Effects of step acid treatment process on the structure and pyrolysis characteristics of Ximeng brown coal: formation of gaseous products

Ximeng brown coal was leached by HF, HNO₃ and HCl solution step by step in order to study the effects of acid pretreatment on the formation of gaseous products during coal pyrolysis. A fixed bed pyrolysis-gas chromatography was used to analyze the release behaviors of gaseous products from pyrolysis of raw and the pretreated coals. The results show that there is an obvious influence of acid pretreatment on the release behaviors of hydrogen-, oxygen-and sulfur-containing gases during coal pyrolysis. The formation routes, stages and release temperature intervals for various gases are different, which result in that the influences of minerals removed by acid leaching behave significantly different. The gaseous products were also dramatically influenced by oxidative characteristic during HNO₃ pretreatment, which causes that the release amount of hydrogen-containing gases decreases, and that of oxygen-containing gases increases. There is a discrepant influence on the release of H₂S and COS by HNO₃ pretreatment; the former decreases and the later increases.