Preliminary understanding of initial reaction process for subbituminous coal pyrolysis with molecular dynamics simulation

A series of molecular dynamics simulations using the ReaxFF reactive force field was carried out to investigate the mechanism of initial thermal decomposition associated with pyrolysis of a kind of subbituminous coal. The calculation results show that the primary decomposition reactions of Hatcher subbituminous model begin with intramolecular changes such as the cleavage of unstable CC and CO bonds. The formation mechanisms for typical pyrolysis products were explored. For example, the initial pathway for the formation of CO is by the decarbonylation of carbonyl or carboxyl group, while CO2 is mainly produced by hydrogen transfer and decarboxylation of carboxyl groups. CH4 can be formed mainly by CH3 free radical abstracting a hydrogen atom from the hydroxyl group. H2 is formed by two hydrogen atoms from one or two groups bonding together, which makes the residue fragments more stable. Hydrogen can also react with oxygen-containing free radicals or unsaturated bonds. Combining ReaxFF molecular dynamics (RMD) simulation and density functional theory (DFT) calculation, we find that the free radical C9H9O is an important fragment during the pyrolysis process of Hatcher subbituminous model. As a precursor for cresol, it can capture hydrogen radical to form intermediate C9H10O and may continue to produce o-cresol and ethylene in the presence of hydrogen resource. These simulation results for the initial pyrolysis process and the reaction mechanisms agree with previous experimental observations.