Hydrous pyrolysis reactions of sulphur in three Australian brown coals

Abstract

Hydrous pyrolysis of three Australian brown coals in the presence of cadmium acetate (CdOAc) indicates that as much as 77% of the original organic sulphur may be generated as H 2S. In the absence of CdOAc much of the generated H 2S was re-adsorbed back into the insoluble coal residues. GC–MS analyses of the solvent extracts of the coals and of the solvent extracts of the hydrous pyrolysates at 110–330°C both in the presence and absence of cadmium acetate reveal a little parent thiophene and benzothiophene totally dominated by a broad range of methyl derivatives. Yields of these sulphur compounds tend to be greater when cadmium acetate is absent. Benzothiophenes are favoured over thiophenes at the higher hydrolysis temperatures. Analyses of the solvent extracted residues by FP–GC–MS and of mixtures of low sulphur coal and elemental sulphur, produced a similar range of parent thiophene and benzothiophene and their methylated derivatives. The ubiquity of these sulphur compounds suggests these to be the thermodynamically favoured products of sulphur/organic matter interaction. The only other class of sulphur compounds seen, the longer chain n-alkyl thiophenes, occurred in the soluble 200°C hydrous pyrolysis products obtained in the absence of cadmium acetate. Only these appear to reflect structures in the original brown coals. The implication of this study is that the earliest formed organic sulphur compounds in immature sediment will, with increasing temperature, continue to decompose and release H 2S and elemental sulphur. The released sulphur species will, in turn, undergo reaction with the organic matter to form more stable sulphur compounds. Sulphur incorporation is likely to be a continuing process throughout diagenesis and catagenesis.