Sulfur transformation during the pyrolysis of coal with the addition of CaSO4 in a fixed-bed reactor

•Higher H2S and COS yields could be obtained by the presence of CaSO4 at high temperature.•CaSO4 could reduce tar sulfur at high temperatures.•High proportions of CaSO4 could promote the decomposition of organic sulfur in the char at high temperatures.•A significant increase of sulfide sulfur was observed with the addition of CaSO4 above 600 °C.

The present paper was devoted to investigate the sulfur transformation during pyrolysis of the mixture of coal and CaSO4 in a fixed bed reactor at a temperature range of 500-800 °C. The results indicated that the presence of CaSO4 could promote the evolution of H2S and COS at high temperature, which should be due to the higher decomposition rate of organic sulfur and the reactions between CaSO4 and pyrolysis products, e.g., H2 and CO. Moreover, when the blending ratio(CaSO4/coal) exceeded 20%, CaSO4 could greatly promote the decomposition of organic sulfur at 800 °C. The mechanism of organic sulfur decrease due to CaSO4 was proposed. In comparison of raw coal, sulfide sulfur was significantly increased with the addition of CaSO4 above 600 °C, which should be mainly due to CaS formation through CaSO4 decomposition.