Molecular Simulation on Competitive Adsorptions of CO2, CH4, and N2in Deep Coal Seams

In this paper we have simulated the competitive adsorption of CO2, 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 CO2> CH4> N-2. For the CH4/CO(2)competitive adsorption, absorbed CO(2)can reach saturation at low pressure conditions. CO(2)has an adsorptive advantage compared with CH4. It is shown that CO(2)can promote the CH(4)desorption by the displacement mechanism. For CH4/N(2)competitive adsorption, the adsorption capacity of N(2)is weaker than that of CH4, 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 H2O has a greater impact on the gas with a stronger adsorption capacity in the binary component system. For the CH/CO2/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 CO2/N(2)injection does not promote CH(4)desorption. H2O also has a significant impact on the competitive adsorption in the ternary component system. The strong interaction between H2O and CO(2)weakens the CO(2)adsorption capacity.