1556
Energy & Fuels 2006, 20, 1556-1564
Analytical and Characterization Studies of Organic and Inorganic Species in Brown Coal G. Domazetis,* M. Raoarun, and B. D. James Department of Chemistry, La Trobe UniVersity, Victoria 3086, Australia
J. Liesegang, P. J. Pigram, and N. Brack Department of Physics, La Trobe UniVersity, Victoria 3086, Australia
R. Glaisher Department of Applied Physics, La Trobe UniVersity, Bendigo, Victoria 3552, Australia ReceiVed July 21, 2005. ReVised Manuscript ReceiVed February 20, 2006
Detailed studies have been carried out on the distribution of organic functional groups and inorganic species in as-received (ar) and acid-washed (aw) brown coals using elemental analysis, energy dispersive X-ray analysis (SEM-EDX), X-ray photoelectron spectroscopy (XPS), and Time-of-flight-secondary ion mass spectrometry (TOF-SIMS). Surface concentrations of the various carbon groups, organic oxygen, and inorganic hydroxide were obtained using XPS, but oxygen from clay and quartz, if present, interfered with organic oxygen determinations for the coals. A comparison of ar and aw coals using XPS and SEM-EDX is provided in terms of inorganic and organic sulfur groups. Chloride in these coals is present mainly as acid extractable forms, but small amounts of chloride in the organic matrix were indicated by the elemental analysis of ultra low-ash coals. TOF-SIMS fragments from brown coals were indicative of polymers consisting mainly of single aromatic groups linked by hydrocarbons with carboxyl and phenol functional groups. Sulfur fragments were from inorganic sulfur, thiols, organo-sulfates, and S-N-organic species. Numerous fragments containing organically bound chloride were observed. Fragments of the inorganic species Na, Mg, Al, Si, K, Ca, Ti, Cr, Fe, Mn, Ni, Cu, and Ga were also observed. Environmentally undesirable species, particularly from organo-sulfur and organochloride groups in brown coal, are likely to emerge from processes that heat coal-water mixture.
Introduction Brown coal is a heterogeneous organic substance containing oxygen, sulfur, nitrogen, chloride, and inorganic species at concentrations ranging from a percentage of the coal weight down to parts per million. Increasingly, more information is needed on the major and minor constituents that are characteristic of brown coal to ensure its efficient and clean utilization.1-6 For example, the combined effects of volatile inorganic species, including chloride and sulfur, may cause corrosion, fouling, and emissions of pollutants, while trace elements are likely to have an adverse environmental impact. The constituents in brown coal also impact on environmental aspects of coal process technologies; for example, treatment processes that heat coal and water mixtures to high temperatures, such as the hydrothermal process, release a variety of water-soluble organic * Corresponding author. E-mail:
[email protected]. Fax: +61 3 9479 1399. Tel: +61 3 9479 2811. (1) Vassilev, S. V.; Tasco´n, J. M. D. Energy Fuels 2003, 17, 271-281. (2) Benson, S. A.; Hurley, J. P.; Zygarlicke, C. J.; Steadman, E. N.; Erickson, T. A. Energy Fuels 1993, 7, 746-754. (3) Srinivasachar, S.; Helble, J. J.; Ham, D. O.; Domazetis, G. Prog. Energy Combust. Sci. 1990, 16, 303-309. (4) Pavageau, M.-P.; Pea´cheyranc, C.; Krupp, E. M.; Morin, A.; Donard, O. F. X. EnViron. Sci. Technol. 2002, 36, 1561-1573. (5) Danihelka1, P.; Volna, Z.; Jones, J. M.; Williams, A. Int. J. Energy Res. 2003, 27, 1181-1203. (6) Ting B. T. G.; Manahan, S. E. EnViron. Sci. Technol. 1979, 13, 15371540.
species into wastewater, and these are derived from organooxy, organo-sulfur, and organo-chloride groups in the coal.7 Additionally, processes using acidic coal and water mixtures, such as that of Clean Coal Technology Pty Ltd., release significant amounts of inorganic species and some organic species (depending on the temperature employed) into wastewater streams.8 It is important, therefore, to be able to characterize organic and inorganic species in these coals to obtain an assessment of likely environmental impacts. It is equally important to develop methodologies and suitable techniques to identify such constituents over a large concentration range. Although a great deal has been reported on the composition of high rank coals, less is known of the composition of brown coal, particularly about the forms of chloride and sulfur. Chloride exists mainly as inorganic chloride but has also been thought to be associated with the coal matrix. Sulfur has been classified as inorganic sulfide, sulfate, and a variety of organic forms (thiols, sulfides, thiophenes, and sulfones) with little information on the type and extent of the various sulfur functional groups.9-20 A useful methodology is that of conventional (7) Favas, G.; Jackson, W. R. Fuel 2003, 82, 59-69. (8) (a) Domazetis, G. Chem. Aust. 2004, 17, 11-13. (b) Domazetis, G. Treatment of Wastewater from Coal Cleaning; Clean Coal Technology Pty Ltd.: 2001; Report RN02. (9) Barman, B. N.; Cebolla, V. L.; Mehrotra, A. K.; Mansfield, C. T. Anal. Chem. 2001, 73, 2791-2804.
