Volatilization characteristics and relationship of arsenic and sulfur during coal pyrolysis

Abstract

Similarities concerning the occurrence of arsenic and sulfur in coal are the reasons for studying the relationship between the volatilization of arsenic and sulfur during the pyrolysis process. For this purpose, three coals with different arsenic and sulfur contents were selected. Combined application of electron probe microanalysis (EPMA) and sequential chemical extraction determined the relationship between the speciation of arsenic and sulfur (especially pyrite) in coal. Effects of temperature and speciation on the volatilization of arsenic and sulfur were discussed. Results show that arsenic in all coals is significantly enriched in the distribution area of pyrite. The volatilization ratio of sulfur and arsenic increases with temperature, and their volatilization mainly occurs at 300–600 °C. Before 500 °C, aliphatic sulfur and organic arsenous are the main contributors to the volatilization of sulfur and arsenic, respectively. At 500–600 °C, the volatile sulfur and arsenic mainly come from the decomposition of pyrite. A characteristic peak is observed for the volatilization rate of arsenic and sulfur, and it almost coincided with the weight loss peak of coal. Moreover, the simultaneous volatilization behavior of arsenic and sulfur is observed in Gansu (GS) coal at 400 °C due to the devolatilization of coal and in Chongqing (CQ) coal at 550 °C due to the decomposition of pyrite. Sulfate in coal has a negative effect on the volatilization of arsenic. The possible reaction paths were predicted by the thermodynamic analysis of FactSage 7.3 software. Fe2O3 and CaS could adsorb arsenic to form stable arsenate (FeAsO4 and Ca3(AsO4)2) during the pyrolysis process.