Modeling Methane Adsorption Distance Using Carbon Nanotubes and Bituminous Coal Pore Models

To simplify the analysis of the adsorption of methane by coal, single-walled carbon nanotubes (SWCNTs), slit graphene lamellae, and bituminous coal pore models were constructed, and the adsorption of methane molecules in the three models was studied using molecular dynamics and density functional theory. The results show that methane molecules cannot be adsorbed within carbon nanotubes with a pore size of 0.5 nm. In carbon nanotubes with a pore size of 1 nm, adsorbed methane is influenced by the inner wall of the SWCNTs to induce dipoles and to interact with nearby methane molecules. In large-pore-size carbon nanotubes and graphene sheets, methane molecules adsorb and accumulate on the wall surface to form adsorption rings, the thickness of which gradually increases with the pore size under the effect of curvature, ranging from 0.455 to 0.521 nm. This criterion for adsorption was applied to the pore model of bituminous coal, and 52 adsorbed methane states were determined. Furthermore, the Brunauer-Emmett-Teller equation was used to fit the methane isothermal adsorption curve of the bituminous coal pore model, and the adsorption quantity of methane in a single molecular layer was 54, confirming that the adsorption criterion of 0.521 nm is reasonable.