Effects of Pore Parameters and Functional Groups in Coal on CO2/CH4 Adsorption

The mechanisms of CO₂/CH₄ adsorption in coal are the theoretical foundation for CO₂ sequestration in coal seams targeted for enhanced coalbed methane recovery. Herein, by changing the model (low rank coal: WMC, middle rank coal: XM and high rank coal: CZ) with plenty of side aliphatic chains and functional groups established in the literature, the influence and mechanism of pore parameters and functional groups(-CH₃, -OH, -C₂O, -C=O) on the adsorption of CO₂ and CH₄ in different rank coals are systematically studied. Using the Connolly surface algorithm to calculate the pore volume (V ^F) and the specific surface area (S ^SA) of coal with different functional groups, it can be seen that the influence of the functional group change on the pore structure is related to the coal rank. Changing the various functional groups in the original coal structure to a unified functional group (-CH₃, -OH, -C₂O, or -C=O) will increase the accessible pore volume (V ^F) and the specific surface area (S ^SA), except in low-rank and middle-rank coal, where the ordered arrangement of -C=O will decrease V ^F and S ^SA. The adsorption capacities of different pore parameters and functional groups were calculated by Grand Canonical Monte Carlo simulation and density functional theory. On pure adsorption, the pore parameters exert greater influence than the functional groups. By comparing the adsorption energy of the original pore structure containing functional groups and that of modified pores without functional groups, the contributions of the pore structure and original functional groups on CO₂/CH₄ adsorption are 71 and 29% and 83 and 17%, respectively. Small-diameter pores and -C₂O have a strong adsorption capacity. In terms of competitive adsorption, the -C=O functional groups and pore diameters ranging from 1.0 to 2.0 nm can significantly enhance the selectivity of CO₂ over CH₄. The CH₄ and CO₂ adsorption does not occur via rigorous monolayer adsorption; multilayer adsorption can occur for CH₄ and CO₂ with pore diameters of 1.0-2.0 and 1.0-2.2 nm, respectively, thus causing micropore filling. These quantitative results establish a foundation for the development of adsorption theory for CO₂/CH₄ in coal.