Oxidation characteristics of coal gangue under oxygen-lean atmosphere: Linking thermal effects, gas evolution, and functional group transformations

Spontaneous combustion in coal gangue dumps typically occurs under oxygen-lean conditions, in which oxygen availability controls the coupled evolution of mass loss, heat release, gaseous products, and surface functional groups. In this study, thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR), C600 calorimetry, and in situ Fourier transform infrared (FTIR) are combined to investigate the oxidation characteristics of coal gangue under oxygen-lean atmosphere. The TG/DTG results reveal a clear oxygen-dependent grouping: at 1-5% O 2 , TG curves nearly overlap, the peak mass-loss rate stabilizes at about −4.60% min −1 , and oxidation remains incomplete even at 800 °C, indicating a strongly oxygen-supply-limited regime. Increasing oxygen concentration enhances combustion parameters, while promoting more concentrated heat release. Within the 30-600 °C C600 experimental window, oxygen-lean atmospheres prolong the slow exothermic stage and delay the fast exothermic stage; the observed fast-stage fraction increases from 20% at 1% O 2 to 47% at 21% O 2 . CO 2 is the dominant gaseous product, with decreasing O 2 causing weaker, delayed and incomplete release. In situ FTIR further indicates complementary transformations between hydroxyl, aliphatic/aromatic hydrocarbons and oxygen-containing groups. Under extra oxygen-lean conditions, oxygen-containing structures accumulate while CO 2 evolution remains weak and incomplete. As oxygen availability increases, broader functional-group participation and faster turnover of oxygen-containing intermediates accompany stronger CO 2 evolution. Correlation analysis shows strong associations among mass loss, heat release, CO 2 evolution, and functional-group changes, indicating a shift from a common oxygen-supply constraint toward more distributed multi-pathway oxidation. These findings clarify the mechanistic role of oxygen-lean atmospheres in coal gangue oxidation and provide a basis for spontaneous-combustion prevention.