XANES study on effect of acid treatment on sulfur forms in Yima coal and their transformation behavior during pyrolysis

Effect of acid treatment on specific surface area, sulfur forms and their transformation during pyrolysis of Yima coal were investigated by XANES technique. Results show that the specific surface area of acid-treated coals is higher than that of raw coal due to removal of some mosaic minerals in coal matrix and opening of some pores. Most of the minerals and inorganic sulfur are removed by the acid treatment, the relative percentages of sulfide, sulfoxide and sulfone in YMN are higher than that of YMR and YMD due to the higher oxidability of HNO₃. Sulfur forms on the coal surface are more modified than that in the coal bulk by the acid treatment. The release amount of sulfur-containing gases during pyrolysis of acid-treated coals decreases, but the accumulated release yields increase due to removal of most alkaline minerals and the relative increase of thermally decomposable sulfur forms in coal. The main sulfur forms show a more even distribution in acid-treated coal chars due to the inter-conversions between different sulfur forms during pyrolysis. HCl-HF-HNO₃ process could remove most of the minerals and inorganic sulfur, and change distribution of sulfur forms, which provides a suitable guidance for utilization of high ash, pyrite-rich high-sulfur coals.