10.1021/ef0502251 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/28/2006
Organic/Inorganic Species in Brown Coal
elemental chemical analysis and non-intrusive examinations of as-received (ar) samples, supplemented by a similar study of the same coal samples after acid treatment (aw). Suitable acid treatment may remove inorganic species, leaving the unaffected coal matrix. Information on the functional groups in coal may be obtained using XPS and TOF-SIMS. XPS detects major elements and their chemical states on the surface of the coal particles and may be used to differentiate between organic and inorganic species, while high-resolution TOF-SIMS may detect inorganic and organic fragments and also fragments from trace constituents.21-24 This paper presents studies of a number of ar and aw brown coal samples using conventional elemental analysis, XPS, SEMEDX, and TOF-SIMS. These techniques are shown to be suitable for detecting and characterizing carbon, oxygen, sulfur, and chloride groups (including organic chloride and organic S-N functional groups) in brown coals. Experimental Section Coal. The Australian coal samples used in this study were obtained from the Loy Yang and Yallourn open cut mines at the La Trobe Valley in Victoria, the Lochiel and Bowmans coal fields in South Australia, and the Esperance coal field in Western Australia. German brown coals were supplied by Rheinbraun GBT. Each coal sample was crushed with an IKA-Werke MF10 impact analytical mill, then passed through a Mesh 10 sieve (1.68 mm opening), thoroughly mixed, and stored in an airtight polyethylene container. Coal was acid-washed by mixing 100 g of raw coal with 400 mL of a 1:1 solution of deionized water and concentrated HCl, heated, and then stirred at 90-100 °C for >5 h, similar to that previously described.25 The mixture was then filtered, and the coal was again washed with deionized water and filtered through a Bu¨chner funnel until the pH of the filtrate remained unchanged. Samples of coal were also acid-washed using H2SO4 (for chloride determinations) and also with HCl (for sulfur determinations). All chemicals were AR grade. The aw coal was stored in an airtight polyethylene container. Selected samples of Loy Yang and German coals with ultra low ash were provided by Clean Coal Technology Pty Ltd. (Australia). Additional samples were prepared for XPS studies to assist in the assignment of oxygen, sulfur, and chloride. These included (a) humic acids extracted from Esperance coal, (b) dried coals mixed (10) Huggins, F. E. Int. J. Coal Geol. 2002, 50, 169-214. (11) Hodges, N. I.; Ladner, W. R.; Martin, L. G. J. Inst. Energy 1983, 56, 158-169. (12) Ward, C. R. Int. J. Coal Geol. 2002, 51, 135-168 (13) Huggins, F. E.; Huffman, G. P. Fuel 1995, 74, 556-569. (14) Huggins, F. E.; Huffman, G. P. Coal Sci. Technol. 1991, 17, 4361. (15) Martinez-Tarazona, M. R.; Palacios, J. M.; Cardin, J. M. Fuel 1988, 67, 1624-1628. (16) Jime´nez, A.; Martinez-Tarazona, M. R.; Sua´rez-Ruiz, I. Fuel 1999, 78, 1559-1565. (17) Vassilev, S. V.; Eskenazy, G. M.; Vassileva, C. G. Fuel 2000, 79, 923-938. (18) Huffman, G. P.; Shah, N.; Huggins, F. E.; Stock, L. M.; Chatterjee, K.; Kilbane, J. J., II; Chou, M.-I. M.; Buchanan, D. H. Fuel 1995, 74, 549555. (19) Huggins, F. E.; Srikantapura, S.; Parekh, B. K.; Blanchard, L.; Robertson, J. D. Energy Fuels 1997, 11, 691-701. (20) Gorbaty, M. L.; Kelemen, S. R. Prepr. Symp. Am. Chem. Soc.; DiV. Fuel Chem. 2000, 45, 177-180. (21) Buckley, A. N.; Lamb, R. N. Int. J. Coal Geol. 1996, 32, 87-106 (22) Mclntyre, N. S.; Martin, R. R.; Chauvina, W. J.; Winder, C. G.; Brown J. R.; MacPhee, J. Fuel 1985, 64, 1705-1712. (23) Sun, X. Int. J. Coal Geol. 2001, 47, 1-8. (24) Dai, S.; Hou, X.; Ren, D.; Tang, Y. Int. J. Coal Geol. 2003, 55, 139-150. (25) Domazetis, G.; Liesegang, J.; James, B. D. Fuel Process Technol. 2005, 86, 463-486.
