Simulations and experimental investigations of the competitive adsorption of CH 4 and CO 2 on low-rank coal vitrinite

Abstract

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.