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Energy & Fuels 2003, 17, 1416-1422
Study on Organic Sulfur Functionalities of Pyridine Extracts from Coals of Different Rank Using Reductive Pyrolysis Piotr Rutkowski,† Graz˘ yna Gryglewicz,*,† Steven Mullens,‡,§ and Jan Yperman‡ Institute of Chemistry and Technology of Petroleum and Coal, Wrocław University of Technology, ul. Gdan´ ska 7/9, 50-344 Wrocław, Poland, and Laboratory of Applied Chemistry, CMK, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium Received April 2, 2003
The organic sulfur functionalities in the pyridine extracts obtained from lignite (lignite A), subbituminous coal (subA), and bituminous coal (mvb) were studied by atmospheric pressuretemperature-programmed reduction (AP-TPR) method. The extraction yield was in the range of 6.7-29.3 wt % with a maximum for medium volatile bituminous coal. The H2S AP-TPR recovery for the pyridine extracts was relatively low, not higher than 45%. The coupling of the AP-TPR reactor with a different detection system from potentiometric one, i.e., with a mass spectrometer, enables the detection not only of H2S but also of other sulfur-containing compounds, which were released in the volatile products during reductive pyrolysis. Alkanethiols, thiophene, and its C1C2 alkylated derivatives were detected in the volatiles. Both the char and the tar produced during an AP-TPR experiment were also studied by atmospheric pressure-temperature-programmed oxidation (AP-TPO) to monitor the sulfur compounds left and captured in the AP-TPR solid and liquid residues, respectively. The obtained extracts were characterized by high sulfur content, 1.18-3.74 wt %. The organic sulfur functionalities distribution of the pyridine extract reflects the rank of coal subjected to extraction. The AP-TPR analysis showed that for the extract from lignite thiols, alkyl and alkyl aryl sulfides dominate, whereas for the extract from subbituminous coal a comparable proportion of alkyl and alkyl aryl sulfides and thiophenes is found. Thiophenes are the major organic functionalities in the pyridine extract from bituminous coal.
Introduction The extraction process with different solvents is widely used in the investigation of coal constitution.1 Solvent extraction of coal allows obtaining high amounts of extract at ambient conditions. However, it should be remembered that the examination of extract gives information only on the extractable part of coal. The extraction yield highly depends on the properties of both solvent and coal. An efficient solvent is that which easily penetrates the coal structure and causes the swelling of coal.2,3 It has been shown that the solvent containing nitrogen (pyridine, amines) or oxygen (tetrahydrofuran) gives a high extraction yield.4 Larsen et al.5 have proved that hydrogen-bond accepting solvents such as pyridine and THF may easily break the hydrogen bonds in the organic matrix of coal and replace them with coalsolvent hydrogen bonds. However, the extract obtained * Corresponding author. E-mail:
[email protected]. † Wrocław University of Technology. ‡ Limburgs Universitair Centrum. § Present address: Material Technology, VITO, Boeretang 200, B-2400 Mol, Belgium. (1) Van Krevelen, D. W. Coal (Typology-Chemistry-Physics-Constitution); Elsevier: Amsterdam, 1993; p 549. (2) Dryden, I. C. G. Fuel 1951, 30, 39-44. (3) Larsen, J. W.; Baskar, A. Energy Fuels 1987, 1, 230-232. (4) van Bodegom, B.; van Veen, J. A. R.; van Kessel, G. M. B.; Sinnige-Niyssen, M. W. A.; Stuiver, H. C. M. Fuel 1985, 64, 59-63. (5) Larsen, J. W.; Gurevich, I.; Glass, A. S.; Stevenson, D. S. Energy Fuels 1996, 10, 1269-1272.
with nitrogen-containing solvent is much richer in nitrogen, even more than 20% compared with the raw sample.1 A lot of information on sulfur functionalities in solid materials, i.e., coal,6,7 clays,8 and gums9 was reached by applying AP-TPR. The method based on Attar’s pioneer works10,11 on organic sulfur distribution in coal was developed by Majchrowicz et al.12 In recent years, many important improvements and optimizations were realized.13,14 To determine the characteristic temperatures for reduction/hydrogenation of different sulfur functionalities, several silica-immobilized model com(6) Majchrowicz, B. B.; Yperman, J.; Reggers, G.; Francois, J. P.; Gelan, J.; Martens, H. J.; Mullens, J.; Van Poucke, L. C. Fuel Process. Technol. 1987, 15, 363-376. (7) Maes, I. I.; Gryglewicz, G.; Machnikowska, H.; Yperman, J.; Franco, D. V.; Mullens, J.; Van Poucke, L. C. Fuel 1997, 76, 391-396. (8) Mullens, J.; Yperman, J.; Carleer, R.; Franco, D. V.; Van Poucke, L. C.; Van der Biest, J. Appl. Clay Sci. 1993, 8, 91-99. (9) Mullens, S. Ph.D. Thesis, LUC, Diepenbeek, 2000. (10) Attar, A.; Dupuis, F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1978, 23, 44-53. (11) Attar, A.; Dupuis, F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1979, 24, 166-177. (12) Majchrowicz, B. B.; Yperman, J.; Martens, H. J.; Gelan, J.; Wallace, S.; Jones, C. J.; Baxby, M.; Taylor, N.; Bartle, K. D. Fuel Process. Technol. 1990, 24, 195-202. (13) Yperman, J.; Maes, I. I.; Van der Rul, H.; Mullens, S.; Van Aelst, J.; Franco, D. V.; Mullens, J.; Van Poucke, L. C. Anal. Chim. Acta 1999, 395, 143-155. (14) Mullens, S.; Yperman, J.; Reggers, G.; Carleer, R.; Buchanan, A. C., III; Britt, P. F.; Rutkowski, P.; Gryglewicz, G. J. Anal. Appl. Pyrolysis 2003 (in print).
