Adsorption and transport of methane and carbon dioxide mixture in anthracite

Deepening the understanding of the transport and competitive adsorption of methane and carbon dioxide in coal has important significance for the injection optimization design and injection displacement evaluation of the project that CO2 storage in coal and enhanced coalbed methane recovery CO2-ECBM.The experiments were carried out containing the proximate analysis,pore structure analysis by low temperature liquid nitrogen and CO2 adsorption,and isotherm adsorption of pure methane,carbon dioxide,and their mixture (volume fraction 75% CH4+25% CO2,50% CH4+50% CO2,25% CH4+75% CO2) for three anthracite samples in the southern Qinshui Basin.The variation of absolute adsorption amount and gas separation factor of mixture adsorption in coal with pressure is analyzed based on the calculation of mixture adsorption phase density.Meanwhile,the equivalent diffusion coefficient and the proportion of macropore diffusion of gas in each pressure section in the adsorption experiment were calculated by using the bidisperes diffusion model.Finally,it was studied that the effects of coal composition and pore structure on gas coal migration and competitive adsorption in coal.The results show that (1) The specific surface area of super-micropores in coal is 1-2 orders of magnitude higher than that of micropores,which is the main place for gas adsorption.(2) The adsorption separation factor (S(CO2-CH4)) of anthracite varies from 3 to 20 with equilibrium pressure and equilibrium molar volume fraction of gas components.Generally,S(CO2-CH4) decreases with the increase of pressure,and is positively related to the development of micropores in coal.(3) The bidisperes diffusion model can better describe the diffusion process of the mixed gas.In general,the macropore equivalent diffusivity of coal samples varies as a power function with pressure.Under the same pressure,the macropores equivalent diffusivity increases with the increase of the carbon dioxide content.The micropore equivalent diffusivity have no significant relationship with the gas volume fraction and pressure.And the proportion of macropore uptake changes in a "V" shape with the increase of pressure.The results will help to deeply understand the sorption processes of the methane and carbon dioxide mixture,and to provide guidance for the CO2-ECBM project in the southern Qinshui Basin.