Effect of pore structure on competitive sorption and diffusion of mixed methane and carbon dioxide in anthracite, South Qinshui Basin, China

Abstract

A deep understanding of the competitive adsorption and diffusion of methane (CH4) and carbon dioxide (CO2) in coal is significant for implementing the CO2-ECBM project. For three high-rank coal samples, industrial analysis, coal maceral, and vitrinite reflectance (RO) determination, mercury injection test, and low-temperature nitrogen/carbon dioxide specific surface test were carried out, and the composition and pore structure of coal samples were studied systematically. Meanwhile, through the three mixed gases (75%CH4 + 25%CO2, 50%CH4 + 50%CO2, 25%CH4 + 75%CO2) adsorption and desorption test, the absolute adsorption amount, free phase and adsorption phase of CH4 and CO2 under different equilibrium pressures, and the equivalent diffusion coefficients of gas migration in the process of adsorption/desorption equilibrium were calculated. Finally, the relationship among the coal composition, pore structure, and the competitive adsorption and diffusion characteristics of methane and carbon dioxide mixture in coal were studied. The results show that: (1) the composition of coal has an important influence on the development of coal pore structure and the adsorption and transport of gas in coal. Supermicropore (<2 nm in size) is the prominent adsorption place. Furthermore, its development, which essentially determines the adsorption capacity of coal samples, is positively correlated with fixed carbon content, vitrinite content, and vitrinite reflectance. (2) When the pores >100 nm in size relatively developed, the coal samples have higher porosity, lower micropore diffusion, and a smaller desorption hysteresis index. Because the pore's connectivity is limit, its content has no apparent correlation with the permeability of coal samples and most of the adsorption and diffusion parameters. (3) The desorption hysteresis is mainly caused by the opposite migration direction of gas in the process of adsorption and desorption and the resulting dislocation of coal matrix swelling and shrinkage. The competitive adsorption of CH4 or CO2 and their separation index (SCO2/CH4) in coal samples decreases with the increase of adsorption equilibrium pressure. Also, the desorption process often shows a higher SCO2/CH4 value than that of the adsorption process. (4) During the adsorption process, the equivalent macro-diffusivities of the coal sample decrease as a power function with increasing pressure and increase with the carbon dioxide content increasing in the mixed gas. However, the equivalent micro-diffusivities change little with the different CH4 and CO2 mixtures migration in different coal samples. The equivalent diffusion coefficient of coal is positively correlated with macropore, mesopore, transitional and micropore content in coal samples.