Energy Fuels 2010, 24, 834–843 Published on Web 01/05/2010
: DOI:10.1021/ef900947y
Mineralogy and Chemical Composition of High-Calcium Fly Ashes and Density Fractions from a Coal-Fired Power Plant in China Yongchun Zhao,†,‡ Junying Zhang,*,† Chong Tian,† Hailong Li,† Xinyu Shao,‡ and Chuguang Zheng† †
State Key Laboratory of Coal Combustion, ‡School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Received August 29, 2009. Revised Manuscript Received November 26, 2009
To understand the formation mechanism of high-calcium fly ashes, the mineralogical, physical, and chemical properties of several high calcium fly ashes and their different density fractions (2.89 g/cm3) from a coal-fired power plant were characterized by X-ray diffractometry (XRD), field scanning electron microscopy equipped with energy dispersive X-ray analysis (FSEM-EDX), and X-ray fluorescence spectroscopy (XRF). The occurrence of calcium in coal was determined using sequential extraction tests. The results show that the carbonate-bonded calcium is the dominant species in Xiaolongtan coal, and the ionexchangeable calcium only occupies 19.2% of total calcium. The major calcium-bearing minerals in low temperature ash (LTA) of the feed coal, lignite from the Yunnan province, include calcite, bassanite, and dolomite. The fly ashes examined contained aluminosilicates with a high concentration of calcium oxide. The major minerals include mullite, quartz, lime, anhydrite, and gehlenite, and the minor minerals are comprised of hematite, magnetite, akermanite, portlandite, and larnite. Minerals in the density faction less than 1.0 g/cm3 consist of lime, calcite, anhydrite, and clay; between 1.0-2.5 g/cm3, quartz, mullite, anhydrite, and gehlenite; between 2.5-2.89 g/cm3, anhydrite, lime, gehlenite, hematite, and quartz; and greater than 2.89 g/cm3, larnite, gehlenite, anhydrite, brownmillerite, and some heavy minerals. In accordance with the microstructural characteristics of the fly ash particles, high-calcium fly ash can be classified into several groups, namely hollowed smooth particles, dense particles, agglomerate particles, porous particles, plerosphere, and other particles with complex surface characteristics. On the basis of chemical composition, high-calcium fly ashes can be classified into four groups namely: calcium oxide, calcium sulfates, Ca-Al-Si compounds, and Ca-S-X (X: Fe, Al, Si, Mg, etc.) compounds. Calcium oxide and calcium sulfates are mainly derived from the original calcium-bearing minerals in coal, whereas Ca-Al-Si and Ca-S-X compounds are formed by the secondary reaction of CaO and CaSO4.
helpful for the utilization of fly ash,4,5 and also for understand the mineral behavior during coal combustion in a boiler. Many researchers have studied the physical and chemical characteristics of fly ashes by different analytical methods.1,6-19
1. Introduction Coal-fired electric power generation is still the main electricity supply in China; a large amount of coal combustion produced lots of fly ash, which caused serious environmental pollution and occupied a large area of ground. At the present, the fly ash production from coal-fired power plants is about 160 Mt annually in China, but the utilization of coal ash amounted to only 70 Mt.1 At the same time, with the continual increase of coal consumption, lots of low-quality coals, including lignite, have been used in power plants in China. The low-rank lignite combustion not only produced a large amount of fly ash with high-calcium content, but also made significant impact on safe boiler operation.2 High-calcium fly ash (with a higher CaO content limited to 15-30 wt %) is largely utilized as an admixture for concrete, and as a filler, due to its cementing properties.3 Glassy spheres, lime, and anhydrite are the most prominent reactive phases.4 Most research has indicated that a detailed knowledge about the mineralogical and chemical composition of fly ashes is more
