Effects of demineralization on the surface morphology, microcrystalline and thermal transformation characteristics of coal

•Surface morphology of macerals in raw coal and acid washing coal were compared.•The demineralization treatment significantly increased surface roughness of coal.•Crystalline carbon of acid washing coal was more disorder compared to the raw coal.•Minerals promote the crack of aliphatic and polycyclic aromatic hydrocarbon of tar.•The pyrolysis conversation rate of coal increased with base-acid ratio of minerals. In this study, experiments were conducted to systematically determine how the surface morphology, microcrystalline and thermal transformation characteristics of coal change during acid treatment. HCl-HF acid washing was applied to pretreat the raw coal. The effects of the demineralization treatment on the carbonaceous structure and functional groups were first tested using X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The results show that the demineralization treatment significantly removes the peaks of inherent mineral matter (based on XRD), enhances the disorder in the structure of the coal (based on Raman spectroscopy), and changes the structure of CO, the aromatic structure and the aliphatic side chains (based on FTIR). The surface morphologies of the raw coal and demineralized coal were then studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show that the demineralized coal has a greater surface roughness than the raw coal. The surface morphology of the macerals show that the demineralization treatment transforms the vitrinite peaks to valleys and increases the depths of the valleys; furthermore, the demineralization treatment cause the peaks and valleys of the surface roughness to interchange. Finally, pyrolysis experiments of the raw coal and demineralized coal were conducted with thermogravimetric analysis (TGA) and a fixed-bed reactor, and the subsequent gas and tar compositions were characterized by gas chromatography (GC) and gas chromatography–mass spectrometry (GCMS). The results show that the pyrolysis reactivity of coal is decreased by demineralization treatment. The pyrolysis experiments in the fixed-bed reactor indicate that the pyrolysis conversion rate increases with the base-acid ratio of the inherent mineral matter. The pyrolysis products show that the amounts of H2 and CH4 increases due to the presence of the inherent mineral matter. The inherent mineral matter can also promote the cracking of aliphatic hydrocarbons, polycyclic aromatic