Analysis of effects of microstructure of tectonically-deformed coal on the CO ₂ and CH ₄ competitive adsorption capacity

In order to clarify the mechanism of the competitive adsorption of CO ₂ and CH ₄ 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 CO ₂ and CH ₄ were investigated. The results show that the adsorption amount of CO ₂ and CH ₄ 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 CO ₂ and CH ₄ on the surfaces of secondary defects are stronger than that on the surfaces of single defects. The carboxyl is the most unfavorable for CO ₂ adsorption on coal surface, and the hydroxyl is the most unfavorable for CH ₄ adsorption on coal surface. The selectivity coefficients of CO ₂/CH ₄ are always greater than 1, the smaller the slit pore size, the more significant advantage of CO ₂ competitive adsorption is. Furthermore, the peak value of the electron density of states of CO ₂ is larger than that of CH ₄ at Fermi level after CO ₂ and CH ₄ are adsorbed on the coal surface, indicating that CO ₂ plays a dominant role in the competitive adsorption process with CH ₄. Overall, the competitive adsorption mechanism of CO ₂ and CH ₄ 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.