Energy & Fuels 2006, 20, 1089-1096
1089
Minerals Transformations in Northeastern Region Coals of India on Heat Treatment Samit Mukherjee* and Sunil Kumar Srivastava Fuel Science DiVision, Central Fuel Research Institute (CSIR), PO-FRI - 828 108, Dhanbad, Jharkhand, India ReceiVed May 23, 2005. ReVised Manuscript ReceiVed February 3, 2006
Studies on transformations of minerals in Baragolai, Ledo, and Tipong coals of the Makum coalfield of the northeastern region of India on heat treatment in the presence of air at 350 and 850 °C have been carried out using XRD, FTIR, DTA, TGA, and DTG. XRD patterns of the above three coals show that the samples contain both amorphous and crystalline phases. The coal samples exhibit a number of peaks. The most dominant peaks in all the coal samples are due to quartz. The coal samples also exhibit very small peaks assigned to calcite, gypsum, pyrite, marcasite, and chlorite. The XRD patterns of the coal samples are found to change on heating in the presence of air until ash is prepared at 350 °C. These samples show some new peaks assigned to kaolinite, illite, and feldspar. These are present in raw coals too but could not be detected due to the presence of these minerals in small amounts in comparison to that of the amorphous material. The large amount of amorphous carbonaceous material possibly envelops the minerals which slowly get oxidized on heating in the presence of air at 350 °C resulting in an increase in the concentrations of the minerals kaolinite, illite, and feldspar. The XRD patterns of the 350 °C heated ash sample exhibit peaks due to quartz, calcite, and marcasite. Thus, these are not affected by heat treatment of coal in the presence of air at 350 °C. There was no peak present due to gypsum because the water of crystallization was lost. Some of the peaks observed in the raw coal and the ash produced at 350 °C were found to disappear in the ash prepared at 850 °C. The XRD patterns of ash prepared at 850 °C exhibit some new peaks due to iron oxide which might have been formed as a product of thermal transformation reactions of some phases present in the coal. The sample did not show peaks due to pyrite, marcasite, and calcite. Pyrite undergoes oxidation at 370 °C and marcasite at 430-480 °C forming iron oxide and sulfur dioxide. Calcite decomposes in the range of 675-750 °C yielding calcium oxide and carbon dioxide. The 850 °C heated ash sample also did not exhibit peaks due to kaolinite and illite. Kaolinite transforms into metakaolin at 550-600 °C, and illite undergoes dehydroxylation at 450-550 °C. Quartz is the dominant phase in the coal and its ash prepared at 350 and 850 °C, i.e., no change. This is expected as R-quartz is the best known most stable crystalline form of silica, and it transforms to another crystalline form, cristobalite, through an amorphous transition phase when heated above 1470 °C for a considerable time. The FTIR spectra of the raw coal samples, ash obtained at 350 °C, and ash prepared at 850 °C were recorded and compared. The general characteristics of the FTIR spectra of all the coal samples are almost similar. On comparison of the spectra, it was observed that on heating the coal samples in the presence of air all the stretching, bending bands due to coaly matter functional groups disappear. The FTIR peaks due to presence of different functional groups of minerals support the findings of XRD summarized above. Similarly the TGA, DTA, and DTG results support our XRD findings too as stated above.
Introduction Coal consists of organic matter (macerals) and inorganic matter, i.e., minerals. The mineral matter constituents in coal may be both inherent and detrital and exist in different quantities and forms.1,2 Some of the mineral matters have originated from the inorganic constituents of the parent vegetation and become a part of the coal matrix (inherent) either during the early stages of coalification in the peat bog or at the subsequent maturation process.3 The other major inorganic constituents in coal are detrital and are due to the depositional environment, transportation by wind, erosion, and percolation through cracks or fissures. * To whom correspondence should be addressed. Phone: + 91-3262381001-10,ext.362.Fax: +91-326-2381113.E-mail:
[email protected]. (1) Brane, J. S. S., King, J. G., Eds. Fuel, Solid, Liquid and Gaseous; London, 1961; p 82. (2) Himus, G. W., Ed. Fuel Testing, Laboratory Methods in Fuel Technology; London, 1954; p 106. (3) Karr, C., Jr., Ed. Analytical Methods for Coal and Coal Products; Academic Press: New York, 1978; Vol. II, p 266.