(5) Tsimas, S.; Moutsatsou-Tsima, A. Cem. Concr. Compos. 2005, 27 (2), 231–237. (6) Goodarzi, F. Fuel 2006, 85 (10-11), 1418–1427. (7) Hower, J. C.; Robertson, J. D.; Thomas, G. A.; Wong, A. S.; Schram, W. H.; Graham, U. M.; Rathbone, R. F.; Robl, T. L. Fuel 1996, 75 (4), 403–411. (8) Koukouzas, N.; Hamalainen, J.; Papanikolaou, D.; Tourunen, A.; Jantti, T. Fuel 2007, 86 (14), 2186–2193. (9) Lecuyer, I.; Bicocchi, S.; Ausset, P.; Lefevre, R. Waste Manage. Res. 1996, 14 (1), 15–28. (10) Moreno, N.; Querol, X.; Andres, J. M.; Stanton, K.; Towler, M.; Nugteren, H.; Janssen-Jurkovicova, M.; Jones, R. Fuel 2005, 84 (11), 1351–1363. (11) Pires, M.; Querol, X. Int. J. Coal Geol. 2004, 60 (1), 57–72. (12) Tishmack, J. K.; Olek, J.; Diamond, S. Cem., Concr. Aggregates 1999, 21 (1), 82–92. (13) Vassilev, S. V.; Menendez, R. Fuel 2005, 84 (7-8), 973–991. (14) Vassilev, S. V.; Menendez, R.; Alvarez, D.; Diaz-Somoano, M.; Martinez-Tarazona, M. R. Fuel 2003, 82 (14), 1793–1811. (15) Vassilev, S. V.; Menendez, R.; Borrego, A. G.; Diaz-Somoano, M.; Rosa Martinez-Tarazona, M. Fuel 2004, 83 (11-12), 1563–1583. (16) Vassilev, S. V.; Menendez, R.; Diaz-Somoano, M.; MartinezTarazona, M. R. Fuel 2004, 83 (4-5), 585–603. (17) Vassilev, S. V.; Vassileva, C. G.; Karayigit, A. I.; Bulut, Y.; Alastuey, A.; Querol, X. Int. J. Coal Geol. 2005, 61 (1-2), 35–63. (18) Vassilev, S. V.; Vassileva, C. G.; Karayigit, A. I.; Bulut, Y.; Alastuey, A.; Querol, X. Int. J. Coal Geol. 2005, 61 (1-2), 65–85. (19) Dai, S.; Zhao, L.; Peng, S.; Chou, C.-L.; Wang, X.; Zhang, Y.; Li, D.; Sun, Y., Int. J. Coal Geol. 2009, DOI:10.1016/j.coal.2009.03.005
*To whom correspondence should be addressed. Fax: 86-2787545526. E-mail:
[email protected]. (1) Koukouzas, N. K.; Zeng, R.; Perdikatsis, V.; Xu, W.; Kakaras, E. K. Fuel 2006, 85, 2301–2309. (2) Huffman, G. P.; Huggins, F. E.; Shah, N.; Shah, A. Prog. Energy Combust. Sci. 1990, 16 (4), 243–251. (3) Grzeszczyk, S.; Lipowski, G. Cem. Concr. Res. 1997, 27 (6), 907– 916. (4) Enders, M. Cem. Concr. Res. 1995, 25 (6), 1369–1377. r 2010 American Chemical Society
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: DOI:10.1021/ef900947y
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Table 1. Characteristics of Xiaolongtan Coal (wt %) proximate analysis (air-dried basis)
ultimate analysis (air-dried basis)
M
VM
Ash
FC
C
H
N
S
O
14.99
39.75
17.86
27.4
45.4
4.3
1.2
1.9
14.4
Ash Chemical Composition Na2O
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
TiO2
MnO
Fe2O3
1.67
3.05
15.61
34.65
1.11
3.77
0.91
30.88
0.92
0.04
7.38
plant (6 100 MW) in Yunnan Province in China. The plant was a typical pithead power plant, and its feed coal was Xiaolongtan lignite. The proximate and ultimate analyses are presented in Table 1. To understand the occurrence of calcium in coal, 1 g of Xiaolongtan coal particles was subjected to sequential leaching first with deionized water followed by 50 mL of 1 N ammonium acetate (NH4OAc), and finally with 50 mL of 1:3 hydrochoric acid (HCl). Each sample was agitated at room temperature in each of the solvents for 18-24 h. After leaching with each solvent, the filtrate was analyzed by inductively coupled plasma spectroscopy (ICP) and the residue was examined by X-ray fluorescence spectroscopy (XRF). The fly ash samples (FA51 and FA60) were collected from the hoppers of electrostatic precipitators (ESP) of different units in the power plant. FA51 was collected from the common unit, and FA60 was obtained from the unit that contained the flue gas desulfurization equipment before the ESP. To understand the distribution of calcium-bearing minerals and phases, the different density fractions (cenosphere, 2.89 g/cm3) of fly ashes were separated by sink-float experiments using different solutions, including distilled water (1.0 g/cm3), dibromomethane (CH2Br2 with a density of 2.5 g/cm3), and bromoform (CHBr3 with a density of 2.89 g/cm3). After separation, each fraction was washed and dried in an electrical furnace at 150 °C for 8 h. 2.2. Analytical Techniques. Major and minor minerals in the coal and fly ashes were