Thermodynamic Characteristics of CH₄/CO₂ Adsorption in Different Rank Coals and Its Molecular Mechanism

In order to investigate the essence of CH₄/CO₂ adsorption in coal for CO₂-enhanced coalbed methane recovery (CO₂-ECBM), this study established the coal structure models from the chemical composition and structure information on different rank coals to conduct CH₄ and CO₂ adsorption simulation under different environmental conditions. Thus, the differences and connections between integral heat and isosteric heat of CH₄/CO₂ adsorption in coal and its micro-mechanism were discussed. The results show that as the coal metamorphism degree deepens, the integral heat of CH₄/CO₂ adsorption, similar to adsorption capacity, presents a decreasing first and then increasing trend. While the adsorption equilibrium time of high-rank coal gives a significantly decreasing characteristic with pressure. Then, on the basis of adsorption simulation behavior, it finds that because complex functional groups exist in the coal macromolecular structure, the adsorption capacity shows a different characteristic compared with the experimental results; that is, it decreases with the coal metamorphism degree. Meanwhile, compared to CO₂ adsorption, the isosteric heat of CH₄ adsorption appears to have an obvious downward trend with increasing pressure and then gradually stabilizes. Further, there is always a clear linear relationship between CH₄ adsorption capacity, and isosteric heat for aromatic pores in different rank coals. While for slit pores, both CH₄ and CO₂ molecules exhibit significant parabolic relationships between adsorption capacity and isosteric heat. In addition, on the one hand, except for the obvious chemical adsorption of low-rank coal in the high-pressure stage, affected by pore morphology and size, the isosteric heat of CH₄ or CO₂ adsorption manifests lower values in slit pores and large pore sizes. On the other hand, based on the adsorption systems of similar structural fragments with different functional groups, -OH has been identified as the functional group with the strongest adsorption effect on gas molecules and is also the main functional group causing CO₂ chemical adsorption.