Mineral matter formed during the first stage of coalification can alter during the second stage. During mining, large amounts of minerals can also be incorporated in the product from strata above or below the coal seam. The minerals commonly found in coals are silicates, clay minerals, such as kaolinite, illite, etc., quartz, sandstone, pyrites, and carbonates, such as siderites and ankerites. Sulfur in coal exits in the forms of pyrite, sulfate, and organic. The studies of mineral matter transformation during coal combustion have a long history and enabled the understanding of volatile minerals release from coal, chemical transformation of the minerals, and interaction of minerals with organic matter in the coal samples. Coals from various sources contain different compositions of the mineral matter; the understanding of the transformation of individual minerals forming the mineral matter is needed to model the ash deposition. The mineral matter in coal has several detrimental effects on coal utilization.4-6 It affects almost every aspect like mining,
10.1021/ef050155y CCC: $33.50 © 2006 American Chemical Society Published on Web 03/16/2006
1090 Energy & Fuels, Vol. 20, No. 3, 2006
handling, transportation, utilization, etc. Mineral matter undergoes major changes during combustion of coal and leads to atmospheric pollution. Combustion of high-sulfur coal forms SO2 which is toxic and corrosive. SO2 is subsequently converted to SO3 which in contact with water forms H2SO4. The SO2/ acid cause severe environmental problems such as corrosion of boilers, underground pipelines, metallic installations, mine machineries, etc.7 Mineral matter in coal leads to clinker formation and generation of fly ash and bottom ash.8 It also reduces the melting point and loss of sensible heat of coal during combustion. The high ash content of coal creates several engineering problems. These include corrosion and erosion of boilers, frequent down time loss, loss of heat in melting and fusion of coal ash, high maintenance cost of the boiler and other machineries, etc. Coke containing sulfur beyond a certain limit is unsuitable for metallurgical purposes. In weathered coal, oxides of sulfur percolate to groundwater making the water highly acidic. The acid mine drainage poses serious problems to the existence of flora and fauna in the environment. The high ash contents also cause problems related to utilization of coal for liquefaction, gasification, carbonization, etc. Use of coal with high mineral matter content increases the quantity of fly ash and consequently intensifies the problems of their disposal. Ash handling is also an expensive, dirty, and dusty operation causing concern for environmentalists. The mineral matter in coal however has some beneficial effect on coal liquefaction.9-11 Coal Mineral Matter and Its Transformation. The mineral matter of coal may be identified in the form of two major groups. (1) Extraneous minerals: particles containing over 90 wt % of mineral matter, separated from the organic matter as a result of the prior to combustion crushing of coal, usually of finer size, 4-7 µm (top size 40-70 µm), than the organic particles of the size up to 100 µm. (2) Inherent minerals: closely associated with organic coal particles, which do not separate prior to combustion. The content of the inherent minerals is usually below 10 wt % (mostly 2-4 wt %) in the organic particles. The present article reports the transformation of minerals in three different coals of the Makum coalfield of the northeastern region of India on heat treatment. Experimental Section The coal samples used in the investigation were collected from the Baragolai, Ledo, and Tipong collieries of the Makum coalfield, Assam, belonging to the northeastern region of India. The proximate analyses of the coal samples were done by following Indian standard methods (IS:1350 (part I), 1984). The percentage of carbon, hydrogen, and nitrogen were estimated by using a Perkin-Elmer (model 2400) elemental analyzer and total sulfur by following the Eschka method (ASTM D 3177). The percentage of oxygen was calculated by difference. The forms of sulfur were determined by following ASTM D 2492 methods. Ash analyses of the samples were done by standard chemical analysis methods.12-15 Coal ash was prepared at 350 °C (kept for 72 h) and 850 °C (kept for 1 h) (4) Steinmetz, G. L.; Mohan, M. S.; Zingaro, R. A. Energy Fuels 1988, 2, 684. (5) Merrit, R. D. J. Coal Qual. 1988, 7, 95. (6) Karr, C., Jr., Ed. Analytical Methods for Coal and Coal Products; Academic Press: New York, 1978; Vol. II, p 268. (7) Bonnet, R.; Czechechowski, F. Metals and metal complexes in coals. In New Coal Chemistry; Royal Society: London, 1981; pp 51-53. (8) Lowry, H. H., Ed. Chemistry of Coal Utilization; Wiley: New York, 1963; Supplemental Vol., pp 820-891. (9) Whitehurst, D. D., Mitchell, O. T., Farcasiu, M., Eds. Coal Liquefaction; Academic Press: 1980; p 161. (10) Mukherjee, D. K.; Chowdhury, P. B. Fuel 1976, 55, 4. (11) Gray, D. Fuel 1978, 57, 213.