identified using X-ray diffraction (XRD). XRD studies were carried out on a χ’Pert PRO diffractometer equipped with a graphite diffracted-beam monochromator. The accelerating voltage was 40 kV and the current was 40 mA. Diffraction patterns were collected at 5-70° 2θ using Cu KR radiation. The scans have an increment of 0.017° and a counting time of 10 s per step. Final semiquantitative XRD analysis was performed using the reference intensity method (RIM) described by Chung.27,28 The investigations by field emission scanning electron microscopy (FSEM) were carried out on a Sirion200 microscope equipped with a GENESIS energy dispersive X-ray spectroscope (EDX). FSEM-EDX was used to study the morphology and composition of a single high-calcium fly ash particle. The chemical compositions of the bulk ashes and fractions were determined directly in solid samples by X-ray fluorescence spectrometry (XRF).
Fly ashes are a heterogeneous mixture of mineral phases, amorphous glassy phases, and a little of organic unburned carbon.4 More than 188 minerals in fly ash have been identified, and most of them are accessory or trace minerals.20,21 Using X-ray diffractometry (XRD) Tishmack et al. studied the mineralogical characterizations of six high-calcium fly ashes originating from Powder River Basin coal.12 Bayat has investigated the various (mineralogical, morphological, physical, and chemical) properties of fly ashes produced in several Turkish coal-fired power plants.22 Enders studied the CaO distribution of mineral phases in a high-calcium fly ash from Germany.23 The reactivity of high-calcium fly ash was controlled by the occurrence of reactive mineral phases. Reactive mineral phases comprised mainly Ca-bearing mineral phases. Giergiczny used differential thermal analysis and thermogravimetry to evaluate the reactivity of high-calcium fly ash in mixtures with cements.24 He found that the chemical and phase composition was variable and related to the particle size. Antiohos and Tsimas investigated the impact of reactive silica on the activity of high-calcium fly ash.25 Bosbach and Enders studied the microstructures of glassy spheres from high-calcium fly ash, and found that many nanometre-sized anorthite, gehlenite, anhydrite, and magnetite crystals were attached to the glassy spheres.26 The glassy matrix was quite heterogeneous at the lower nanometer scale. However, all of the above researchers studied the bulk fly ash, and these methods cannot differentiate between glassy spheres of varying composition. Recently, Vassilev et al. published a comprehensive study on the phase mineral composition of separated fractions of fly ashes from the power plants in Europe.13,15,16,18 However, little study has been done on the mineralogy and microstructure of density-separated fractions of fly ashes from Chinese coal combustion. The present paper reports our extensive investigation on the occurrence of calcium in coal, the characteristics of calciumbearing minerals, chemical composition, and morphology of high-calcium fly ashes, especially the density fractions of ashes obtained from pulverized coal-fired power plants in China. The purpose is to characterize and elucidate the detailed properties of high-calcium fly ash particles and provide new insight into high-calcium fly ash formation and utilization in China. 2. Samples and Analytical Procedures 2.1. Samples. The coal samples used for this study were collected from the Xiaolongtan pulverized coal-fired power (20) Finkelman, R. B. Modes of Occurrence of Trace Elements in Coal; No. OFR-81-99; U.S. Geological Survey Open-File Report: 1981; p 322. (21) Vassilev, S. V.; Vassileva, C. G. Energy Fuels 2005, 19 (3), 1084– 1098. (22) Bayat, O. Fuel 1998, 77 (9-10), 1059–1066. (23) Enders, M. Cem. Concr. Res. 1996, 26 (2), 243–251. (24) Giergiczny, Z. J. Therm. Anal. Calorim. 2006, 83 (1), 227–232. (25) Antiohos, S.; Tsimas, S. Cem. Concr. Compos. 2005, 27 (2), 171– 181. (26) Bosbach, D.; Enders, M. Adv. Cem. Res. 1998, 10 (1), 17–23.
