ), while the high-ash JCH sample has the highest contents of K-, Fe-, and Ca-bearing aluminosilicates (22.7, 4.9, and 4.2%, respectively). The reasons for this finding have not been clarified yet, but it is helpful to understand the behavior of these aluminosilicates at high temperatures. Montmorillonite is a little more abundant in the low-ash coals (JZH and DT) than in the higher ash coals. As a widely distributed mineral species, quartz was found in all of the coals. However, its content varies significantly. The low-ash coals (JZH and DT) have the highest contents (13.7 and 11.7%, respectively) of quartz, while the JCH coal has the lowest content (0.6%) of quartz. Pyrite is the dominant sulfide mineral in most coals, whose quantity varies considerably as well. The low-ash JZH and DT coals have higher contents (6.3 and 10%, respectively) of pyrite than the medium- and highash coals, while the JCH sample contains the lowest content (0.2%) of pyrite. In addition to pyrite, other S-bearing minerals, such as pyrrhotite, oxidized pyrrhotite, and gypsum, also account for a larger fraction in the JZH and DT coals than in the higher ash ones, as indicated in Table 2. They are believed to result from the oxidation of specific minerals during storage and coal weathering.62 Calcite is the most prevalent carbonate found in these coals, having a higher fraction than dolomite and ankerite. The DT coal has a higher content (2.3%) of dolomite, but no ankerite is determined. However, ankerite accounts for a little higher proportion (1.4-1.5%) in the YZH and JCH coals. Other minor minerals, including iron oxide and rutile, also appear in most coals. The YZH coal has the highest content (3.4%) of iron oxide, and rutile is higher (1.2%) in the DT coal. Apatite, being the major source of P, is higher (1.1%) in the DT coal, with a minor fraction (0.1%) in the ZHJ sample. However, little apatite was found in the other samples. It should be noted that the “unknown” minerals account for a significant fraction (7.517.3%) in the coals studied. They have complex chemical composition and do not fall into any of the prescribed categories. Mineral Distribution and Association. Figure 2 depicts the mineral PSDs for all of the coal samples. It shows that the minerals in the WFG and YZH samples have a smaller size (62) Vassilev, S. V.; Vassileva, C. G. Fuel Process. Technol. 1996, 48, 85.
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Figure 3. Association of mineral matter.
distribution than those in the other four samples. About 50% of the minerals in these two samples are 10 µm. The minerals in the DT sample have the coarsest size distribution, with more than 80% being >10 µm. The minerals < 4.6 µm in the DT, ZHJ, and JCH samples have a similar size distribution. The DT minerals > 22 µm are coarser than the JCH ones in the same size range. The size distribution of inorganic matter in the fuels is one of the most important factors in determining ash size.63,64 Minerals with a finer size distribution tend to produce finer ash particles, although the ash size is also influenced by other factors, such as char fragmentation, mineral composition, mineral-coal association, combustion conditions, etc.7,22,25,65,66 Figure 3 illustrates the mineral-coal association for the coal samples. It can be seen that higher contents (>70%) of included minerals are present in the WFG and YZH samples than in the other four. It was also found that the included minerals < 10 µm in the WFG and YZH samples account for 66.0 and 51.6% of the total minerals, respectively, while the excluded ones < 10 µm account for only 13.4 and 20.3%, respectively. Figure 4 further implies that the size distribution of included minerals is generally finer than that of excluded ones in a certain coal. Obviously, higher included mineral contents should contribute to the prevalence of fine mineral particles in the WFG and YZH samples, as discussed above. The DT sample contains the lowest content (∼20%) of included minerals, resulting in the coarsest size distribution, as shown in Figure 2. The mineral-coal association has significant effects on coal properties, combustion processes, ash formation, and deposition.7,35,49,50,53,67-70 Wells et al.50 gave linear correlations between the mineral matter and
Yu et al.
