Molecular Simulation Study Of The Adsorption Of Methane And Carbon Dioxide In Bituminous Coal Model

Enhanced coal bed methane recovery by injecting CO2 technology(CO2-ECBM) not only is beneficial to promote the large-scale development and utilization of coalbed methane, but also can realize coal seam sequestering CO2, achieve the goal of greenhouse gas emissions. In this paper, the adsorption behavior of single-component CH4 and CO2 and CH4/CO2 binary mixture components were comprehensive studied uder different reservoir conditions by molecular simulations, which is conducive to understand the adsorption mechanisms of CH4 and CO2 on coal and the mechanism of CO2-ECBM from the micro level. The main conclusions of this paper are as follows:1. The Wiser bituminous coal macromolecular was used to build a bituminous coal structure model whose density is 1.21 g/cm3, which proves the rationality of the bituminous coal model.2. With the increasing pressure, the adsorption quantities of both CH4 and CO2 initial increase rapidly and then reach a plateau. The adsorption isotherms of CH4 and CO2 are good fit for the characteristics of type I adsorption isotherm, which can use the Langmuir isothermal adsorption equation fitting. Lower temperature is advantageous to adsorption of CH4 and CO2, higher temperature promotes desorption of CH4 and CO2. Isosteric heats of CO2 is lager than that of CH4, which is the main reason why the adsorption quantity of CO2 on coalsis greater than that of CH4.3. The isosteric heats of both CH4 and CO2 gradually decrease with the increasing moisture content, which results in the adsorption quantity decreasing. But the existence of H2 O molecules does not change the adsorption mechanisms of CH4 and CO2 on bituminous coal model. The adsorption isotherms of CH4 and CO2 still accord with the type I adsorption isotherm.4. In the investigated ranges of the temperature and pressure, the competitive adsorption ability of CO2 is greater than that of CH4. Therefore, CO2 is adsorbed preferentially to CH4 on bituminous coal. Higher temperature and higher pressure will result in the competitive adsorption ability of CO2 relative to CH4 decreasing, and also the efficiency of CO2 sequestration and CO2-ECBM becomes lower.5. Under the different geological depth and bulk CO2 mole fraction conditions, the competitive adsorption ability of CO2 is greater than that of CH4, and CO2 is adsorbed preferentially to CH4 on bituminous coal. The greater injection site depth and bulk mole fraction of CO2 were, the weaker competitive adsorption ability of CO2 relative to CH4 in bituminous coal was. Too much CO2 injected into the coal seam will cause the ratio of CO2 sequestration and the efficiency of displacement CH4 decrease, and there will be less CO2 sequestered in the deep injection sites than in shallow ones for the same amounts of replaced CH4.6. The presence of H2 O molecules didn’t change the preferential adsorption of CO2, and the competitive adsorption ability of CO2 is greater than that of CH4 in bituminous coal. The existence of a small amount of H2 O molecules can significantly reduce the competitive adsorption ability of CO2 relative to CH4 in bituminous coal. Although the competitive adsorption ability of CO2 gradually enhanced with the numbers of H2 O molecules increasing, it is always lower than that without water adsorption.