Mukherjee and SriVastaVa Table 1. Characterization of Coal Samples characteristic
Baragolai
Ledo
Tipong
Proximate Analysis on an As-Received Basis (%) moisture 5.4 4.9 4.8 ash 8.4 10.4 17.6 volatile matter 41.4 (48.0)a 41.6 (49.1)a 38.6 (49.8)a fixed carbon 44.8 (52.0)a 43.1 (50.9)a 39.0 (50.2)a Ultimate Analysis on an As-Received Basis (%) carbon 68.8 (79.8)a 70.0 (82.6)a 61.9 (79.8)a hydrogen 5.1 (6.0)a 5.2 (6.1)a 4.8 (6.2)a nitrogen 1.5 (1.7)a 1.5 (1.8)a 1.3 (1.7)a sulfur 4.3 (5.0)a 4.3 (5.1)a 3.5 (4.5)a oxygen (diff) 20.3 19.0 28.5 caloric value (kcal/kg) caking index
7526 20
7326 19
7013 18
sulfur distribution (%) pyritic sulfate organic
0.64 0.52 3.11
0.52 0.41 3.38
0.51 0.46 2.49
a
Data within brackets are on a dry mineral matter free basis. Table 2. Characterization of Coal Ash (850 °C) composition (wt %) constituent
Baragolai
Ledo
Tipong
SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O TiO2 LOI
57.9 18.9 11.9 3.9 4.7 0.84 0.13 0.36 0.02 0.06
54.8 21.8 12.7 2.6 3.8 1.3 0.19 1.17 0.01 0.03
56.0 21.2 11.7 2.7 4.7 1.2 0.10 1.44 0.01 0.02
in an ordinary muffle furnace. The data on characterization of the above coal samples are presented in Table 1. The data on coal ash characterization (850 °C sample) are provided in Table 2. XRD patterns were used to characterize the minerals present in the coal and its ash. FTIR spectra were used to identify the functional groups present in coal and its ash. X-ray diffraction patterns were obtained using a Philips Analytical X-ray B.V. diffractometer. In one case (Tipong coal ash at 850 °C, Figure 3 b) the pattern is recorded in a computer-controlled X-ray diffractometer type JDX-11 P 3A, JEOL, Japan. The FTIR spectra in the range of 4000-375 cm-1 were recorded in KBr disk using an FTIR 2000 Perkin-Elmer spectrophotometer. The thermal analysis (DTA, TGA, DTG) of the samples was recorded using a SDT-2960 thermal analyzer supplied by M/S TA Corporation, U.S.A. The samples (10-20 mg) were taken in a platinum crucible and heated at the heating rate of 1012 °C per min up to 1000 °C in a nitrogen or an air atmosphere.
Results and Discussion Minerals in Coal, LTA, and Ash at 800 °C. The principal minerals found in a coal and LTA are divided into six categories,16 viz., (1) silicatessquartz (SiO2), chalcedony (SiO2); clay minerals, such as kaolinite [Al2Si2O5(OH)4], illite [K1.5Al4(Si6.5Al1.5)O20(OH)4], smectite [Na0.33(Al1.67Mg 0.33)Si4O10(OH)2], chlorite [(MgFeAl)6(AlSi)4O10(OH)8]; interstratified clay minerals, such as feldspar (KAlSi3O8, NaAlSi3O8, CaAl2Si2O8), (12) Himus, G. W., Ed. Fuel Testing, Laboratory Methods in Fuel Technology; London, 1954; pp 119-130. (13) Vogel, A. I. A Textbook of QuantitatiVe Inorganic Analysis, Including Elementary Instrumental Analysis; Longman: 1969. (14) Scott, W. W. Standard Methods of Chemical Analysis, 5th ed.; Howell, N., Furman, D., Eds.; Van Nostrand Company: 1961. (15) Kodama, K. Methods of QuantitatiVe Inorganic Analysis; Interscience Publishers: 1963. (16) Ward, C. R. Int. J. Coal Geol. 2002, 50, 135.
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Figure 1. XRD patterns of (a) Baragolai coal, (b) ash at 350 °C, and (c) ash at 850 °C.
