Investigation of Overall Pyrolysis Stages for Liulin Bituminous Coal by Large-Scale ReaxFF Molecular Dynamics

Deep understanding of the detailed coal pyrolysis process is very important for clean coal utilization. The overall stages in coal pyrolysis were investigated by ReaxFF MD simulations of large-scale coal models combined with reaction analysis of a cheminformatics approach. Analysis of slow heat-up ReaxFF molecular dynamics (MD) simulations shows that the Liulin coal pyrolysis process can be divided into four stages based on the thermal cleavage of bridge bonds: the activation stage of the coal structure (Stage-I), the primary pyrolysis stage (Stage-IIA), the secondary pyrolysis stage (Stage-IIB), and the recombination dominated stage (Stage-III). The transition from the dominant cleavage of the ether bridged bond into breaking of the aliphatic bridged bonds corresponds to the transition of Stage-IIA to Stage-IIB in Liulin bituminous coal pyrolysis. Further investigation of the relationship between radicals and gas production suggests that temperatures for the transition of gas generation rates can be used as indicators for pyrolysis stage transitions, namely H₂O for Stage-I and Stage-IIA, and CH₄ for the primary and secondary pyrolysis reactions, provided such production rate transitions could be detected experimentally. In addition, the compromise between the competition reactions of decomposition and recombination as well as radical generation and consumption plays a significant role along the entire pyrolysis process, and the slight differences of the reactions in competition determine the yield, species, and distribution of final pyrolyzates, which seems consistent with the mesoscale structure theory.