Sulfur Transformation in a South Australian Low-Rank Coal during pyrolysis

Abstract

Sulfur transformation during pyrolysis of a high sulfur low-rank coal from South Australia has been studied. Three experimental techniques covering a wide range of conditions, namely, temperature-programmed pyrolysis, fixed-bed pyrolysis, and fluidized-bed pyrolysis, have been employed to investigate the effect of pyrolysis conditions on the interactions between different forms of sulfur and mechanisms of sulfur evolution during coal pyrolysis. Both chemical analysis method following Australian Standards and SEM with an energy dispersive X-ray detector are used for sulfur analysis of the char. The results reveal that sulfur evolution is a net result of organic and inorganic sulfur decomposition and interaction. The presence and conversion of inorganic sulfur into complex organic sulfur compounds provide a major mechanism for sulfur retention in char during pyrolysis. The difference in heating rates in the different pyrolysis experiments does not change the natural of the sulfur transformations but affects the extent to which they occur. Faster heating rates do not necessarily imply greater sulfur evolution due to formation of complex organic sulfur. Coal samples pretreated by acid washing and Ca and Na ion exchange are also used to examine the role of inorganic matter in sulfur transformation. At low temperatures (<400°C) acid washing shows little effect on sulfur retention, but at higher temperatures, sulfur retention is greatly reduced. While Na ion exchange enhances sulfur retention compared to the acid washed coal particularly at high temperatures (>400°C), Ca ion exchange shows the opposite trend. An increase in sulfide formation in the Ca ion-exchanged coal at high temperatures is observed, indicating that organic sulfur decomposition is enhanced in the presence of Ca. The effect on sulfur retaining of potential reactions involving Ca ions with sulfur may be offset by the catalyzing influence of Ca ions on organic sulfur decomposition.