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Energy & Fuels 1991,5,582-586
Pyrolytic Desulfurization of Some High-Sulfur Coals Roberto Garcia and Sabino R. Moinelo Instituto Nacional del Carbon y Sus Derivados, Apartado 73, 33080 Oviedo, Spain
Christopher J. Lafferty and Colin E. Snape* Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow GI IXL,UK Received February 11, 1991. Revised Manuscript Received April 1, 1991
The extent of sulfur removal achieved in coal pyrolysis and hydropyrolysis at 520 "C in a fixed-bed reactor has been compared for three high-sulfur coals, namely a Spanish and a Turkish lignite and a US hvc bituminous coal (PSOC1495)containing significantly different amounts of pyritic and organic sulfur. For all three coals, the level of desulfurization increased with conversion and was thus higher in hydropyrolysis than in pyrolysis at 150 bar. The use of a sulfided molybdenum catalyst in hydropyrolysis gave rise to levels of desulfurization in excess of 90% excluding unreacted sulfatic sulfur. However, a number of subtle differences were found for the coals investigated. The ease of reduction of pyrite to pyrrhotite correlated with increasing grain size for the three coals investigated. The Spanish lignite tar prepared at low pressure contained significant amounts of thiophenes but, in going to hydropyrolysis, the concentrations of thiophenes decreased relative to those of benzoand dibenzothiophenes as anticipated from the relative rates of hydrodesulfurization.
Introduction For the effective utilization of high-sulfur coals, a thorough understanding of the chemical transformations of organic and pyritic sulfur forms during devolatilization is required. The extent of sulfur removal achieved during coal pyrolysis has been reported in a number of investigations'-" but, in most cases, only one regime has been considered for a given coal. The extent of sulfur removal broadly increases with increasing total conversion as found, for example, by Feldman et al.' in fluidized bed pyrolysis. Fallon et a1.2 established that, in hydropyrolysis, the organic sulfur is more difficult to eliminate than pyritic sulfur in accord with the reduction of pyrite to pyrrhotite at relatively low temperatures (-400-450 "C). In contrast, Cypres and Furfari3 found that for a high-sulfur coal rich in dolomite, the extent of desulfurization was extremely low due to capture of hydrogen sulfide to yield calcium and magnesium sulfides. Sugawara et al.4 have recently determined pyrrhotite concentrations in a series of hydropyrolysis chars and their results clearly demonstrate that, a t 600 "C and above, pyrrhotite is reduced to iron. It is well established that, in steam, pyrrhotite can react to form magnetite with the elimination of hydrogen sulfide at (1) Feldman, H. F.; Mima, J. A.; Yarorsky, P. M. Adu. Chem. Ser. 1974, No. 131, 108. (2) Fallon, P. T.; Bhatt, B. L.; Steinberg, M., Fuel Process. Technol. 1980. 3. 155. (3j Cypres, R.; Furfari, S. Fuel 1982, 61,447. (4) Sugawara, T.;Sugawara, K.; Ohaehi, H. Fuel 1988, 67, 1263. (5) Khan, M. R. Fuel 1989,68, 1439. (6) Khan, M. R.; Najjar, M. S.; Chakravarty, T.;Solomon, P. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990,35(1), 150. (7) Calkins, W. H. Energy Fuels 1987,1,59. (8)Oh, M. A.; Bumham, A. K.; Crawford, R. W. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1988, 33(1), 274. (9) Stephenson, M..D.; Rostamagadi, M.; Johnson, L. A.; K m , C. W. In Processing and Utiltsation of High Sulphur Coals; Attia, Y. A., Ed.; Elsevier: New York, 1985; p 353. (IO) Attar, A. Fuel 1978, 57, 201. (11) Cleyle, P. T.;Caby, W. F.; Stewart, I.; Whiteway, S. G. Fuel 1984, 63, 1579. ~
similar t e m p e r a t u r e ~ . ~Whilst J~ organic sulfur is more difficult to completely eliminate than pyritic sulfur, Khan6 has concluded recently that it is the organic sulfur which principally governs the sulfur contents of volatiles. Conversely, pyritic sulfur governs those of chars, particularly at low temperatures where the evolution of sulfur gases generally proceeds that of hydrocarbons.6ig Calkins' demonstrated that, in inert atmospheres, it is only the non thiophenic groups, such as aliphatic and benzylic sulfides and disulfides that are thermally labile. However, it is probable that aliphatic sulfides convert to thiophenes during pyrolysis."' In addition, some of the hydrogen sulfide released from the reduction of pyrite and labile organic sulfur forms is incorporated into chars as thiophenic sulfur especially in inert atmospheres. Oh et al.8 distinguished three broad temperature ranges, namely
