Study on the evolution laws of coal spontaneous combustion products and reaction pathways under different oxygen concentrations based on ReaxFF simulation

Understanding the oxygen-dependent reaction behavior of residual coal is important for evaluating its renewed oxidation after oxygen re-entry into sealed fire zones. In this study, temperature-programmed oxidation experiments coupled with gas chromatographic and FTIR analyses were combined with reactive force field molecular dynamics (ReaxFF-MD) simulations to investigate coal oxidation under different oxygen supply conditions. The experimental results showed that decreasing O 2 concentration suppressed carbon-oxide formation and altered the relative distribution of hydrocarbon products, indicating enhanced competition between oxidation and pyrolysis under oxygen-deficient conditions. FTIR analysis further revealed distinct oxygen-dependent evolution of aliphatic C–H, carbonyl-related, and hydroxyl-related structures. ReaxFF-MD simulations reproduced the overall oxygen-dependent tendency of oxidation-related products and showed that oxygen availability regulates the transformation of oxygen-containing and aliphatic structures as well as the competition among fragmentation, oxidation, and recombination reactions, thereby altering the distributions of gaseous products, tar, and residual carbon. Representative molecular pathways identified from the trajectories further elucidated the formation and transformation of major gaseous products. Overall, oxygen deficiency induces a redistribution of the oxidation–pyrolysis reaction network rather than simply reducing the reaction extent. These findings provide an experimentally supported molecular-level interpretation of the chemical behavior of residual coal under oxygen-deficient conditions and its renewed oxidation following oxygen re-entry.