Supercritical Transition Adsorption Process of CO₂ and CH₄ on Heterogeneous Coal Particle Based on the Gibbs Surface Excess Model

To investigate the CO₂ adsorption-sequestration potential in deep coal seams, coal samples from various coal-bearing regions in China were selected for high-pressure adsorption experiments for CO₂ and CH₄. These experiments utilized the gravimetric method under varying temperature conditions. A segmented adsorption phase density (ρ_a) fitting model was applied to analyze the trends of the absolute adsorption amount (<i>m</i> _a). The results indicate that when CO₂ and CH₄ transition from gaseous to supercritical states, their adsorption mechanisms evolve from micropore filling and monolayer adsorption in mesopores to multilayer adsorption, which is divided into four distinct stages. The segmented fitting model effectively describes the micropore filling at low pressures and the multilayer adsorption in supercritical states. At low pressures, the adsorption behaviors of CO₂ and CH₄ are similar. However, once the supercritical state is entered, the rapid increase in CO₂ density leads to markedly different adsorption behaviors compared to CH₄. CO₂ displays anomalously high <i>m</i> _a values during its transition from a gas-like supercritical state to a liquid-like supercritical state. The liquid-like supercritical promotes a gradual increase in <i>m</i> _a during the later stages of high-pressure adsorption. The <i>m</i> _a of CO₂ reaches a maximum before transitioning to a liquid-like supercritical phase.