3. Results and Discussion 3.1. Occurrence of Calcium in Coal. The occurrence of calcium in coal plays an important role on its transformation during coal combustion. Calcium in coal can be classified (27) Chung, F. H. J. Appl. Crystallogr. 1974, 7, 519–531. (28) Chung, F. H. J. Appl. Crystallogr. 1975, 8, 17–19.
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mineral in coal, as well as bassanite, may be weathering products from carbonates.31 Calcium-containing silicate minerals are mainly detrital and are rarely authigenic minerals or weathered products of various detrital, syngenic, and epigenetic minerals in coals.32 These minerals as well as calcium containing phosphates and other calcium-bearing minerals are rarely found in coals. To identify the minerals of coals directly, the mineral matter was isolated from the coal samples by low-temperature oxygen-plasma ashing, and the yield of low-temperature ash (LTA) was determined as a fraction of the air-dried coal in each case.36 The LTA is a useful method for releasing the minerals that interacted with the organic fraction.37 After the low temperature ashing, the LTA of the feed coal was analyzed by XRD. The main mineral phases present in the LTA of coal are quartz, kaolinite, illite, pyrite, calcite, bassanite, dolomite, and anatase (Figure.2). Calcium-bearing carbonates mainly include calcite and dolomite, which often occur as individually discrete particles in coals. Calcite of syngenetic origin often occurs individually as grains, “roses”, and lenses. However, in others cases epigenetic calcite commonly occurs as veinlets, filling cavities, or as a cement of fractured coal fragments.31,32 Calcium-bearing sulfates mainly comprise bassanite, which may be a weathering product. The bassanite in LTA can also be derived from the dehydration of gypsum, or the combination of sulfur and calcium in the organic matter and water during low temperature ashing process.37,38 3.3. Calcium-Bearing Minerals in Fly Ashes. Table 3 presents the common calcium-bearing minerals and phases in fly ashes that were compiled from various sources.10,14,22,30,38-41 The main categories of calcium-bearing minerals in fly ash include oxides, aluminosilicates, and sulfates. Other calciumbearing minerals are in accessory or trace amount. Calcium oxide is the characteristic phase of the coal combustion product. Sulfate may derive from the dehydration of gypsum in coal. Calcium aluminosilicates are the most common calcium-bearing minerals in fly ash and may be derived from the reaction between Ca and other components. Most calcium-bearing silicates are formed from reactions between lime and quartz, amorphous Si and Al, and the spinel-like phase, especially in high-calcium lignites.41 The fly ashes and density fractionated samples were examined using XRD and are given in Figures 3 and 4. The relative proportions of the minerals in each sample, which are evaluated by semiquantitative estimation based on the RIM and Rietveld methods,42-44 are given in Table 4. The major phase present in the fly ashes is glass phase. The major minerals include quartz, mullite, lime, anhydrite, and gehlenite, and the minor minerals are akermanite, hematite, and magnetite. Larnite and brownmillerite were identified only in sample FA51, and calcite and portlandite were found
Figure 1. Occurrence of calcium in Xiaolongtan lignite.
