Thermodynamic characteristics of low-temperature oxidation of oil-impregnated coal: Gas generation, marginal effect, microscopic mechanism, and implications for safe production

To reveal the mechanism by which crude oil intrusion affects low-temperature oxidation and spontaneous combustion of coal in a coal-oil coexisting system, oil-impregnated coal samples were prepared and characterized by low-temperature N₂ adsorption, SEM，programmed-temperature oxidation, and FTIR. The results show that crude oil preferentially blocks macropores and mesopores, reducing specific surface area and pore volume, thereby inhibiting aliphatic groups (–CH₃, –CH₂–) oxidation into oxygen-containing groups (–C=O, –COO–). Consequently, CPT increases exponentially from 161.2 to 206.8 °C, while FCC decreases power-law from 4.24 to 2.69 (min·°C) −1 , while CO and C₂H₄ cumulative emissions decrease continuously. Low-temperature oxidation liability exhibits a pronounced “marginal effect,” the marginal inhibition efficiency drops from 1.031 to 0.010 (min·°C·%) −1 as oil content rises from O0 → O1 to O7 → O10. CH₄ exhibits a “first-suppressed-then-promoted” trend: below 200 °C, crude oil suppresses coal‑oxygen reactions; above 240 °C, thermal cracking of saturated alkanes in crude oil dominates, causing cumulative CH₄ to increase linearly with oil content. Regarding composite gas indicators, CO/ΔO₂ shows an early-stage peak lag for high-oil coal, C₂H₄/CH₄ rapidly jumps after 200 °C for low-oil coal, and CH₄/CO exhibits an overall “U-shaped” reversal. These findings underscore the necessity of establishing a multiple-composite identification system for safe production in coal-oil coexisting mines.