Figure 2. XRD patterns of (d) Ledo coal, (e) ash at 350 °C, and (f) ash at 850 °C.
tourmaline[Na(MgFeMn)3Al6B3Si6O27(OH)4],analcime(NaAlSi2O6‚ H2O), clinoptilolite [(NaK)6(SiAl)36O72‚20H2O], heulandite (CaAl2Si7O18‚6H2O); (2) sulfidesspyrite (FeS2), marcasite (FeS2), pyrrhotite (Fe(1-x)S), sphalerite (ZnS), galena (PbS), stibnite (SbS), millerite (NiS); (3) phosphatessapatite [Ca5F(PO4)3], crandallite [CaAl3(PO4)2(OH)5‚H2O], gorceixite [BaAl3(PO4)2(OH)5‚H2O], goyazite [SrAl3(PO4)2(OH)5‚H2O], monazite [(Ce,La,Th,Nd)PO4], xenotime [(Y,Er)PO4]; (4) carbonatess calcite (CaCO3), aragonite (CaCO3), dolomite [CaMg(CO3)2], ankerite [(Fe,Ca,Mg)CO3], siderite (FeCO3), dawsonite [NaAlCO3(OH)2], strontianite (SrCO3), witherite (BaCO3), alstonite [BaCa(CO3)2]; (5) sulfatessgypsum (CaSO4‚2H2O), bassanite (CaSO4‚
2H2O), anhydrite (CaSO4), barite (BaSO4), coquimbite [Fe2(SO4)3‚9H2O], rozenite [FeSO4‚4H2O], szomolnokite (FeSO4‚ H2O), natrojarosite [NaFe3(SO4)2(OH)6], thenardite (Na2SO4), glauberite [Na2Ca(SO4)2], hexahydrite [MgSO4‚6H2O], tschermigite [NH4Al(SO4)2‚12H2O]; and (6) otherssanatase (TiO2), rutile (TiO2), boehmite (Al‚O‚OH), goethite [Fe(OH)3], crocoite (PbCrO4), chromite [(Fe,Mg)Cr2O4], clausthalite (PbSe), and zircon (ZrSiO4). The principal minerals identified in ash of coal at 800 °C are quartz (SiO2), cristobalite (SiO2), tridymite (SiO2), metakaolin (Al2O3‚2SiO2), mullite (Al6Si2O13), albite (NaAlSi3O8), anorthite (CaAl2Si2O8), sanidine (KAlSi3O8), corundum (Al2O3), pyrrhotite (Fe(1-x)S), oldhamite (CaS), anhydrite (CaSO4), aragonite (CaCO3), vaterite (CaCO3), portlandite [Ca(OH)2], lime (CaO), periclase (MgO), wuestite (FeO), hematite (Fe2O3), maghemite (Fe2O3), magnetite (Fe3O4), spinel (MgAl2O4), magnesioferrite (MgFe2O4), calcium ferrite (CaFe2O4), srebrodolskite (Ca2Fe2O5), brownmillerite (Ca4Al2Fe2O10), wollastonite (CaSiO3), gehlenite (Ca2Al2SiO7), merarinite [Ca3Mg(SiO4)2], melitite (Ca4Al12MgSi3O14), and whitlockite [Ca3(PO4)2]. X-ray Diffraction Analysis. The X-ray diffraction patterns of the raw coal samples as well as after heating the same at 350 °C in air for 72 h and at 850 °C in air for 1 h are shown in Figures 1-3. The peaks are assigned in accordance with those reported in the literature,17-21 Joint Committee on Powder Diffraction Standards (JCPDS)22 and the International Centre for Diffraction Data (ICDD) powder diffraction file.23 The X-ray diffraction spacings of commonly occurring coal minerals are presented in Table 3. XRD patterns show that the samples contain both amorphous and crystalline phases. The most dominant peaks in all the coal samples are at d ) 3.32-3.30 Å, which has been assigned to quartz. The samples also exhibit other small peaks due to quartz at or near d ) 2.46, 2.27, 2.25, 2.12, 1.98, 1.81, 1.53, 1.37, and 1.22 Å. Some samples exhibit prominent peaks at d ) 4.21-4.19 Å, and all of them are assigned to quartz. Some of the samples exhibit very small peaks at or near d ) 3.04, 2.29, 2.11, and 1.90 Å, which have been assigned to calcite, at d ) 7.55 and 3.06 Å, which are due to gypsum, and at d ) 3.11, 2.69, 2.43, 1.63, 1.43, 1.25, and 1.21 Å, which are due to pyrite. The samples also exhibit peaks at d ) 2.05, 1.90, and 1.68 Å, which are due to marcasite, and at d ) 3.53 and 2.53 Å, which are due to chlorite. The XRD patterns of the coal samples are found to change on heating. Ash samples prepared at 350 °C show some new peaks at around d ) 7.09, 2.55, 2.37, 2.33, 1.86, 1.67, 1.66, 1.57, and 1.49 Å, which have been assigned to kaolinite, and at around d ) 5.02, 3.33, and 2.56 Å, which have been assigned to illite. Some of the ash samples exhibit very small peaks at around d ) 3.20-3.18 Å, which may be due to feldspar. These peaks could not be detected in the original coal samples possibly due to presence of minerals in small amounts. The large amount of amorphous carbonaceous material possibly envelops the minerals. The carbonaceous material slowly gets oxidized on heating to 350 °C in the presence of air for 72 h resulting in increase in concentration of the minerals.
