ReaxFF Molecular Dynamics Study on the High-Temperature Oxidation Mechanisms of High-Sulfur Coal Under Different Oxygen Supply Levels in Goafs

To elucidate the microscopic effects of oxygen supply levels on the high-temperature oxidation pathways of high-sulfur coal in goafs, a molecular structure model of Liupanshui high-sulfur coal (LPS) was constructed and optimized based on ultimate analysis, XPS, and 13C-NMR. Subsequently, four coal–oxygen reaction systems with different oxygen supply levels were constructed to investigate the oxidation behavior of high-sulfur coal under oxygen-limited conditions. ReaxFF molecular dynamics simulations were performed to analyze the influence of oxygen availability on reaction pathways, gaseous product evolution, radical transformation, and structural degradation of coal. The simulation results indicate that oxygen deficiency mainly affects the early oxidation stage by weakening the radical chain reaction network. A reduction in oxygen supply significantly inhibits the formation and transformation of key intermediates, including ·OOH and H2O2, thereby limiting the accumulation of ·OH radicals. In comparison, the termination reaction between ·OH and ·H for H2O formation is less sensitive to oxygen variation. Reduced oxygen availability decreased the overall reaction intensity and slowed the cleavage of C-C and C-H bonds, thereby delaying the degradation of the coal macromolecular structure. The formation of CO2 and H2O was significantly inhibited under oxygen-deficient conditions, while CO exhibited a different evolution pattern due to the balance between its formation and secondary oxidation. Changes in oxygen supply also affected sulfur-containing gas products, with lower SO2 production and increased yields of H2S, COS, and CS2. Reaction pathway analysis further indicated that thiolic and thiophenic sulfur underwent different oxidation processes, and thiophenic sulfur was activated at a later stage than thiol sulfur. These findings clarify the molecular mechanisms governing oxygen-dependent oxidation behavior in high-sulfur coal.