into two parts: discrete mineral matter and organically associated cation.29 Chemical leaching by aqueous solution of ammonium acetate (NH4OAc) was thought to leach only organically associated cations and leave mineral matter intact in low rank coals. Generally, in lignite, calcium is molecularly dispersed in the coal macerals and is bonded to the oxygen anions in carboxyl groups.2 However, the occurrence of calcium in Xiaolongtan lignite is different (Figure 1). Carbonate-bonded calcium is the dominant species in Xiaolongtan coal; the ion-exchangeable calcium only accounts for 19.2% of total calcium. Calcium in water-soluble and residual fractions is 22.7 and 23.6%, respectively. The difference of calcium speciation between Xiaolongtan lignite and other typical American lignites may reflect their complex formation processes. 3.2. Calcium-Bearing Minerals in Coals. More than 316 minerals have been identified in coal, and most of them are accessory or trace minerals.20,21 The calcium-bearing minerals found in coal are listed in Table 2 (compiled from various sources).14,30-32 The categories of calcium-bearing minerals include carbonates, sulfates, silicates, phosphates, and others. Carbonate minerals in coal are mostly authigenic.32 Kortenski (1992) investigated the occurrence and types of carbonate minerals in different ranks of coals from Bulgaria and found that carbonate mineralization depended on the penetration of ready mineral forms and mineralized solutions during peat genesis and the infiltration of mineralized solutions during coalification.33 Calcite and dolomite are the most important and common calcium-bearing minerals in coal.34 Dai et al. investigated the mineralogy of Late Permian coal in Dafang Coalfield, and they found it has a high content of ankerite (up to 10.2 vol %), which was derived from low-temperature calcic hydrothermal fluids.35 The other calcium-bearing carbonates are uncommon and occur in trace amounts in coal. Sulfate minerals in coal have an authigenic and mainly epigenetic origin, and sometimes they may originate from weathering.32 Gypsum, a typical sulfate
(36) Ward, C. R.; Bocking, M.; Ruan, C.-D. Int. J. Coal Geol. 2001, 47 (1), 31–49. (37) Gluskoter, H. J. Fuel 1965, 44, 285–291. (38) Nankervis, J. C.; Furlong, R. B. Fuel 1980, 59 (6), 425–430. (39) Karayigit, A. I.; Onacak, T.; Gayer, R. A.; Goldsmith, S. Appl. Geochem. 2001, 16 (7-8), 911–919. (40) Ural, S. J. Hazard. Mater. 2005, 119 (1-3), 85–92. (41) Vassileva, C. G.; Vassilev, S. V. Fuel Process. Technol. 2005, 86 (12-13), 1297–1333. (42) Mandile, A. J.; Hutton, A. C. Int. J. Coal Geol. 1995, 28 (1), 51–69. (43) Ward, C. R.; French, D. Fuel 2006, 85 (16), 2268–2277. (44) Ward, C. R.; Taylor, J. C.; Matulis, C. E.; Dale, L. S. Int. J. Coal Geol. 2001, 46 (2-4), 67–82.
(29) Matsuoka, K.; Rosyadi, E.; Tomita, A. Fuel 2002, 81 (11-12), 1433–1438. (30) Zhao, Y.; Zhang, J.; Sun, J.; Bai, X.; Zheng, C. Energy Fuels 2006, 20 (4), 1490–1497. (31) Vassilev, S. V.; Yossifova, M. G.; Vassileva, C. G. Int. J. Coal Geol. 1994, 26 (3-4), 185–213. (32) Vassilev, S. V.; Vassileva, C. G. Fuel Process. Technol. 1996, 48, 85–106. (33) Kortenski, J. Int. J. Coal Geol. 1992, 20 (3-4), 225–242. (34) Maes, I. I.; Gryglewicz, G.; Yperman, J.; Franco, D. V.; Mullens, J.; Van Poucke, L. C. Fuel 1997, 76 (2), 143–147. (35) Dai, S.; Chou, C.-L.; Yue, M.; Luo, K.; Ren, D. Int. J. Coal Geol. 2005, 61 (3-4), 241–258.