(17) Crystal Structure of Clay Minerals and their X-ray Identification; Mineralogical Society Monogram No. 5; Brindley, G. W., Brown, G., Eds.; 1980. (18) Vander Marel, H. W. Soil Sci. 1950, 70, 109. (19) Grim, R. E. Clay Mineralogy, 2nd ed.; McGraw Hill: 1968. (20) Karr, C., Jr., Ed. Analytical Methods for Coal and Coal Products; Academic Press: New York, 1978; Vol. II, p 275. (21) Acharya, B. S. Fuel 1992, 71, 346. (22) Powder Diffraction File; Joint Committee on Powder Diffraction Standards: PA, 1971. (23) Powder Diffraction File, Alphabetical Indexes, Inorganic Phases; International Centre for Diffraction Data: 1966; Sets 1-46.
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Figure 3. (a) XRD patterns of (g) Tipong coal and (h) ash at 350 °C. (b) XRD pattern of Tipong coal ash at 850 °C. Table 3. X-ray Diffraction Spacing of Commonly Occurring Coal Mineralsa mineral kaolinite illite montmorillonite chlorite mixed layer illite montmorillonite calcite dolomite siderite aragonite pyrite quartz
gypsum rutile feldspars a
diffraction spacing (Å) 7.09-7.17 (100), 4.36 (6), 4.19 (5), 3.57 (80), 2.50 (5), 2.55 (3), 2.49 (3), 2.38 (25), 2.33 (4), 1.92 (20), 1.91 (5), 1.90 (25), 1.87 (20), 1.84 (25), 1.81 (20), 1.79 (25), 1.71 (25), 1.69-1.63 (25), 1.62 (7), 1.59 (6), 1.49 (9) 10.0-10.08 (9), 5.02 (5), 4.98 (60), 4.48 (2), 3.32 (100), 2.57 (8), 2.01 (5) 12.0-15.0 (100), 5.01 (6), 4.50 (8), 1.50 (6) 14.3-14.0 (100), 7.20-7.18 (40), 4.78-4.68 (60), 3.60-3.50 (60), 2.60-2.55 (20) 10.0-14.0 (100), 7.24 (30) 3.86 (1), 3.04 (100), 2.84 (3), 2.50 (1), 2.29 (18), 2.10 (18), 1.93 (5), 1.91 (2), 1.88 (2), 1.63 (4), 1.60 (1), 1.52 (1), 1.47 (2), 1.44 (5), 1.42 (3), 1.36-1.23 (1), 1.18-1.02 (1), 0.99-0.96 (2) 2.88 (100), 2.19 (30) 3.59 (60), 2.79 (100), 2.35 (50), 2.13 (60) 3.40 (100), 3.27 (52), 1.98 (65) 3.13 (35), 2.71 (85), 2.42 (65), 2.21 (50), 1.91 (40), 1.63 (100), 1.56 (14), 1.50 (20), 1.44 (25), 1.24 (12), 1.21 (14), 1.18 (8), 1.15 (6), 1.10 (6), 1.04 (25), 1.01 (8), 0.99-0.80 (6) 4.26 (35), 3.34 (100), 2.46 (12), 2.28 (12), 2.24 (6), 2.13 (9), 1.98 (6), 1.82 (17), 1.80 (
