Insight into difference in high-pressure adsorption-desorption of CO2 and CH4 from low permeability coal seam of Huainan-Huaibei coalfield, China

Abstract

This study aimed to investigate the differential characteristics of high-pressure adsorption-desorption of supercritical CO2 and CH4 at main temperatures of 36 ℃, 42 ℃, and 48 ℃ and the pressure of 0–20 MPa in low permeability coal, a case from the tectonic coal area of Huainan-Huaibei coalfield, eastern China. Combining the variation characteristics of adsorption capacity with pressure, gas free phase density, the prediction of CO2 adsorbed phase density, the correction method of gas adsorption capacity, the sequestration capacity of CO2 in low permeability coal, and the adsorption hysteresis index of adsorbed gas were assessed. The controlling factors of gas adsorption capacity were revealed based on the general discussion of the relation between the gas adsorption capacity and the conditions such as temperature, pressure, coal maceral, coal quality, and pore structure. The results show that the lowest equilibrium pressure associated with the CH4 excess adsorption peak is 9 MPa, which is larger than most of the experimental pressures for the CH4 isothermal adsorption test. CO2 excess adsorption capacity of the different coal samples has characteristic of an obvious peak at the experimental pressure near 6 MPa under different experimental temperatures, all CO2 excess adsorption capacity changes with CO2 free phase density in a two-step pattern, CH4 or CO2 adsorbed phase density can be predicted by the truncation method. Desorption of CH4 or CO2 in coal has an obvious "hysteresis loop", the cumulative adsorption hysteresis index (CAHI) and the incremental adsorption hysteresis index (IAHI) of CH4 based on the calculation of CH4 excess adsorption capacity are controlled by the temperature and pressure, it has the obvious rise during the stage of low pressure (less than 4 MPa), whether IAHI or CAHI of CO2, it shows characteristics of complex changes. Variations in CAHI and IAHI for CH4 or CO2 are governed by the development of pore structure in coal, and the temperature. Coal maceral, coal quality, and coal grade have an effect on gas adsorption in coal by affecting the growth, filling, and deformation of pores in coal; the adsorption capacity of the gas depends both on the surface area of the pores and the pore volume of the pores in the coal. The differential mechanism for the gas adsorption capacity from the low permeability coal requires further understanding.