Influence of additives on the devolatilization product yield of typical South African coals, and effect on tar composition

Abstract

South African coal is mainly used for electricity generation by means of pulverized fuel combustion and liquid fuels production via indirect gasification technology and Fischer-Tropsch synthesis. In order to expand the utilization potential with regard to the production of high-value compounds, the effect of in situ organic/inorganic salts on the devolatilization behaviour and product spectrum of tars derived from three typical South African coals was investigated. Potassium-based salts and alumina were chosen as additives, based on numerous literature citations stating that these were successful in catalysing coal char devolatilization reactions and influencing tar quantity and quality. Using a modified Fischer Assay analysis experimental method, the solid and liquid product yields were determined at a temperature of 520°C. An increase in additive loading decreased the yield of liquid products by up to 50%, while the char yield increased by up to 10%. The quality of the liquid products formed was also affected by an increase in additive load, with the resultant tars having a lighter average molecular weight and a lower average boiling point, indicating significant changes in the product composition. Due to the potential for the production of high-value chemicals from coal, a study of this nature represents a significant insight into the devolatilization behaviour of typical South African coal, and could serve as a precursor for the development of a technology capable of producing high-value chemicals as well as a coal-derived char suitably catalysed for gasification to produce synthesis gas