Molecular Simulation on Competitive Adsorptions of CO 2 , CH 4 , and N 2 in Deep Coal Seams

In this paper we have simulated the competitive adsorption of CO 2 , CH 4, and N 2 gases in deep coal seams by building a graphite supercell structure and discussed the impact of pressure, pore size, and multicomponent composition on CH 4 desorption. The results show that the adsorption capacity of a single component gas changes is in the order of CO 2 > CH 4 > N 2 . For the CH 4 /CO 2 competitive adsorption, absorbed CO 2 can reach saturation at low pressure conditions. CO 2 has an adsorptive advantage compared with CH 4 . It is shown that CO 2 can promote the CH 4 desorption by the displacement mechanism. For CH 4 /N 2 competitive adsorption, the adsorption capacity of N 2 is weaker than that of CH 4 , demonstrating that improvement in coalbed methane (CBM) production by N 2 injection is achieved by reducing the partial pressure and creating flow channels. The presence of H 2 O has a greater impact on the gas with a stronger adsorption capacity in the binary component system. For the CH/CO 2 /N 2 competitive adsorption, the CO 2 adsorption is dominant in 1 nm slit pores, while CH 4 adsorption is dominant in 2 nm slit pores. This indicates that when the pore diameter increases, the CO 2 /N 2 injection does not promote CH 4 desorption. H 2 O also has a significant impact on the competitive adsorption in the ternary component system. The strong interaction between H 2 O and CO 2 weakens the CO 2 adsorption capacity.