Molecular mechanisms of multicomponent flue gas adsorption and methane displacement in coal nanopores

Hot flue gas injection is considered a promising strategy for enhancing coalbed methane recovery and facilitating carbon dioxide sequestration. However, the microscopic mechanisms governing multicomponent gas adsorption, transport, and displacement within confined coal nanopores remain insufficiently understood. In this study, a multiscale simulation framework integrating grand canonical Monte Carlo, equilibrium molecular dynamics, and non-equilibrium molecular dynamics (NEMDs) was established based on a Wiser-type coal slit nanopore model to systematically investigate the effects of pressure, temperature, pore size, and moisture on gas adsorption, diffusion, and displacement behaviors. The results demonstrate that gas occurrence, diffusion, and displacement behaviors are jointly governed by confinement effects and thermodynamic conditions. Increasing pressure enhances adsorption and promotes the transition from surface adsorption to pore filling while suppressing molecular diffusion. Elevated temperature weakens adsorption stability and gas molecular density while enhancing molecular mobility, thereby producing competing effects on methane displacement efficiency. Larger pores weaken confinement effects and facilitate molecular diffusion, whereas smaller pores strengthen adsorption but hinder gas transport. Water molecules reconstruct the interfacial adsorption environment, thereby enhancing gas retention while reducing diffusion capability. In multicomponent systems, carbon dioxide exhibits stronger adsorption affinity than nitrogen owing to its preferential occupation of high-energy adsorption sites. NEMD simulations further demonstrate that carbon dioxide-rich flue gas enhances methane recovery to 97.7%, while a portion of the injected carbon dioxide remains stably trapped within coal nanopores. These findings provide molecular-scale insights into the coupled mechanisms underlying methane recovery and carbon dioxide sequestration in deep coal seams.