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Table 2. Calcium-Bearing Minerals in Coals14,30-32 category carbonates
sulphates silicates
phosphates others (rarely occur in coals)
a
mineral
chemical formula
calcite dolomite ankerite ferrodolomite aragonite manganocalcite barytocalcite gypsum bassanite allanite actinolite amphibole feldspar garnet hornblende zeolite plagioclase apatite calcium ferrite srebrodolskite brownmillerite scheelite
CaCO3 CaMg(CO3)2 Ca(MgFe)(CO3)2 Ca(Fe, Mg)[CO3]2 CaCO3 (Ca,Mn)CO3 BaCa(CO3)2 CaSO4 3 2H2O CaSO4 3 0.5H2O (Ca, Ce)2(Fe3þ,Fe2þ)Al2[SiO4][Si2O7]O(OH) Na2Ca4(Mg,Fe)10[Si4O11]4O2[OH]2 Ca2Fe2Al2Si2O4 Ca3(Cr, Al, Fe)2Si3O12 CaNa(Mg,Fe)(Al,Fe,Ti)SiO(OH,F) Ca5F(PO4)3 CaFe2O4 Ca2Fe2O5 Ca4Al2Fe2O10 CaWO4
relative abundancea M-A M T T T T T M T T T T T T T T T T T T T T
A, abundant, >3 wt %; M, minor, 1-3 wt %; T, trace, 3 wt %; M, minor, 1-3 wt %; T, trace, 2.89 g/cm3 ash (about 0.8-5.1 wt %) contains mainly larnite, brownmillerite, anhydrite, and some heavy minerals, namely, hematite and magnetite, most of them coming from the conversion of authigenic minerals in the coal. The complex composition is the result of melting and the combination of different minerals during coal combustion. Within the density fractions, we found some uncommon minerals that were not found in the fly ashes, including oldhamite in the cenosphere; andradite in the light ash; and larnite, brownmillerite, and grossular in the sunken ash. Although the mineral phases in fly ash cannot be separated completely by flotation, the density fractionation method is still a useful method for separating and partly identifying the components.
(46) Kolay, P. K.; Singh, D. N. Cem. Concr. Res. 2001, 31 (4), 539– 542. (47) Anshits, A. G.; Kondratenko, E. V.; Fomenko, E. V.; Kovalev, A. M.; Anshits, N. N.; Bajukov, O. A.; Sokol, E. V.; Salanov, A. N. Catal. Today 2001, 64 (1-2), 59–67. (48) Blanco, F.; Garcia, P.; Mateos, P.; Ayala, J. Cem. Concr. Res. 2000, 30 (11), 1715–1722. (49) Ngu, L.-n.; Wu, H.; Zhang, D.-k. Energy Fuels 2007, 21 (6), 3437– 3445. (50) Sokol, E. V.; Maksimova, N. V.; Volkova, N. I.; Nigmatulina, E. N.; Frenkel, A. E. Fuel Process. Technol. 2000, 67 (1), 35–52. (51) Vassilev, S. V.; Vassileva, C. G. Fuel Process. Technol. 1996, 47 (3), 261–280. (52) Huffman, G. P.; Huggins, F. E.; Levasseur, A. A.; Durant, J. F.; Lytle, F. W.; Greegor, R. B.; Mehtat, A. Fuel 1989, 68 (2), 238–242. (53) Fernandez-Turiel, J. L.; Georgakopoulos, A.; Gimeno, D.; Papastergios, G.; Kolovos, N. Energy Fuels 2004, 18 (5), 1512–1518.
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Figure 3. X-ray diffraction patterns of high-calcium fly ashes (cps = counts per second) (mineral abbreviations: A = anhydrite; Ak = akermanite; B = brownmillerite; C = calcite; G = gehlenite; H = hematite; L = lime; La = larnite; Ma = magnetite; Mu = mullite; P = portlandite; Q = quartz).
