Experimental and molecular simulation study on the competitive adsorption characteristics of CO₂ and CH₄ in coals of different rank

To investigate the competitive adsorption mechanism of CO₂ and CH₄ in coal seams of different ranks, lignite (low-rank), long-flame coal (medium-rank), and anthracite (high-rank) were selected as research subjects. Competitive adsorption experiments were conducted under various temperatures, pressures, and gas composition ratios. The molecular structures of the coal samples were characterized using elemental analysis, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and solid-state nuclear magnetic resonance (NMR). Based on the characterization results, corresponding coal macromolecular models were constructed. Grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods were employed to simulate the adsorption process, and the adsorption capacity, interaction energy, and accessible pore distribution characteristics of different gases in the coal molecular models were calculated. The results show the following: (1) With increasing coal rank, the absorption peak area of aromatic structures increases. A higher content of aromatic structures leads to stronger gas adsorption capacity, meaning that high-rank coals possess greater adsorption capacity. As coal rank deepens, the aromatic structure evolves directionally: trisubstituted benzene ring structures dominate and their proportion continuously increases, while tetrasubstituted structures decrease, and disubstituted and pentasubstituted structures increase. This reflects the evolution of coal macromolecules toward condensed aromatic hydrocarbons.(2) CO₂/CH₄ competitive adsorption is a dynamic process jointly controlled by the partial pressure effect and adsorption affinity. The gas composition ratio determines the initial pattern of competitive adsorption: the higher the CO₂ proportion, the more obvious its advantage in the initial stage. System pressure significantly regulates the dominant mechanism: the low-pressure stage is mainly controlled by the partial pressure effect, whereas the high-pressure stage is mainly controlled by adsorption affinity. Low temperature and high pressure are favorable for the preferential adsorption of CO₂ and the displacement of CH₄.(3) Molecular simulation results show that the degree of aromatic ring condensation and the C—C/C—H bond ratio in coal molecules are positively correlated with CO₂ adsorption energy. In all different ranks, the interaction energy of CO₂ (from -143.09 kJ/mol to -128.87 kJ/mol) is significantly lower than that of CH₄ (-89.96 kJ/mol to -82.42 kJ/mol), with the absolute value being 41.8 kJ/mol to 54.4 kJ/mo l higher. This indicates that the binding strength between CO₂ and the coal surface is much higher than that of CH₄, which fundamentally explains why CO₂ always maintains an advantage in competitive adsorption.