Determination of organic sulfur forms in some coals and kerogens by high pressure temperature-programmed reduction Stuart C. Mitchell, Colin E. Snape, Roberto Garcia, Khudtir lsmail and Keith D. Bartle” Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow Gl IXL, UK “School of Chemistry, University of Leeds, Leeds LS2 9JT, UK (Received 22 June 1993; revised 2 November 7993) The principal inherent drawbacks associated with temperature-programmed reduction (TPR), for specifying the distribution of organic sulfur forms in solid fuels, in that first, thiophenic sulfur is largely undetectable and second, sulfides interconvert readily into thiophenes, have been overcome by the use of a well-swept, fixed-bed reactor, operating at relatively high hydrogen pressure (15 MPa). The high pressure technique was applied previously to two high sulfur lignites, Rasa (Croatia) and Mequinenza (Spain), and indicated that thiophenic forms account for -70% of the total organic sulfur. The study has now been extended successfully to samples of varying organic sulfur content which include three of the Argonne Premium coals i.e. Wyodak-Anderson, Illinois No. 6 and Upper Freeport; a UK bituminous coal i.e. Gedling; two kerogens i.e. Goynuk (Type I, Turkey) and Kimmeridge Dorset-Cuddle (Type II, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP IJK); and a liptinite concentrate of Mequinenza lignite. For those samples in which pyritic sulfur represents a significant proportion of the total sulfur, treatment with lithium aluminium hydride was necessary, in order to obtain any meaningful information on the organic sulfur forms present. From the subsequent H,S evolution profiles, the relative contributions from thiophenic and non-thiophenic forms have been resolved satisfactorily. In common with other pyrolysis techniques and X-ray techniques (XPS and XANES), high pressure TPR indicates that thiophenic sulfur increases with rank. However, the proportions of thiophenic sulfur derived from high pressure TPR are consistently higher than by other techniques. (Keywords: organic sulfur: kerogeas; high pressure temperature programmed reduction) The need to improve current knowledge of the role of sulfur in maturation and conversion processes has generated much interest in the speciation of the organic sulfur forms present in coals, petroleum source rocks, and related carbonaceous materials. Analytical approaches have included the use of X-raysims, temperature- programmed reduction (TPR)’ -’3, flash pyrolysisi4,’ ‘, temperature-programmed oxidation (CAPT0)16,17 and various means of selective chemical modification18-20. X-ray photoelectron spectroscopy (XPS)‘-4 and X-ray abso~tion near edge structure (sulfur K-edge XANES)“*3.5p7 have been used extensively to distinguish between thiophenic and non-thiophenic sulfur forms in coals, but both techniques suffer from relatively small binding energy differences between individual chemical forms, leaving interpretation heavily reliant on curve-fitting procedures. The use of the sulfur L-edge in XANES8 and the selective chemical modification’8-2o of particular sulfur forms (both oxidation and reduction), have recently offered improved resolution. The main findings to date have indicated that the proportion of thiophenic sulfur increases with maturity, with low-rank coals containing higher concentrations of sulfides. Temperature-programmed reduction (TPR) is based on the principle that different organic sulfur forms present in solid fuels have different characteristic reduc- tion temperatures at which hydrogen sulfide (H2S) evolves. In previous studies’-“, sulfur balances were poor, with virtually all the thiophenic sulfur remaining in the char due to the low hydrogen pressures and low- boiling reducing agents used. Further, little account was taken of the reduction of pyrite to pyrrhotite, while the nature of the reactor design, coupled with the slow heating regimes employed, almost inevitably resulted in the conversion of sulfides into thiophenes’. The effect of these inherent drawbacks has been minimized by using a high pressure TPR technique, in which evolved hydrogen sulfide and other volatiles are measured on-line, with a quadrupole mass spectrometer. In the first report of this technique13, satisfactory resolution was achieved between thiophenic and non-thiophenic forms for two high sulfur lignites (Mequinenza, Spain and Rasa, Croatia), with - 70% of the organic sulfur being reduced to H,S; the remainder being found as predominantly thiophenic compounds in the product tars*l. In either case, there was little evidence for the evolution of other sulfur-containing gases (COS or mercaptans). Mercaptans were detected in trace amounts, but their overall contribution is anticipated as being extremely small with the majority being reduced to H,Sz2. The presence of COS in pyrolysis gases is well documented as being derived from the oxidation of pyrite’4,23. ~16-~361~94~~7~1159-4% i 1994 Butt~rw~rth-Heinemann Ltd Fuel 1994 Volume 73 Number 7 1159