Molecular simulation study on the effect of coal metamorphism on the competitive adsorption of CO2/CH4 in binary system

To explore the competitive adsorption characteristics of a CO2/CH4 binary gas in coal with different metamorphic degrees, macromolecular-structure coal models with different metamorphic degrees (anthracite coal, coking coal, and long-flame coal) were constructed based on elemental analysis, X-ray photoelectron spectroscopy (XPS), and carbon-13 nuclear magnetic resonance (13C NMR) results. Then, the competitive adsorption behaviors of CO2 and CH4 molecules in the different coal molecular models were simulated using molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) methods, and the changes of the competitive adsorption capacity, density distribution, adsorption selectivity, adsorption heat, and energy distribution were investigated. The results showed that the variation characteristics of the total adsorption of mixed gas were similar to those of pure gases, and the increase in the pressure led to an increase in the total adsorption, which also increased with the increase in the coal deterioration degree. The distribution ranges of the gas adsorption sites in coal with different metamorphic degrees were significantly different, with anthracite coal being the most widely distributed and long-flame coal being the least widely distributed. With the increase in the coal rank, the competitive adsorption selectivity of any proportion of CO2/CH4 binary mixed gas increased. The difference in the equivalent adsorption heat of single-component gases CO2 and CH4 in the mixed gas increased with the increase in the coal metamorphism and decreased with the increase in the CO2 gas proportion coefficient. This paper provides a theoretical basis for the application of CO2 enhanced coal bed methane in coal seams with different metamorphic degrees.