Study Of The Adsorption Behavior And Thermal Effect Of CO₂ And CH₄ On Different Metamorphic Degree Coal

CO₂ Sequestration in Deep Coal Seams with Enhanced Coal-bed Methane Recovery(CO₂-ECBM)is a technology that integrates greenhouse gas emission reduction and new energy development.It could store CO₂ and promote CH₄ recovery simultaneously,which is beneficial for the environment and the economy.Compared with CH₄,coal has a preferential adsorption for CO₂,but the thermal effect caused by the adsorption difference between the two gases on the coal would have significant effect on the displacement of CH₄ by CO₂,the accurate description of the adsorption heat plays a positive role in further revealing the mechanism of the displacement of CH₄ by CO₂ .In this work,weakly caking coal,gas coal,coking coal,and anthracite were selected as the research objects,.and a high pressure gas adsorption and desorption-microcalorimeter combination method was employed to systematically investigate the effects of temperature(30,40,50?),pressure(0?20 bar),and the degree of metamorphism on the adsorption behavior and thermal effect of CO₂ and CH₄ on coal.All the adsorption isotherms of CO₂ and CH₄ on weakly caking coal,gas coal,coking coal,and anthracite belong to type I adsorption isotherms,and they are subject to the Langmuir model.As the temperature increases,the amounts of CO₂ and CH₄ adsorbed on coal decreases.Under the same conditions,with the degree of metamorphism of coal samples increases,the adsorption amounts of CO₂ and CH₄ on coal decreases first and then increased,presenting a U-shaped trend.Further,the adsorption amount of CO₂ on the four coal samples is significantly larger than that of CH₄,indicating that CO₂ is more easily adsorbed on coal.The isosteric adsorption heats of CO₂ and CH₄ on coal demonstrates that the adsorption processes of CO₂ and CH₄ on coal are both physical adsorption.With the increase of metamorphic degree of coal samples,the isosteric adsorption heat of CO₂ on coal decreases first and then increases,while the isosteric adsorption heat of CH₄ on coal gradually decreases.The limit isosteric adsorption heat of CO₂ on the four coal samples is 54.02,37.63,17.88,and 32.42 k J/mol,respectively.It is evident that with the metamorphic degree increasing,the limit isosteric adsorption heat of CO₂ on coal decreases first and then increases,showing a U-shaped trend.The limit isosteric adsorption heat of CH₄ on coal samples also presents the same trend.The limit isosteric adsorption heat of CO₂ is significantly higher than that of CH₄,indicating that the interaction force between CO₂ and coal molecules is stronger than that of CH₄ and coal molecules.Under the same experimental conditions,the actual measured adsorption heats of CO₂ and CH₄ on weakly caking coal,gas coal,coking coal,and anthracite decrease with increasing temperature.And as coal rank increases,the actual measured adsorption heat show a trend of decreasing first and then increasing.The actual measured heats of CO₂ adsorption on four coal samples is greater than these of CH₄,suggesting that the force between coal molecules interacting with CO₂ is stronger than that with CH₄,which is consistent with the results obtained by the limit isosteric adsorption heat.The research results obtained in this paper could provide data support for improving CO₂ storage performance and CH₄ recovery.