Parameter selection of factors affecting methane adsorption for coal molecules under the action of interlayer water molecules

Methane is one of the main disaster sources in mine accidents. Given the significant impact of moisture content in the actual coal seams on adsorption capacity. To comprehensively explain the microscopic process of water molecules transitioning from static to dynamic through interlayer structure and to ensure controllability of macroscopic experiments, this manuscript constructed the interlayer outside water-coal molecular model (IOW-CMM) and interlayer inside water-coal molecular model (IIW-CMM) by molecular simulation method. Criteria for judging the two models were put forward, methane adsorption differences were revealed and optimal combination of synergistic effects were clarified among water molecular content, interlayer distance, temperature and pressure influencing factors via response surface methodology (RSM); coal pore size experiment under the macroscopic conditions and micro-optimized interlayer distance were verified. Results demonstrated that the negative electrostatic potential energy of water molecules in IIW-CMM was higher than that in IOW-CMM, leading to low methane absolute and saturation adsorption capacity. When temperature was at 288.15 K, saturation adsorption capacity both reached the maximum. The optimal response combinations were 2% water molecular content, 4 nm interlayer distance, temperature 288.15 K and pressure 30 MPa. Under the above conditions, predictive model results were similar to simulation results, the test reasonableness of maximum coal pore size (3.8 nm) under macroscopic conditions got closer to interlayer distance (4 nm) after optimization by microscopy. The change rates of pore volume for coal samples after water treatment increased significantly, which inhibited methane adsorption. This research provides scientific basis for reduction of gas content and rational parameter selection, and is of guiding significance for in-depth discussion on prevention and control of coal and gas herniation.