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.