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.