Construction of different metamorphic degree coal models and corresponding adsorption characteristics based on molecular simulation

Coal samples with different metamorphic degrees have different gas-adsorption capacities. Most of the existing studies analyze differences in gas-adsorption characteristics from a macro perspective, based on experimental methods, and there has been limited research on the micro level. Using industrial and elemental analyzes, Fourier-transform infrared spectroscopy, 13C-NMR, and X-ray photoelectron spectroscopy, we tested and analyzed the molecular-structure characteristics, such as elemental composition, atomic ratio, and functional-group type and distribution, of three coal samples: anthracite (WYM), coking coal (JM), and lignite (HM), with different metamorphic degrees and constructed their macromolecular-structure models. The carbon contents and densities of the models were consistent with the measured values. We also constructed a coal matrix model using a molecular simulation based on the constructed coal molecular model. By comparing and analyzing the trends of adsorption amount, isosteric heat, and selective adsorption coefficient, we explored the differences in the adsorption characteristics of single and binary mixtures on coals with different metamorphic degrees. The adsorption of CH4 and CO2 on the coal matrix conformed to the Langmuir adsorption model. With increasing metamorphic degree, the adsorption amount increased in the order WYM > JM > HM, and the adsorption capacity of the coal matrix for CO2 was higher than that for CH4. The isosteric adsorption heat of CH4 was proportional to the degree of coal metamorphism; however, its relationship with pressure was not clear. Under different temperature and pressure conditions, the selective adsorption of CH4/CO2 on coal with various metamorphic degrees decreased with an increasing CO2 molar ratio. Conditions of 303.15 K (30 °C), 2–4 MPa, and CH4/CO2 = 0.1 were more conducive to the competitive adsorption of CO2 than that of CH4.