Study on the Effect of Coal Slit Pore Size on CH₄ and CO₂ Adsorption Characteristics Based on Grand Canonical Monte Carlo

CO₂ Enhanced Coalbed Methane Mining (CO₂-ECBM) technology integrates multiple economic, resource, environmental, and safety benefits such as carbon fixation, enhanced coalbed methane recovery, promotion of natural gas development, and reduction of greenhouse gas emissions. The study found that, based on the adsorption potential theory, the adsorption potentials of CH₄ and CO₂ in the slit pore were calculated, and the pore region was divided based on the potential energy distribution in the pore and the average kinetic energy of gas molecules. The location of positive adsorption potential φ_sf was defined as the repulsion region, the region with |φ_sf /<i>E</i>_k|>2 as the strong adsorption region, the region with 1<|φ_sf /<i>E</i>_k|≤2 as the weak adsorption region, and the region with |φ_sf /<i>E</i>_k|≤1 as the free<i>-</i>phase region. The present study investigates the adsorption and diffusion characteristics of CH₄ and CO₂ in slit pores of different sizes. This investigation is achieved by conducting Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations, with the objective of determining the effects of pressure, temperature and slit pore size on the quantity of adsorption were analyzed, and adsorption configurations were obtained under different pressure and pore size conditions. It was found that CH₄ mainly existed in the pores of coal below 3.5 nm, which mainly showed microporous filling and monolayer adsorption. When the slit pore size increased from 2.5 to 8.0 nm, the adsorption isotherm of CO₂ conformed to Langmuir type IV, and the adsorption behaviors included microporous filling, monolayer adsorption and multilayer adsorption, and capillary coalescence phenomenon was observed as the pressure increased. The following essay will seek to demonstrate an understanding of the adsorption-diffusion mechanism of CH₄ and CO₂ in slit pores for improving the recovery rate of CBM, for predicting CBM reserves, and for increasing the potential of CO₂ sequestration.