Distribution of the adsorbed density of supercritical CO2 onto the anthracite and its implication for CO2 geologic storage in deep coal

Abstract

CO2 geological storage in deep coal has been a research hotspot in carbon capture and storage under the background of carbon neutrality. Understanding supercritical CO2 adsorption onto coals has been significant for accurately evaluating deep coal's CO2 geological storage capacity and further determining the deployment of a CO2-ECBM project. The supercritical CO2 adsorption experiments on dry Qinshui anthracite coal were conducted. The adsorbed CO2 density and average number of adsorbed molecule layers were calculated. The effects of temperature, pressure, and pore size on the density of the adsorbed phase were discussed using the simplified local density (SLD) adsorption model, and the distribution of adsorbed CO2 on the coal surface was disclosed. Finally, the modified method for calculating CO2 geological storage in coal was proposed. The results show that: (1) The average adsorbed density of supercritical CO2 onto the anthracite at the experimental temperatures ranged from 1.073 to 1.088 g/cm3, and the average number of adsorbed molecular layer ranged from 1.15 to 1.26, indicating the coexistence of micropore filling and multilayer adsorption. (2) The SLD theory demonstrated that the distribution of the adsorbed CO2 density is jointly controlled by temperature, pressure, pore size, and CO2 phase. The supercritical CO2 can form a second layer of adsorbed molecules. The minimum pore size to keep the adsorbed CO2 density unchanged on the pore wall is around 0.82 nm for subcritical CO2 and 3 nm for supercritical CO2. (3) The real average adsorbed CO2 density is always lower than that of the first adsorbed molecular layer and the density under extreme pressure. There are four different distributions of free and adsorbed CO2 on the surface of the coal with the changes in temperature and pressure, consisting of subcritical, gas-like supercritical, liquid-like supercritical, and extreme states. (4) The modified method of CO2 storage capacity for coal, based on excess adsorption and equivalent free CO2 in the total pore, avoids the error caused by the uncertainty of adsorbed CO2 density and free space volume. It is recommended that this method should be preferentially adopted during the selection of the potential target area for CO2 geological storage in deep coal. The study aims to provide a new idea for the mechanism of supercritical CO2 adsorption on coal and the assessment of supercritical CO2 geologic storage capacity in deep coal.