Analysis of effects of microstructure of tectonically-deformed coal on the CO₂ and CH₄ competitive adsorption capacity

In order to clarify the mechanism of the competitive adsorption of CO₂ and CH₄ influenced by microstructure of tectonically-deformed coal, based on Grand Canonical Monte Carlo, Molecular Dynamics method, and Density Functional Theory, the effects of the extension of basic structural unit (L_a), the number of basic structural unit (N), slit aperture (d), surface defects (V), and oxygen-containing functional groups (M) on the adsorption behavior of CO₂ and CH₄ were investigated. The results show that the adsorption amount of CO₂ and CH₄ and the average isosteric heat of adsorption increase with the increase of L_a and d but decrease with the increase of N. The presence of V and M in tectonically-deformed coal reduces the adsorption amount and average isosteric heat of adsorption. The adsorptions of CO₂ and CH₄ on the surfaces of secondary defects are stronger than that on the surfaces of single defects. The carboxyl is the most unfavorable for CO₂ adsorption on coal surface, and the hydroxyl is the most unfavorable for CH₄ adsorption on coal surface. The selectivity coefficients of CO₂/CH₄ are always greater than 1, the smaller the slit pore size, the more significant advantage of CO₂ competitive adsorption is. Furthermore, the peak value of the electron density of states of CO₂ is larger than that of CH₄ at Fermi level after CO₂ and CH₄ are adsorbed on the coal surface, indicating that CO₂ plays a dominant role in the competitive adsorption process with CH₄. Overall, the competitive adsorption mechanism of CO₂ and CH₄ in different tectonically-deformed coal structures is revealed from the microscopic point of view, and the results can provide a theoretical basis for the efficient exploitation of coalbed methane.