Adsorption behavior of carbon dioxide and methane in bituminous coal: A molecular simulation study

The adsorption behavior of CO2, CH4 and their mixtures in bituminous coal was investigated in this study. First, a bituminous coal model was built through molecular dynamic (MD) simulations, and it was confirmed to be reasonable by comparing the simulated results with the experimental data. Grand Canonical Monte Carlo (GCMC) simulations were then carried out to investigate the single and binary component adsorption of CO2 and CH4 with the built bituminous coal model. For the single component adsorption, the isosteric heat of CO2 adsorption is greater than that of CH4 adsorption. CO2 also exhibits stronger electrostatic interactions with the heteroatom groups in the bituminous coal model compared with CH4, which can account for the larger adsorption capacity of CO2 in the bituminous coal model. In the case of binary adsorption of CO2 and CH4 mixtures, CO2 exhibits the preferential adsorption compared with CH4 under the studied conditions. The adsorption selectivity of CO2 exhibited obvious change with increasing pressure. At lower pressure, the adsorption selectivity of CO2 shows a rapid decrease with increasing the temperature, whereas it becomes insensitive to temperature at higher pressure. Additionally, the adsorption selectivity of CO2 decreases gradually with the increase of the bulk CO2 mole fraction and the depth of CO2 injection site.