Influence of clay mineral changes on the methane adsorption characteristics of anthracite coal under acidification

The study uses a combination of laboratory tests, theoretical studies and molecular simulations, taking clay mineral-rich coal rock as the research object. Proximate analysis and X-ray diffraction experiments were performed to qualitatively and quantitatively analyze the ash and mineral contents in coal samples before and after acidification. A simulation software was used to build coal pore models with/without clay minerals, characterize the coal samples before and after acidification, and perform adsorption simulations, relative concentration simulations, and interaction energy calculations. The adsorption constants were measured through Isothermal adsorption experiments to verify the results of the simulation calculations. The ash content of the coal samples was reduced by 73.64% after acidification; muscovite completely disappeared and kaolinite content reduced by 75.32%. The adsorption simulation results revealed that the methane adsorption by the muscovite- and kaolinite-slit models was approximately 1.46 and 1.06 times that by the slit model, respectively. From the relative concentration simulation results, it can be seen that the relative concentration distribution peaks of methane appeared on the surface of muscovite and kaolinite in the clay slit model, indicating that the clay minerals have the ability to adsorb methane. The results of the interaction energy calculations show that the interaction energies of the slit model, muscovite-slit model and kaolinite-slit model with methane were −20.49 kJ/mol, −55.51 kJ/mol and −38.69 kJ/mol, respectively, which indicated that the interaction energies of the clay-slit model with methane were greater than those of the slit model, and that the presence of clay minerals was favorable for the adsorption of methane by the coal pore model. The results of isothermal adsorption experiments showed that the adsorption constant (a) decreased by 23.50%, indicating that the substantial decrease of muscovite and kaolinite in the experimental coal samples under the effect of acidification would lead to the weakening of their methane adsorption capacity, which confirms the results of the previous analysis, simulation, and calculations.