Adsorption of Coalbed Methane in Dry and Moist Coal Nanoslits

Coalbed methane (CBM) is regarded as an important unconventional natural gas to relieve the world from energy shortage. Clearly understanding the adsorption mechanisms of methane in coal formations is fundamental to reserve estimation and recovery enhancement. Here, molecular models of the bituminous coal nanoslit (BCNS) were constructed on the basis of the elemental analysis of the coal samples. Molecular dynamics (MD) was utilized to investigate CBM adsorption in BCNSs and graphene nanoslits (GRANSs) with different widths. The adsorption of the nanoslits increases with decreasing the width. The absolute gas adsorption increases with pressure. In 2 and 3 nm BCNSs, the adsorption under lower pressure is larger than that of GRANSs with the same width, while the adsorption under higher pressure is almost the same as that of GRANSs with the same width. In 1 nm BCNSs, the adsorption is larger than that of GRANSs with the same width in the simulation pressure range. It is the existence of dissolved phase in BCNSs that results in the discrepancy. The dissolved phase can be found whatever the BCNS width is but its contribution declines with the pressure. The critical width for the existence of free phase is 2 rim for BCNSs and GRANSs. When water and gas coexist in GRANSs, gas accumulates at the water-solid interface spontaneously. In contrast, for BCNSs, water preferentially adsorbs on the hydrophilic sites by hydrogen bonds and forms clusters, while gas preferentially adsorbs on the hydrophobic sites. The water bridge will form as the water content increases further.