10.1021/ef030072m CCC: $25.00 © 2003 American Chemical Society Published on Web 09/16/2003
Organic S Functionalities of Pyridine Extracts from Coals Table 1. Temperature Ranges of Reduction/ Hydrogenation of Different Sulfur Functionalities in AP-TPR [9] sulfur group
temperature (°C)
elemental sulfur alkanethiols aryl thiols disulfides dialkyl sulfides alkyl aryl sulfides pyrite diaryl sulfides thiophenes troilite iron(II) sulfate iron(III) sulfate other sulfates (Mg, Ca, Ba)
180-220 180-320 300-400 400-460 380-490 430-560 480-590 500-640 g600 >740 470 and 750 470, 535, and 750 g800
pounds were analyzed using AP-TPR.14-16 It was found that under 620 °C nonthiophenic and above 620 °C thiophenic compounds are reduced/hydrogenated.7 A detailed list of the characteristic reduction/hydrogenation temperatures of sulfur groups is given in Table 1. The AP-TPR sulfur recovery problem was also faced in the case of organic sulfur forms. Attar in his review works17,18 on the fundamental reactions of sulfur groups in coal during pyrolysis pointed out that, although hydrodesulfurization is a favorable reaction, some H2S can be fixed again with organic groups. Lafferty et al.19 have given some evidence that a part of the sulfides are being lost in secondary reactions as thiophenes at low hydrogen pressure. Moreover, Maes et al.20 proved that the presence of calcium compounds, i.e., calcite (CaCO3) and dolomite (CaCO3‚MgCO3) in coal, leads to a decrease in sulfur recovery due to the capture of H2S by these minerals and their thermal decomposition products. The same was observed for calcium bound to carboxylic groups of organic coal matrix. The formed CaS is stable within AP-TPR conditions.20 The AP-TPR technique still requires some improvements toward sulfur recovery, in particular, the determination of other more volatile sulfur compounds and the problem of highly oxidized materials. Despite some drawbacks, at present the AP-TPR is one of not too many techniques that gives such a widespread information on the distribution of inorganic and organic sulfur groups in solid materials. Many methods have been proposed for this purpose, and the majority of them are discussed in the review works by Davidson,21 Calkins,22 Stock et al.,23 and in some recent papers. The most often used are X-ray absorption near-edge structure (XANES),24,25 X-ray photoelectron spectroscopy (XPS),26,27 (15) Ismail, K.; Mitchell, S. C.; Brown, S. D.; Snape, C. E.; Buchanan, A. C., III; Britt, P. F.; Franco, D. V.; Maes, I. I.; Yperman, J. Energy Fuels 1995, 9, 707-716. (16) Van Aelst, J.; Yperman, J.; Franco, D. V.; Van Poucke, L. C.; Buchanan, A. C., III; Britt, P. F. Energy Fuels 2000, 14, 1002-1008. (17) Attar, A. Fuel 1978, 57, 201-211. (18) Attar, A. Coal Process. Technol. 1978, 4, 26-34. (19) Lafferty, C. J.; Mitchell, S. C.; Garcia, R.; Snape, C. E. Fuel 1993, 72, 367-371. (20) Maes, I. I.; Gryglewicz, G.; Yperman, J.; Franco, D. V.; Mullens, J.; Van Poucke, L. C. Fuel 1997, 76, 143-147. (21) Davidson, R. M. Fuel 1994, 73, 988-1005. (22) Calkins, W. H. Fuel 1994, 73, 475-484. (23) Stock, L. M.; Wolny, R.; Bal, B. Energy Fuels 1989, 3, 651661. (24) Brown, J. R.; Kasrai, M.; Bancroft, G. M.; Tan, K. H.; Chen, J. M. Fuel 1992, 71, 649-653. (25) Huffman, G. P.; Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, S.; Lu, F. Energy Fuels 1991, 5, 574-581.
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temperature-programmed oxidation (PTO, CAPTO),28,29 temperature-programmed pyrolysis (TPP),30 flash pyrolysis,31 temperature-programmed reduction (TPR),32 and its atmospheric-pressure (AP-TPR)13 and highpressure variants (HP-TPR).33 The more conventional methods include a stepwise oxidation of sulfur groups with HClO434,35 and reaction of coal with HI36 (quantitative thiol determination). Comprehensive analysis of the above literature data proves that the methods proposed are not perfect and at present there is no method which would permit true and verifiable quantitative determination of particular species of organic sulfur. In our previous report,37 we pointed out that the presence of pyrite in coal makes the investigation of organic sulfur groups using AP-TPR more difficult. The reason is that the H2S evolving as a product of pyrite reduction overlaps the evolution of hydrogen sulfide from hydrogenation/reduction of some organic sulfur functionalities. In view of the optimal determination of organic sulfur functionalities, it is necessary to remove pyrite from coal or separate an organic part of coal using a nondestructive method. For the latter method, the original organic sulfur groups can be saved from destruction or modification. In this work, solvent extraction was applied to obtain nonaltered coal-derived material totally free of mineral constituents. This is important because the knowledge about organic sulfur functionalities in coal of different coalification degree is still relatively scant. This paper reports the results of sulfur functionalities in pyridine extracts of different rank coals using reductive pyrolysis coupled with different detection systems monitoring volatile sulfur compounds. Experimental Section Materials. Three Polish coals of different rank, i.e., Bełchato´w (lignite A), Siersza (subA), and 1 Maja (mvb) were investigated. All coals were ground to