the abrasive and erosive wear by considering only the excluded mineral occurrences in the pulverized coal that are harder than steel and have a size > 25 µm. Ozbas et al.70 indicate that removing excluded minerals can improve the combustion characteristics of the lignite. It is concluded that excluded minerals are converted into ash by direct transformation or fragmentation, while included minerals usually undergo coalescence because of interactions within the coal particle.7,35 The detailed distribution and association of individual mineral species (>2 wt %) identified in the six coals are illustrated in Figure 5. The contents of the mineral species are based on the total mineral area determined by CCSEM. It shows that quartz occurs primarily as excluded particles. The JZH coal has a higher proportion (11.2%) of excluded quartz, with the included quartz accounting for 3.5%. Similarly, the excluded quartz in the DT coal accounts for 10.3% of the total minerals, while the included quartz only accounts for 2.4%. In comparison to quartz, kaolinite has somewhat of a random association with carbonaceous matter. In the JZH, DT, and JCH coals, the excluded kaolinite accounts for a larger fraction of the total minerals than the included kaolinite. However, the WFG and YZH coals have a higher content of included kaolinite. In the ZHJ sample, kaolinite is distributed more uniformly between the included and excluded fractions. With regard to the other clay minerals, including montmorillonite and K-, Fe-, and Ca-bearing aluminosilicates, they tend to be present primarily as excluded minerals. One exceptional case is the YZH coal, in which the included montmorillonite and K-bearing aluminosilicate account for a higher proportion. Major carbonates, such as calcite and dolomite, and S-bearing minerals, such as pyrite, pyrrhotite, oxidized pyrrhotite, and gypsum, as well as iron oxides, are also present mainly as excluded minerals. The nature of specific mineral species has an important influence on their behavior during coal utilization. Bryers6 discussed the thermal behavior of coal minerals in detail. The excluded quartz is less reactive, while the included quartz may vaporize at lower temperatures in the presence of carbon and other mineral species. Kaolinite and the other clay minerals within the coal particle play an important role in ash deposition as an absorbent for alkali and alkaline elements. However, in case they are present as excluded minerals, fewer interactions between them and other mineral species are expected. The behavior of other minerals, such as pyrite and carbonates, is also largely determined by their association with the carbon matrix.71,72 Occurrence and Distribution of Inorganic Elements. The occurrence and distribution of major inorganic elements in coal are also very important for coal combustion, gasification,
Figure 4. Particle-size distribution of included and excluded minerals in the coals.
Mineral Matter in Six Typical Chinese Coals
Energy & Fuels, Vol. 21, No. 2, 2007 473
Figure 5. Distribution of major mineral species in the coals.
liquefaction, and coking, as well as environmental considerations. It is indicated that, for some coals, the forms of the elements play a more important role in ash deposition than their contents.7 Although the principal forms of the inorganic elements (63) Miller, S. F.; Schobert, H. H. Energy Fuels 1993, 7, 532. (64) Miller, S. F.; Schobert, H. H. Energy Fuels 1994, 8, 1197. (65) Helble, J. J.; Sarofim, A. F. Combust. Flame 1989, 76, 183.
in most coals have been reported, their properties are highly dependent upon coal type and vary greatly.73-76 In this study, (66) Wall, T. F.; Liu, G.-s.; Wu, H.-w.; Roberts, D. G.; Benfell, K. E.; Gupta, S.; Lucas, J. A.; Harris, D. J. Prog. Energy Combust. Sci. 2002, 28, 405. (67) Liu, Y.; Gupta, R.; Sharma, A.; Wall, T.; Butcher, A.; Miller, G.; Gottlieb, P.; French, D. Fuel 2005, 84, 1259.
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Figure 6. Occurrence and distribution of Si and Al.
Figure 7. Occurrence and distribution of Fe and S.
