Effect of ash removal on structure and pyrolysis/gasification reactivity of a Chinese bituminous coal

Abstract

In this study, the effect of ash removal on Shenfu bituminous coal was investigated. The coal was pretreated by hydrofluoric acid (HF) pickling, and the raw/pretreated coal chars were prepared at 900 °C in a fixed bed reactor. The structure of coal and char were detected by Fourier transform infrared (FTIR) and Raman spectroscopy. The reactivity was tested in a thermogravimetric analyzer, including coal pyrolysis and char gasification. The reaction kinetics was analyzed through the Coats–Redfern method, master plots, the model-free and model-fitting method. The results show that the HF pickling can remove silicon from coal efficiently, and the macromolecular framework of coal is quite stable according to FTIR. The Raman parameters imply some carbonaceous structure on coal surface changed. For slow pyrolysis of coal, the effect of heating rate is considered. The changes of pyrolysis characteristics and kinetics are insignificant. For char gasification, the reactivity under isothermal and non-isothermal condition are discussed with an emphasis in different residence time of devolatilization process. In kinetic control region (low temperature), the activation energy (Ea) is very close (about 240 kJ/mol) for all chars. With the temperature increases, the reactivity of raw coal char is more easily suffered by diffusion. The random pore model is more suitable for the ash-free coal char, and the char with long residence time has a larger value of structural parameter ψ and smaller value of pre-exponential factor A. The Ea calculated by model-fitting and model-free method were in good agreement.