Investigation on the evolution characteristics and gas generation mechanism of coal spontaneous combustion-extinction-reignition in sealed fire zones based on ReaxFF simulation

The reopening and management of sealed fire zones in coal mines is essential for achieving efficient resource utilization. However, the rapid reignition of extinguished coal upon fire zone reopening presents a critical challenge to fire control. To elucidate the underlying mechanisms, this study first characterized BNM coal using XPS, ¹³C NMR and FTIR, followed by the construction and optimization of a coal macromolecular model. Subsequently, ReaxFF molecular dynamics simulations were employed to investigate the dynamic process of coal spontaneous combustion, extinction, and reignition under oxygen-deficient conditions in sealed fire zones. The results indicate that in the spontaneous combustion stage, oxidation and pyrolysis interact significantly, leading to a substantial reduction in char content and a sharp increase in H₂O and CO concentrations. During the extinction and cooling phase, aromatic condensation and crosslinking reactions dominate, resulting in a rise in char mass fraction, with CO and C₂H₂ being the primary gas products. In the reignition phase, active free radicals and unsaturated carbon structures significantly accelerate coal-oxygen reactions, causing an exponential increase in CO as the major gas product. The evolution of H/C and O/C atomic ratios and key reactive functional groups indicates that the spontaneous combustion stage is primarily governed by aliphatic structures and critical active functional groups. The active chemical memory structures formed during the cooling phase serve as precursors for rapid oxidation upon reignition. In the reignition stage, the accumulated diene-type unsaturated carbon species and abundant alkyl free radicals from the spontaneous combustion and extinction phases undergo autocatalytic oxidation, ultimately leading to coal reignition upon fire zone reopening. These findings provide important theoretical support for the safe reopening of fire zones in coal mines.