Effect of fractional step acid treatment process on the structure and pyrolysis characteristics of Ximeng brown coal

To investigate the effects of acid pretreatment on the structure and pyrolysis performance of brown coal with high sulfur content, the brown coal from Ximeng, Inner Mongolia was selected, and leached by HF, HNO₃ and HCl solution step by step. The pyrolysis performance and structure characteristics of Ximeng raw coal and different acid treatment coal samples were studied in detail by means of thermal gravimetric analysis and FT-IR technique, and the pyrolysis kinetics was also analyzed. The results show that the structure of coal sample is not significantly affected by the HF treatment process, but an obvious organic structure change is found in the coal samples further treated by HNO₃ solution. The intensity variation of absorption peaks of aliphatic groups CH₃ caused by HNO₃ is weakened by the HCl treatment process. The pyrolysis performance of Ximeng brown coal can be changed by the fractional step acid treatment. HF and HNO₃ treatment processes promote the reactivity of coal pyrolysis, and the further treatment of HCl solution only has a slight influence. The kinetics analysis of coal pyrolysis reveals that the effect of different acid treatments on the pyrolysis characteristics of Ximeng brown coal can not be described only by the apparent activation energy of pyrolysis reaction. There is a compensation effect between pyrolysis activation energy and pre-exponential factor, besides, the pyrolysis activation energy relates to the weight loss of coal pyrolysis.