Sulphur transformation during pyrolysis of an Australian lignite

Transformation of various sulphur forms, including inherent and added pyrite, sulphates (CaSO4 and FeSO4) and organic sulphur, during pyrolysis of an Australian lignite was studied using TGA, TGA-MS and a fixed bed reactor, supplemented by sulphur form analysis. It was shown that hydrogen sulphide (H2S) and a small quantity of sulphur dioxide (SO2) were released during the pyrolysis of the pyrite-lignite blend. However, only SO2 was detected during the pyrolysis of the lignite with high pyrite content. Inorganic matter was found to help retaining some of the inorganic sulphur, including pyrite, in the char. Inherent sulphates decomposed at much lower temperatures than the added sulphates, releasing SO2 rather than H2S. The inherent sulphates in the lignite were dominated by iron sulphates which started to decompose and release SO2 at around 500 K and all sulphate had been decomposed at 1073 K. The retention of organic sulphur in the high organic sulphur lignite was higher than in its acid washed lignite sample, due to the interaction between inherent inorganic matter with the organic sulphur retaining the organic sulphur in the solid phase. SO2 was the only sulphur gas produced during pyrolysis of acid washed lignite. A comprehensive mechanism of sulphur transformation during pyrolysis of lignite was proposed.