10 major inorganic elements, including Si, Al, Ca, Mg, Fe, S, Ti, Na, K, and P, are considered. According to the ash composition in Table 1, Si is the most abundant element (47.756.3%), followed by Al (16-34.3%). Ca accounts for the largest fractions in the JZH and ZHJ coals, while it has the smallest contents in the WFG and YZH samples. The JZH and DT coals have the highest contents of Fe and S because of the enrichment of iron sulfides within them. K is enriched in the DT and JCH coals. Other elements only account for a small fraction (20%) of Si in the JZH and DT samples, which have higher contents of quartz than the other four. Because the JCH sample has the lowest content of quartz, Si in quartz for this sample only accounts for a small fraction (1.1%). Montmorillonite contains a minor fraction of Si and Al for all samples. The higher concentrations of Si and Al in K-Al-silicate for the DT and JCH coals is due to the presence of higher contents of this mineral in them. With regard to the JZH and JCH coals, other silicates including Fe- and Ca-Al-silicates also contain appreciable amounts of Si and Al. In comparison to Al, the form of Si can have more influence on its transformation. Quartz is relatively less reactive, while clays will react rapidly with other minerals or inorganic species, contributing significantly to ash deposition.7,24 Iron and Sulfur. Fe occurs mainly as iron sulfides (pyrite, pyrrhotite, and oxidized pyrrhotite), iron oxide, and Fe-Alsilicate, with a small fraction present as ankerite. However, S is partitioned into iron sulfides and gypsum, as shown in Figure 7. Generally, iron sulfides contain most of S (63.2-87.8%) and a significant fraction of Fe (10.4-80.8%), indicating that the two elements usually occur together in these coals. In the coals that contain a higher content of iron oxide, ankerite, or Fe-Alsilicate, such as the WFG, YZH, ZHJ, and JCH samples, the concentrations of Fe in iron sulfides are relatively low (48.9%) present as calcite, with a minor fraction in the forms of dolomite, ankerite, and clays. Little or no Ca appears as gypsum in these three coals. However, in the JZH coal gypsum is the primary source of Ca (52.6%), with calcite and dolomite being the second. With regard to the DT coal, Ca occurs mainly as calcite and dolomite, with a smaller fraction present as clays and gypsum. In the JCH sample, Ca is relatively more homogeneously distributed among calcite, ankerite, and clays. Mg is mainly present as clays in the JZH, YZH, ZHJ, and JCH samples, while dolomite is the major source of Mg in the DT coal. Additionally, a minor fraction of Mg is present as ankerite in the YZH and JCH coals. Similarly, appreciable Ca and Mg are also contained in the “unknown” minerals. The carbonates prefer to decompose at temperatures around 600 °C and release CO2 during combustion, forming agglomerates of small oxide particles with compositions reflecting the parent mineral.72
Interactions between Ca and Mg and other mineral species, mainly silicates, can contribute to slagging and ash deposition during coal combustion.6 Sodium and Potassium. As shown in Figure 9, kaolinite is the primary source hosting Na in the WFG, YZH, ZHJ, and JCH coals, as well as the primary source of K in the WFG, YZH, and ZHJ samples. For the DT coal, most of the Na (76.9%) is present as the “unknown” minerals. The minor source of Na is K-Al-silicate (13.5%), which also contains most of the K (79.3%). Na or K in kaolinite only accounts for a small fraction, and no Na-Al-silicate was detected in this sample. However, in the JZH coal, Na occurs mainly as the “unknown” mineral and Na-Al-silicate, with kaolinite and K-Al-silicate being its minor sources. In contrast, kaolinite and K-Al-silicate are enriched with K in this sample. Similarly, K in the JCH coal is primarily present as K-Al-silicate (58.3%), with kaolinite as its second source (26.9%). However, less K is present in the “unknown” minerals. As indicated, Na and K are primarily associated with aluminosilicates. These compounds can lead to the sintering of silicates at low temperatures, resulting in slagging and fouling.6 Phosphorus and Titanium. The presence and behavior of phosphorus in fossil fuels have received increasing concerns in recent years despite its low concentration in most coals, because of its potential to cause slagging, deactivation of catalysts, contamination of natural waters, and PM emissions.77-80 It has been reported that P in coal may occur as apatite, crandallite (77) Beck, J.; Muller, R.; Brandenstein, J.; Matscheko, B.; Matschke, J.; Unterberger, S.; Hein, K. R. G. Fuel 2005, 84, 1911. (78) Beck, J.; Unterberger, S. Fuel 2006, 85, 1541. (79) Stout, W. L.; Sharpley, A. N.; Gburek, W. J.; Pionke, H. B. Fuel 1999, 78, 175. (80) Zhang, L.; Ninomiya, Y. Proc. Combust. Inst. 2007, in press.
