Macromolecular structure evolution mechanism of tectonically deformed coal under different deformation mechanisms

Tectonically deformed coal (TDC) is product of tectonic stress in one stage or multiple stages. The physical and chemical properties of tectonic coal have changed greatly during its formation, which significantly influence the coal and gas outburst and coalbed methane exploitation. To investigate the organic macromolecular structure evolution of TDC under different stress and strain, the Fourier transform infrared spectroscopy (FTIR) was used to compare the macromolecular structure and evolution of different types of TDCs in Huaibei mining area. The results show that the organic macromolecular structure of TDC is sensitive to tectonic stress. As the deformation degree of TDCs increases, the aliphatic hydrocarbons and the long aliphatic chains decrease by 13.28% and 1.29 respectively, while the aromatics and the polycondensation of aromatic rings increase by 25.45% and 4.9 respectively. Different stress-strain environments have different effects on the TDCs. Compared with the undeformed coal, the brittle-deformation sequence of coal (include flake and granulated coal) of aliphatic hydrocarbons is reduced by 9.24% and aromatics increase by 18.89%. The ductile deformation sequence (winkle and mylonitic coal) of aliphatic hydrocarbons is 6.36% higher than that of brittle deformed coal and 4.02% higher than that of aromatic hydrocarbons, indicating that tectonic stress is particularly effective in the TDC in ductile deformation series. The ductile deformation process is more likely to cause cracking and recombination of the fat chain, and promote the condensation of the detached small molecules into a stable aromatic structure.