A new constructed macromolecule-pore structure of anthracite and its related gas adsorption: A molecular simulation study

The microscopic analysis of methane (CH4) adsorption in the heterogeneous pore structure of coals is vital for the success of the coalbed methane (CBM) projects. In this work, to make up for the limitations of laboratory testing conditions, grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were performed to systematically study the mechanism of CH4 adsorption on dry and moist coals under reservoir temperature and pressure conditions. A novel strategy, named the diamond-filling method, was proposed for the construction of macromolecular pores with specific diameter and morphology. The simulation results demonstrated that the state of CH4 molecules constrained in pores changes from homogeneous filling to multilayer adsorption with the increase of pore size. During the variation of the state of CH4 molecules, the interaction between CH4 and coals gradually weakened, resulting in a significant reduction in CH4 concentration, which stabilized after the pore size reached 5 nm. In addition, it was found that the spherical pore showed the highest adsorption capacity, followed by cylindrical pore and flat pore. In the case where the moisture content ranged from 1 to 3 wt%, the results showed that H2O molecules kept gathering towards the walls. Meanwhile, the CH4 molecules gradually moved towards the bulk interior part along with the declining concentration due to the deeper adsorption potential of H2O molecules in nanopores. The molecular simulation was shown to be a valuable and efficient tool for revealing the mechanism of CH4 adsorption in coal and producing results that have practical applications.