product of the reaction of the CaO and SiO2 system is C2S, the other silicates such as C3S and CS being formed by the combination of C2S and other reactants.55 The XRD results showed that larnite is contained in some fly ash samples (Table 4), which proves that speculation. Hurley and Schobert indicated that the source of the calcium used to form calcium silicate was most likely an organically associated form.56 (3) The calcium aluminosilicate minerals and amorphous phases that comprise most of the high calcium fly ash, are probably the result of coalescence of minerals during combustion, as well as the reaction between organically bound calcium and minerals in the coal.56 The typical Ca-Al-Si minerals, including gehlenite (C2AS) and anorthite (CAS2), etc., may originate from the reaction between CaO derived from calcium-bearing minerals and clay minerals. These minerals are commonly found in high-calcium fly ash. (4) The Ca/Al/Si/X (X: Fe, Mg, et al.) components include brownmillerite (C4AF), crossular ferroan (C3AS3-F), etc. These compounds often occur as dense particles that are found in sunken ash. These Ca/Al/Si/X amorphous phases may derive from the fusion and coalescence of minerals in coal. The Ca-S-X (X: Si, Al, Fe, et al.) type (Figure 5d) may form through the sulfurization of calcium-bearing minerals, or they may be derived from the reaction of calcium sulfates and aluminosilicates. These types of particles have a rough surface, with a diameter of 5-10 μm. Some typical components include CaS, calcium sulfosilicate (2C2S 3 CaSO4), and
calcium is mainly associated with oxygen and sulfur or phosphorus.54 As shown in Figure 5c, the particles are derived from a combination of different components and coalescence of minerals. The spatial distribution of reactive and inert mineral phases attached to fly ash particle surfaces provide information about their high-temperature formation, which may be relevant to their subsequent reactivity.26 The EDX analysis shows that this type of particles contains >70% CaþSiþAl and also includes a small amount of Fe, Mg, and K impurities. Particles of this type differ in composition from each other. In a study of the microchemistry of glassy spheres in lignite fly ash,4 Enders found that the origin of the glassy spheres was from kaolinite and that with decreasing CaO content the reactivity of the glassy spheres decreased. On the basis of the major-element contents, the calcium aluminosilicates can be classified into four groups: (1) The calcium aluminates, such as mayenite (C12A7), dicalcium aluminate (C2A), tricalcium aluminate (C3A), and so on often occur as cubic crystalline grains. These may be derived from heating calcium oxide and alumina together at high temperatures. The stoichiometric ratio of calcium and aluminum in crystal in fly ash are irregular, so they can easily form calcium and aluminum hydroxide through hydration, which is valuable for fly ash utilization. (2) The calcium silicates include mainly larnite (C2S), alite (C3S), and some other silicates; Liu thought that the initial (54) Kutchko, B. G.; Kim, A. G. Fuel 2006, 85 (17-18), 2537–2544. (55) Liu, H. Heterogeneous Reaction Mechanism and Modeling of Inorganic Constituents during Coal Combustion with Desulfurization and Solid Residues Utilization; Huazhong University of Science and Technology: Wuhan, 2006.
(56) Hurley, J. P.; Schobert, H. H. Energy Fuels 1993, 7 (4), 542–553.
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Figure 4. X-ray diffraction patterns of separated fraction from fly ashes (cps = counts per second) a: cenosphere (2.89 g/cm3) (mineral abbreviations: A = anhydrite; Ad = andradite; Ak = akermanite; B = brownmillerite; C = calcite; G = gehlenite; Gr = grossular; Gy = gypsum; H = hematite; L = lime; La = larnite; Ma = magnetite; Mu = mullite; O = oldhamite; P = portlandite; Q = quartz;). Table 4. Mineral Composition and Semiquantitative Content of Different Density Fractions Fly Ashes (wt %) bulk FA density amorphous mineral matter quartz mullite hematite magnetite lime anhydrite gehlenite akermanite calcite larnite brownmillerite andradite grossular portlandite gypsum oldhamite
2.63 3.16 5.29 5.22 3.37 2.96 3.02 3.18 2.71 3.33 3.68 3.9 3.57 2.24 2.31 2.61
cenosphere
light ash
heavy ash
sunken ash
FA51
FA60
FA51
FA60
FA51
FA60
FA51
FA60
FA51
FA60
82 18 19 11 5 4 19 13 12 8
80 20 13 11 4 4 11 10 12 9 10
77 23 11 15 4
74 26 2 14 3
82 18 35 21 4
79 21 18 13
80 20 20 12 6
83 17 2
85 15
42 15
40 29
7 14 9
7 6 9 18
10 6 8 14
11
12
3
79 21 8 9 6 2 5 13 12 6 13
19 19 12 8 4
4 5 4 31
