Simulations and experimental investigations of the competitive adsorption of CH4 and CO2 on low-rank coal vitrinite.

The mechanism for the competitive adsorption of CH4 and CO2 on coal vitrinite (DV-8, maximum vitrinite reflectance R o,max = 0.58%) was revealed through simulation and experimental methods. A saturated state was reached after absorbing 17 CH4 or 22 CO2 molecules per DV-8 molecule. The functional groups (FGs) on the surface of the vitrinite can be ranked in order of decreasing CH4 and CO2 adsorption ability as follows: [-CH3] > [-C=O] > [-C-O-C-] > [-COOH] and [-C-O-C-] > [-C=O] > [-CH3] > [-COOH]. CH4 and CO2 distributed as aggregations and they were both adsorbed at the same sites on vitrinite, indicating that CO2 can replace CH4 by occupying the main adsorption sites for CH4-vitrinite. High temperatures are not conducive to the adsorption of CH4 and CO2 on vitrinite. According to the results of density functional theory (DFT) and grand canonical Monte Carlo (GCMC) calculations, vitrinite has a higher adsorption capacity for CO2 than for CH4, regardless of whether a single-component or binary adsorbate is considered. The equivalent adsorption heat (EAH) of CO2-vitrinite (23.02-23.17) is higher than that of CH4-vitrinite (9.04-9.40 kJ/mol). The EAH of CO2-vitrinite decreases more rapidly with increasing temperature than the EAH of CH4-vitrinite does, indicating in turn that the CO2-vitrinite bond weakens more quickly with increasing temperature than the CH4-vitrinite bond does. Simulation data were found to be in good accord with the corresponding experimental results.