The Effect Of CO₂ Fluid On The Physical And Chemical Properties And Adsorption Performance Of Coal Under Simulated Formation Conditions

The emissions of carbon dioxide(CO2),a typical anthropogenic greenhouse gas,have caused serious climate change issue,which is a huge threat to human being and ecosystem.Thus,how to mitigate CO2 emissions gains broad interest.Hitherto,a wide variety of methods can control CO2 emissions.Among them,CO2 sequestration in deep coal seams(CO2-ECBM)can not only reduce CO2 emissions,but also recover CH4 resource at the same time.Previous investigations indicate that CO2 exists in a supercritical fluid under the optimum coal reservoir conditions for CO2 storage.Given the particular characteristics of the supercritical CO2 fluid and the physical and chemical properties of the coals,complex fluid-solid interactions do exist between CO2 and coal matrix besides the well-known adsorption phenomenon.The above-mentioned interactions will change the physical and chemical properties of the coals,and further influence CO2 storage in the target coal seams.It should be noted that whether the coal seams can effectively store CO2 in geologic time scale is crucial for CO2-ECBM.The current investigations always focus on the short-term interaction of CO2 with coals.However,the long-term interaction between CO2 and coals is scarcely reported.Therefore,the dynamic interaction of supercritical CO2 with six moisture-equilibrated coal samples was performed.The interaction of high-pressure CO2 with four moisture-equilibrated coal samples for 6 months was also carried out.The effects of the above-mentioned two kinds of interaction process on the physicochemical properties and CO2 adsorption capability of coals were carefully examined.The related conclusions of this study are helpful to gain further insight of CO2-ECBM,and to provide some guidance for the implementation of CO2-ECBMThe main conclusions derived from this study are summarized as below.(1)The interaction between supercritical CO2 and six moisture-equilibrated coal samples and their effects on the physicochemical properties of coals were studied.Results show that supercritical CO2 exposure can change the pore volume,specific surface area and pore shape of coals.Particularly,the microporous structure of six moisture-equilibrated coal samples after interaction with supercritical CO2 changes slightly,while the meso/macropores parameters change significantly.The interaction obviously decreases the meso/macropore surface area of bituminite and lignite,while increases that of anthracites.Both the meso/macropore volume and the average meso/macropore diameter of all the coal samples after supercritical CO2 exposure decrease.XPS analysis results reveal that the CO2 interaction can also affect surface chemistry property of coals,the oxygen-containing functional groups in particular.The supercritical CO2 exposure reduces the content of C-O and C=O of all coal samples.Moreover,the exposure decreases the content of COOH of bituminite,while increases that of anthracites(2)The long-term interaction between high-pressure CO2 and four moisture-equilibrated coal samples and its effects on the physicochemical properties of coals were studied.The results indicate that the mineral content in four moisture-equilibrated coal samples decreases after a long-term interaction with high-pressure CO2.The micropore surface area and volume of anthracite and lignite decrease due to a long-term high pressure CO2 exposure,while that of bituminous increase.In addition,the high-pressure CO2 exposure decreases meso/macropore surface area and pore volume of all coal samples.The XPS analysis results reveal that the content of C-O and C=O of all coal samples after a long-term high-pressure CO2 exposure decreases,while that of COOH presents an opposite trend(3)The effects of long-term CO2 exposure on the adsorption equilibrium and adsorption kinetics for CO2 on dry coal samples were discussed.Study shows that Ono-Kondo lattice model is applicable to describe the adsorption equilibrium for CO2 on the dry coal samples.The adsorption capability of dry anthracite and lignite after high-pressure CO2 exposure decreases,while the opposite trend exists for bituminite.The high-pressure CO2 dependence of the adsorption capability of coals is closely related to the change of pore structure and oxygen-containing functional groups of coal samples.The bidisperse diffusion model provides higher predictive accuracy for the adsorption kinetics of CO2 on the coals The effective diffusion coefficients of CO2 within the coals after high-pressure CO2 exposure display different trends.Particularly,the effective diffusion coefficient of meso/macropores and micropores of CO2 within anthracite increases,while that decreases with respect to lignite coals.The effective diffusion coefficient of meso/macropores and micropores presents abnormal change in bituminous coals(4)The effect of long-term CO2 exposure on moisture-equilibrated coals for CO2 adsorption capacity and the influence of moisture on CO2 adsorption capacity of coals were studied.Results indicate that the maximum CO2 adsorption capacity of the four moisture-equilibrated coal samples after high-pressure CO2 exposure reduces.The presence of moisture does not favor CO2 adsorption.The maximum adsorption capacity of the four moisture-equilibrated coal samples is significantly lower than that of dry coal samples.