Molecular Modeling of Weakly Caking Coal and the CO2 Inhibition Mechanism of Coal–Oxygen Complexation

To elucidate the molecular structural characteristics of weakly caking coal and the microscopic mechanism by which CO2 inhibits coal–oxygen complexation, a weakly caking coal sample from the Dahaize coal mine in Shaanxi, China, was investigated using proximate and ultimate analyses, FTIR, XPS, and 13C NMR. On this basis, a representative coal macromolecular model was constructed and further analyzed using density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations. The molecular formula of the representative weakly caking coal from the Dahaize mine (RNM) unit was determined as C176H156N2O19S2. The aromatic carbon fraction was 65.41%, and the bridge carbon/peripheral carbon ratio was 0.25, indicating a certain degree of aromatic condensation but a limited content of highly fused aromatic structures. DFT calculations revealed that the reactive sites were mainly located around edge oxygen-containing functional groups and bridging structures, with a maximum Fukui index of approximately 0.024. Adsorption simulations showed that O2 and CO2 adsorption on RNM followed Langmuir-type behavior over 303.15–363.15 K: adsorption capacity increased with pressure and decreased with temperature. At 8000 kPa, the CO2 uptake was approximately 1.6 times that of O2. In the binary O2-CO2 system, CO2 preferentially occupied pore surfaces and high-energy adsorption sites, reducing the local enrichment of O2. These results provide a molecular-level explanation for the inhibition of coal–oxygen complexation by CO2 through competitive adsorption, site shielding, and decreased oxidation probability at active sites.