The Influence of Different Pore Shapes on Methane Adsorption Characteristics of Coal

Pore shape has a significant impact on methane adsorption. In this paper, graphene was used to construct coal pore models with different pore shapes. The adsorption of methane was simulated to investigate the effect of pore shape on the adsorption characteristics of coal methane. The results show that methane adsorption varies significantly among different pore models, and the influence of pore openness depends on the adsorption metric considered. In terms of absolute adsorption capacity, the open cylindrical pore exhibits a higher methane adsorption capacity than the semi-closed cylindrical pore, whereas for maximum excess adsorption capacity, the semi-closed cylindrical pore is slightly higher than the open cylindrical pore. The maximum methane adsorption of wedge-shaped pores was 0.6767 mmol/g, while the adsorption effect of ink bottle pores was the worst, with a maximum of only 0.5686 mmol/g. The methane concentration distribution inside the pores shows higher values at the edges and lower values at the center. The adsorption of methane molecules with the four pore models was in the range of 2.75–6 Å. The two cylindrical pore models exhibit lower potential energy than the wedge-shaped and ink-bottle pore models, indicating higher structural stability and smaller adsorption-induced swelling under the present simulation conditions. However, lower swelling and lower energy do not necessarily correspond to higher methane adsorption capacity, as the wedge-shaped pore exhibits the highest adsorption among the four models. With the increase in pressure, the adsorption positions of methane molecules in cylindrical pores gradually changes from a high-energy adsorption site to a low-energy adsorption site.