Multiple Models Characterize the Dynamic Adsorption Behavior of Supercritical CO₂ on Medium to High Rank Coal

Injecting CO₂ into deep coal seams can seal part of the CO₂ permanently and increase the production of coalbed methane. At present, many supercritical adsorption models can be used to evaluate the adsorption capacity of CO₂ in deep coal seams, but they are all based on the inherent adsorption form. The adsorption behavior of CO₂ changes dynamically with an increase in pressure or temperature. To analyze the dynamic adsorption behavior of supercritical CO₂ on medium to high rank coal and compare the fit degree of supercritical adsorption characterization model for excess adsorption capacity at different stages of pressurization or warming, isothermal adsorption experiments of anthracite and coking coal are carried out by a magnetic levitation weighing isothermal adsorption device at 305.15, 308.15, 318.15, 328.15, and 338.15 K. The results show that the average fit degrees of the supercritical L-F and D-A adsorption models are the highest, which are 0.2% higher than the supercritical Langmuir adsorption model and 2.1% higher than the traditional Langmuir monolayer adsorption model, and they can accurately characterize the dynamic adsorption behavior of supercritical CO₂ on medium to high rank coal. The adsorption results of both kinds of coal show that there are three intersections between the supercritical Langmuir and D-R adsorption models and the isothermal adsorption curve. The adsorption process is divided into three sections: low pressure, medium pressure, and high pressure, based on the boundary of free phase densities of 0.55 and 3 mol/L. The fitting results of the Langmuir adsorption model from a low to high critical range are undersized, oversized, and severely undersized, while the fitting results of the supercritical D-R adsorption model are slightly oversized, undersized, and oversized, indicating that the main adsorption behaviors of CO₂ molecules during the pressure rise process are micropore filling, unsaturated multilayer adsorption, and transition pore filling, respectively. When the free phase density continues to increase to 16 mol/L (305.15 K, 308.15 K), 14 mol/L (318.15 K), 12 mol/L (328.15 K), and 10 mol/L (338.15 K), the pore size that can be completely filled reaches the maximum. Under the same pressure, the number of adsorbed layers of CO₂ molecules on the surface of anthracite decreases by 13.5% when the temperature rises from 308.15 to 318.15 K, by 20.8% when the temperature rises from 318.15 to 328.15 K, and by 6.67% when the temperature rises from 328.15 to 338.15 K. However, the temperature has a low impact on coking coal.