Molecular Dynamics (MD) and Grand Canonical Monte Carlo (GCMC) Simulation of Competitive Adsorption Mechanisms of CH 4 /CO 2 /O 2 on Anthracite: Effects of Temperature and Pressure

Coal spontaneous combustion is a critical underground mining hazard, driven primarily by O₂ adsorption/activation on coal pore surfaces. MD and GCMC simulations using Materials Studio investigated competitive adsorption of CH₄, CO₂, and O₂ on an anthracite model (C₁₄₂H₆₂N₂O₁₀S) from Yangmei No. 5 Mine, Shanxi. The model was built from 13C NMR, FTIR, and XPS. Simulations ran at 283.15, 298.15, and 313.15 K, 0–25 MPa, covering single, binary, and ternary mixtures. A previously unreported finding is the identification of O₂-specific adsorption sites within anthracite that selectively accommodate CO₂ while excluding CH₄, revealing a displacement mechanism by which CO₂ replaces pre-adsorbed O₂. At 298.15 K and 25 MPa, saturated capacities follow CO₂ (6.4) > O₂ (5.6) > CH₄ (3.6 molecules/cell). At low pressures, CO₂ adsorption exceeds 85% of saturation, whereas CH₄ and O₂ reach only ~50%. Isosteric heats (CO₂: 35.2; O₂: 27.6; CH₄: 19.8 kJ·mol⁻1) confirm physisorption. In mixtures, CO₂ exhibits the highest selectivity, with CO₂/CH₄ >1.0 across all conditions; CO₂/O₂ shows minimal temperature dependence, indicating effective O₂ displacement across a wide temperature range. These findings provide a molecular-level foundation for optimizing CO₂ injection. Limitations include the use of a single coal model; future work should incorporate varying ranks and moisture effects.