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of Ti (92.4, 61, and 76.7%, respectively). In the other three samples, kaolinite contains a higher or at least similar Ti content, which shows that more Ti is associated with kaolinite in them. The “unknown” mineral also contains a significant fraction of Ti in most coals. Conclusions
Figure 10. Occurrence and distribution of P and Ti.
group, or other phosphate minerals. The top panel of Figure 10 shows that P in the coals is mainly associated with kaolinite, apatite, and Ca-Al-silicate. Apatite is detected in the JZH, DT, ZHJ, and JCH coals, but its content varies widely. Apatite is the primary source of P only in the DT coal, which has a low concentration of kaolinite, as noted before. With the increase of kaolinite content in coals, the fraction of P associated with it tends to increase, too. This trend shows that P has a strong affinity with kaolinite. Kaolinite in the YZH coal is the main source of P, and the WFG coal contains no apatite, also with kaolinite being the primary source of P. Ca-Al-silicate that contains minor P might be crandallite group minerals that cannot be identified by the CCSEM technique. It should be noted that a significant amount of P (up to 79.5%) is also present in the “unknown” minerals in the coals studied. This indicates the complex association of P with other elements. Titanium is also a major metal contaminant in the aged catalysts.81,82 It is often referred to as being among the lithophile elements and commonly associated with aluminosilicates, such as clay minerals. Another significant mode of the occurrence of Ti is rutile. As indicated in the bottom panel of Figure 10, rutile in the JZH, DT, and WFG coals is the dominant source (81) Freeman, G. B.; Adkins, B. D.; Moniz, M. J.; Davis, B. H. Appl. Catal. 1985, 15, 49. (82) Robbat, J. A.; Finseth, D. H.; Lett, R. G. Fuel 1984, 63, 1710.
Clay minerals (kaolinite, montmorillonite, and K-, Fe-, and Ca-bearing aluminosilicates), quartz, pyrite, and calcite form the bulk of mineral matter in six typical Chinese coal samples. Other minor minerals, including dolomite, ankerite, rutile, pyrrhotite, oxidized pyrrhotite, and gypsum, are also detected in some samples, with a varying content from coal to coal. The PSD of included minerals is generally finer than that of excluded ones in a certain coal. The WFG and YZH samples have a higher content of included minerals than the other coals, and the DT coal is, however, rich in excluded minerals. As a consequence, the WFG and YZH samples have more fine mineral particles than the DT coal. Kaolinite is partitioned into the included and excluded fractions randomly. On the other hand, quartz, other clay minerals (montmorillonite and K-, Fe-, and Ca-bearing aluminosilicates), carbonates (calcite and dolomite), S-bearing minerals (pyrite, pyrrhotite, oxidized pyrrhotite, and gypsum), as well as iron oxides tend to occur primarily as excluded minerals. Quartz and clay minerals are the primary source of Si, while most of the Al is associated with silicate minerals. Fe in the coals occurs mainly as iron sulfides (pyrite, pyrrhotite, and oxidized pyrrhotite), iron oxide, and Fe-Al-silicate, with a small fraction present as ankerite. S is abundant in iron sulfides, with a significant proportion in the form of gypsum in the JZH and JCH coals. Most of the Ca occurs as carbonates, except in the JZH coal, where gypsum is its primary source. A minor fraction of Ca is associated with clay minerals as well. Mg is mainly present as dolomite and clay minerals, with a minor fraction present as ankerite. Na is manly associated with kaolinite and K-bearing aluminosilicates, with a minor fraction in the form of Na-bearing aluminosilicates. Most of the K occurs as kaolinite and K-bearing aluminosilicates. P is abundant in kaolinite, with a small fraction present as Ca-bearing aluminosilicates. In the DT coal, where apatite is the most abundant, it is the major source of P. Ti is maily present as rutile and kaolinite in the coals. Acknowledgment. The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (number 50325621) and the National Key Basic Research and Development Program of China (number 2002CB211602). This work was also partially supported by the Programme of Introducing Talents of Discipline to Universities (“111” project, number B06019), China, and the Natural Science Foundation of Hubei Province (number 2006ABC002). L.Z. acknowledges the support from the Opening Foundation of the State Key Laboratory of Coal Combustion (number 200501) at Huazhong University of Science and Technology, Wuhan, China. EF060419W
