Effect Of Supercritical Carbon Dioxide On Physical And Chemical Properties And Adsorption Properties Of Coal

Global warming mainly caused by the most important anthropogenic greenhouse gascarbon dioxide (CO2) is harmful to the environment ecosystems and human society. Thus, it is necessary to adopt several approaches to mitigate CO2 emissions into atmosphere. Among them, CO2 sequestration in deep coal seams with enhanced coalbed methane recovery (CO2-ECBM) is considered as a potential way. Based on the coal characteristics and CO2 fluid properties under suitable reservoir conditions, there exist complex interactions between coal and CO2 during the process of CO2 sequestration in coal seams which will influence on CO2 sequestration on coal and cause potential environmental problems. Therefore, this work concentrated on the influences of CO2 exposure on the phy-chemical properties of different coal rank coal, and further researched the influences of CO2 exposure on CO2 dynamic and static mechanical behavior on coal.The main study contents and conclusions from this work are summarized as follows;1) Supercritical fluid extraction device was used to study the effect of CO2 exposure on the physical and chemical properties of coal. Ash content, pore structure of coal samples, coal pore heterogeneity and the change of oxygen containing functional groups were analyzed by simulating the dynamic process of CO2 and coal under reservoir conditions (Temperature and Pressure). The results indicate that the pore shape of coal has a various change by CO2 exposure, and the meso/macropore volume and the micropore specific surface area of the coal after CO2 exposure are less than that of the raw coal except ED coal. The analysis of the multifractal theory shows that supercritical CO2 effects can make change in heterogeneity of coal pore structure. Moreover, the change of surface chemical property was researched by the Fourier Transform Infrared (FTIR). The analyses show that there is a decrease in the oxygen-containing functional groups mainly including carbonyl (-C=O) and carboxyl (-COOH) of coals after CO2 exposure. The above changes are probably due to the extracting ability or reactivity of supercritical CO2 fluid.2) Volumetric method was applied to test high-pressure methane and CO2 adsorption behavior of the coal samples before and after CO2 exposure, and further analysis possible reasons. The results indicate that the maximum methane adsorption capacities of various rank coals after CO2 exposure increase by 3.45%-10.37%. However, the maximum CO2 adsorption capacities of various rank coals decrease by 9.99%-23.93%. Upon further analysis, the effect of CO2 exposure on pore and the oxygen-containing functional groups on coal surface may account for the interchange of the maximum adsorption capacities of methane and CO2. Specifically, the decrease of coal pore structure parameters caused by CO2 exposure has a negative effect to the adsorption of methane and CO2 on coal. The investigation has found that the oxygen-containing functional groups on coal surface do not favor methane adsorption. However, the opposite relationship is applicable for CO2 adsorption. Thus, it is deduced that primarily injected high-pressure CO2 will hinder the sequestration of the injected CO2.3) In this work, we also studied the effect of CO2 exposure on the gas molecules absorption diffusion behavior of coal, and further analyzed the possible reasons. The bidisperse diffusion model was selected to describe the methane and CO2 diffusion and adsorption on coal. The results show that high-pressure CO2 exposure causes a decrease in both methane and CO2 diffusion-adsorption rates on coals indicated by both macropore and micropore apparent diffusion coefficients. Further investigations indicate that the effect of high-pressure CO2 exposure on adsorption kinetics of methane and CO2 on coals is related to the common influence of the decrease of pore structure and the oxygen-containing functional groups on coal surface and swelling of coal matrix.4) Based on the influences of CO2 exposure on gas molecule dynamic and static mechanical behavior on coal, it is deduced that primarily injected high-pressure CO2 may hinder the CO2 sequestration and methane recovery in the target coal seams. It is necessary to take some measures to mitigate the adverse impacts caused by CO2 exposure. Thus, the recommendation of CO2 injection with a proportion of N2 is proposed to mitigate CO2 sequestration and methane recovery.