Effects of supercritical CO2 extraction on adsorption characteristics of methane on different types of coals

Abstract

Adsorption characteristics of methane on coals before and after supercritical CO2 extraction are closely related to CO2 injection in coal seams for enhanced coalbed methane (ECBM) recovery and CO2 sequestration. Using self-developed extraction facility of supercritical fluids and micro-calorimetric measurement system, this work studied the adsorption behavior of methane on six types of coals which were pretreated with supercritical CO2 at 45 °C and 10 MPa for two weeks. The effects of supercritical extraction, coal rank and pressure on adsorption amount of methane, integral heat of adsorption and pore structure of coal samples have been analyzed. To interpret the experimental data, dynamical models are developed to describe the variations of gas pressure and heat released in the process of adsorption, and corresponding isotherm models are proposed to characterize the adsorption amount and adsorption heat for pretreated and untreated coal samples. The correlation between the integral adsorption heat and adsorption amount was discussed for different coal samples. It is found that compared with untreated coals, the adsorption amount and integral adsorption heat for pretreated coals are reduced, and the decrements are largely dependent on the coal rank. The integral adsorption heat relates linearly with the adsorption amount, and the ratio coefficients for medium rank coals decrease after supercritical CO2 extraction, whereas an increasing trend is found for low-rank and high-rank coals. In addition, it is revealed that the volume of micropores is reduced by supercritical CO2 extraction for low-rank and medium-rank coals, while no significant change is found for high-rank coals.