The influence of water-soluble and acid-soluble inorganic matter on sulphur transformations during pyrolysis of low-rank coals

Temperature-programmed pyrolysis (TPP) experiments are conducted on several samples prepared from a South Australian Bowmans coal, viz. raw, water-washed, acid-washed and Ca++ and Na+ ion-exchanged acid-washed samples, to study the influence of the physically absorbed (water-soluble) and organically bound (acid-soluble) inorganic matter on sulphur transformations during pyrolysis. Changes in sulphur forms between 200 and 900°C were monitored according to Australian Standards, wet chemical analysis of chars. Interpretation of sulphur transformation was aided by scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX). During TPP, raw Bowmans coal is observed to evolve less sulphur than the treated coals between 400 and 500°C. Water-soluble sulphates above other water-soluble inorganics, are found to contribute to the sulphur retention by suppressing the decomposition of organic sulphur in the same temperature region. Acid-soluble inorganics are observed to increase the retention of sulphur between 500 and 600°C. Chemical analysis did not detect a corresponding formation of sulphides, suggesting that the presence of acid-soluble inorganics suppressed the decomposition of organic sulphur between 500 and 600°C. SEM-EDX and iDXi X-ray mapping of TPP chars removed at 800 and 900°C revealed that sulphur does interact with the organically-bound inorganics. Na interacts with silica and forms highly concentrated constituents in char particles during pyrolysis. Both types of inclusions were found to contain highly concentrated areas of sulphur. Calcium becomes enriched in the char to initiate decomposition of organic sulphur and form calcium sulphide in the char prepared between 800 and 900°C.