Impacts of chemical fractionation on Zhundong coal's chemical structure and pyrolysis reactivity

The investigation to the structure change of coal during demineralization is the prerequisite in evaluating the minerals' catalytic action in pyrolysis. In the study, Zhundong raw coal samples were first treated using the chemical fractionation method, and then the pyrolysis reactivities of the samples were measured on a thermogravimetric analyser (TGA). In order to understand the chemical structures of the samples, the Fourier transform infrared spectroscopy (FT-IR) and a series of original infrared structure parameters were employed. Finally, a correlation was established between the infrared structural parameters and the pyrolysis reactivity. It is indicated that the characteristic of Zhundong coal is different from other low-rank coals in inorganic mineral composition and chemical structure of organic matter. Dilute sulfuric acid washing can increase the carboxyl group and phenolic hydroxyl group contents, but has insignificant impacts on aliphatic hydrogen content, aromatic hydrogen content, and hydrogen atom distributions in aliphatic and aromatic structures. Washing process using concentrated HCl/HF can further increase the carboxyl group content but has little impact on the phenolic hydroxyl group content. In addition, the washing can also shorten the aliphatic side chains and bridge bonds, causing the aromatic ring fusing lower and substitution higher. Dilute sulfuric acid and concentrated HCl/HF washings increase the pyrolysis reactivity of Zhundong coal. Finally, a correlation was regressed between the infrared structural parameter S and the pyrolysis reactivity, which was proved to be effective to predict the pyrolysis reactivity of coal.