CO₂ passivation and functional group evolution in coal spontaneous combustion under low oxygen environment

To investigate the dynamic evolution of functional groups during coal spontaneous combustion under varying CO₂ concentrations and elucidate the chemical passivation mechanism of CO₂ on coal oxidation, this study selected a type I bituminous coal prone to spontaneous combustion from Liangdu Mine. Temperature-programmed oxidation experiments were conducted to compare the oxidative heat release characteristics of coal samples under four oxygen concentrations (5%, 10%, 15%, and 20%), determining the critical oxygen concentration for observing CO₂ passivation effects. Fourier transform infrared spectroscopy was applied to quantitatively analyze variations in major coal functional groups during temperature elevation under different CO₂ environments. Pearson correlation analysis was employed to investigate competitive and synergistic relationships among functional groups during low-temperature coal oxidation. Density functional theory calculations were performed to simulate reaction pathways between CO₂ and phenolic hydroxyl groups as well as phenylacetic acid. Results indicate that the critical oxygen concentration is 10%; below this threshold, the oxygen consumption rate decreases by 49.2% and heat release intensity declines by 67.3%. Under 90% CO₂ conditions, coal aromaticity increases by 69.17%, aliphatic chain length parameter rises by 31.06%, ether bond content remains stable, and the temperature corresponding to maximum values shifts from 95 °C to 185 °C. Aromatic structures exhibit synergistic evolution with adjacent oxygen-containing groups during coal oxidation, while aliphatic chains compete with oxygen-containing groups for consumption. High CO₂ environments suppress coal oxidation chain reactions, reducing competitive interactions while enhancing synergistic effects, significantly delaying aliphatic hydrocarbon thermal cleavage and inhibiting hydroxyl consumption and C=O decarboxylation. Reaction energy barriers between phenolic hydroxyl groups/phenylacetic acid and CO₂ are lower than those with oxygen, facilitating carboxylic compound formation with reduced heat release, thereby achieving chemical passivation of coal spontaneous combustion.