Comparative study on effect of microstructure characteristics on adsorption of CH₄ and CO₂ in anthracite based on molecular simulation

In order to explore the effects of pore size, aromatic lamellar stacking degree, aromatic lamellar ductility, gas temperature and gas pressure on the competitive adsorption of CH₄ and CO₂, the No.3 coal from Jincheng mining area was taken as the research object. The adsorption process of binary mixed gas of CH₄ and CO₂ in coal under different temperatures, pressures, pore diameters, aromatic lamellar stacking degrees and aromatic lamellar ductility was studied. The microstructure of coal was tested and analyzed based on industrial analysis and X-ray diffraction experiments. Molecular simulation of the process of gas mixture adsorption in coal using the Grand Canonical Monte Carlo (GCMC) method. The results shown that, the increase of gas pressure was favorable for adsorption, and the adsorption capacity of CO₂ was close to saturation at a pressure of 5 MPa. The increase of temperature led to a decrease in the maximum adsorption capacity and adsorption heat of CO₂ in coal. The increase of pore size led to the increase of CH₄ adsorption capacity in coal, with the fastest increase in CH₄ adsorption occurred when pore size increased from 1 nm to 2 nm. With the increase of aromatic lamellar stacking degree, the number of gas molecules adsorbed in coal did not change significantly, but CO₂ adsorption capacity of per unit mass in coal decreased rapidly, and the CH₄ adsorption capacity decreased slightly. With the increase of aromatic lamellar ductility, the number of CH₄ molecules adsorbed in coal increased slowly and the number of CO₂ molecules adsorbed increased rapidly, and the change of the two gases adsorption capacity in coal per unit mass was not significant. From the perspective of adsorption capacity and adsorption heat, CO₂ is in a dominant position in the competitive adsorption process, so injecting CO₂ into coal seam can effectively displace CH₄. The results of this paper strengthened the molecular understanding of the adsorption of binary mixed gas of CH₄ and CO₂, which can lay a theoretical foundation for the enhancement of coalbed methane extraction by CO₂ injection.