Energy & Fuels, Vol. 20, No. 4, 2006 1557 with inorganic sulfates and chloride, (c) poly(acrylic acid) (PAA) and a mixture of known amounts of PAA and tartaric acid, (d) a precipitate obtained by mixing PAA and a solution of iron chloride, (e) a mixture of acid-washed coal with added NaCl and MgSO4, (f) chars obtained by heating acid-washed coal with known amounts of iron hydroxide species in a stream of nitrogen at 300 °C, and (g) ash samples. Analysis. Coal samples were analyzed for C, H, O, N, Cl, S, ash, and moisture by the Campbell Microanalytical Laboratory in the University of Otago, New Zealand. The analytical method used is based on the complete and instantaneous oxidation of the sample by “flash combustion”, which converts all organic and inorganic substances into combustion products under conditions sufficient to completely oxidize the coal sample. The sample is held in a tin capsule (containing the catalyst tungstic oxide and copper) and dropped into a vertical quartz tube maintained at a temperature of 1020 °C. The helium carrier gas is temporarily enriched with pure oxygen as the sample is dropped into the tube. Quantitative combustion is achieved by passing the mixture of gases over a catalyst layer and then through copper to remove excess oxygen and reduce nitrogen oxides to nitrogen. The resulting mixture passes into a chromatographic column where carbon dioxide, water, sulfur dioxide, and nitrogen are separated, and quantitative analysis is performed using a thermal conductivity detector. Oxygen is determined using the same methodology, but without added oxygen, in a helium carrier gas and is reported as total oxygen for the sample. Duplicate results were obtained for each element in the coal sample. Error for all analyses is (0.3 wt %, with duplicates within 0.3 wt %. Ultimate analysis of selected coals was also carried out by the Australian Coal Industry Research Laboratory Ltd. using standard methods.26 Total ash for coals with high amounts of sodium was determined by heating a known weight of dry coal to 300 °C to remove volatiles, followed by heating the char to 900 °C to a constant weight. For coals with low amounts of sodium, the char was ashed at 1050 °C to a constant weight. Ash was analyzed using XRF and, for samples with small amounts of ash, using ICP-OES.27,28 Coal samples were dried, and moisture was determined by heating the sample in an oven at 105-110 °C for 24 h and then allowing it to cool in a desiccator over P2O5. This was repeated to a constant sample weight. Analysis of inorganics, sulfur, and chloride for coal samples supplied by Clean Coal Technology Pty Ltd. was performed at the CSIRO Lucas Heights Science and Technology Centre by dissolving a known weight of coal sample in a fusion flux and performing elemental analysis of the solution using ICP-OES. XPS, SEM-EDX, and AAS. XPS experiments were performed using a Kratos Axis Ultra XPS spectrometer with monochromatized Al KR radiation (hν ) 1486.6 eV) operating at 150 W. The spectrometer energy scale was calibrated using the Au 4f7/2 photoelectron peak at binding energy (BE) ) 83.98 eV. Spectra were charge corrected with reference to C-C species at BE ) 285.0 eV. Such surface charge neutralization was used to effect a slight improvement in peak resolution. Survey and region spectra were collected at 100 and 40 eV pass energies, respectively. The analysis area was 700 µm × 300 µm. Spectra were quantified using Kratos XPS elementary sensitivity factor data after background subtraction and the fitting of Gaussian (70%)/Lorentzian (30%) component peaks. The full width at halfmaximum (fwhm) of the peaks was maintained constant or within a chosen range for all components in a particular spectrum. Uncertainties for all fitted spectra were estimated to be (10% of the measured atomic concentrations. The practical detection limit for elements is e 0.5 at. % (26) Brockway, D. J.; Higgins, R. S. In The Science of Brown Coal; Durie, R. A., Ed.; Butterworth Heinmann: Oxford, 1991; Chapter 5. (27) Metz, J. G. H.; Domazetis, G. ACHEMA 2000; DECHEMA: Frankfurt, 2000. (28) Domazetis, G.; Raoarun, M.; James, B. D.; Liesegang, J. Energy Fuels 2005, 19, 1047-1055.
1558 Energy & Fuels, Vol. 20, No. 4, 2006
Domazetis et al.
Table 1. Composition of Coal Samples (dry, (0.3%) sample
%C
%H
%N
%O
%S
% Cl
% ash
Esperance Esperancea Esperanceb German Germanb Bowman Bowmanb Lochiel Lochielb Loy Yanga-A Loy Yang-B Loy Yangb Loy Yangc humic acidd
43.30 40.53 60.82 59.49 63.77 48.61 59.71 53.42 56.59 63.34 62.56 65.22 65.45 53.95
3.26 3.72 4.97 4.37 4.27 3.65 4.83 4.28 4.58 5.19 4.88 5.01 5.03 4.51
0.37 0.28 0.74 0.59 0.93 0.43 0.68 0.20 0.59 0.65 0.28 0.51 0.52 1.07
19.97 21.15 27.63 30.42 30.82 28.24 28.79 25.16 30.49 28.20 26.94 28.64 28.78 30.06
3.95 4.57 3.71
