Molecular simulation of CH4 and CO2 adsorption behavior in coal physicochemical structure model and its control mechanism

Abstract

In order to clarify the microscopic coupling mechanism among chemical composition and structure, nanopores and gas adsorption, this study used molecular simulation to investigate the adsorption behavior of CH4 and CO2 molecules in different coal physicochemical structure models under various environment conditions. The results show that with the dissolution of low molecular weight compounds (LMWCs), the absolute adsorption capacity for CO2 and CH4 in aromatic pores gradually increases, and slit pores show a more obvious promotional effect on gas adsorption. On the other hand, observing the radial distribution function(RDF) between the main functional groups in coal and CH4/CO2, oxygen-containing functional groups and aromatic structures display a significant impact on the distribution of gas molecules on coal surface, especially –COOH and -O- groups. While free LMWCs are easy to generate competitive adsorption with gas molecules. Further, on the basis of adsorption heat and energy changes in coal-gas system, it concludes that CH4 molecules on the coal surface always keep physical adsorption, but CO2 molecules are dominated by physical adsorption with weak chemical adsorption. Finally, the relationship among chemical composition and structure, nanopores and gas adsorption is established to reveal the microscopic coupling effect during gas adsorption.