Effect Of Coal Metamorphism On Supercritical CH₄ And CO₂ Competitive Adsorption Characteristics

The CO₂-ECBM technology can effectively reduce the CO₂emission and promote the recovery of CH₄.The competitive adsorption characteristics of mixed gases in the subcritical to supercritical state are not perfect.Aiming at this problem,this paper relies on the gravimetric high-pressure isothermal adsorption system to carry out the isothermal adsorption experiments of supercritical pure CH₄and pure CO₂on three kinds of metamorphic coals under the conditions of 40℃and 1-21 MPa from subcritical to supercritical pressure.Competitive adsorption experiments of three supercritical CH₄-CO₂gas mixtures with different components.Combined with the pore structure characteristics of coal,its adsorption characteristics and competitive adsorption characteristics were explored.The main results are as follows:（1）The gas adsorption amount measured by gravimetric method is all excess adsorption amount,among which the excess adsorption amount of supercritical pure gas increases with the increase of coal rank,and the excess adsorption amount of supercritical CH₄decreases linearly under high pressure,the excess adsorption of supercritical CO₂changes with pressure in four stages;the excess adsorption of supercritical mixed gas with the proportion of CO₂increases gradually from pure CH₄to pure CO₂.（2）The absolute adsorption capacity of CH₄after correction by intercept method shows that the absolute adsorption capacity of anthracite to supercritical CH₄increases with the increase of the degree of metamorphism,and the absolute adsorption capacity of anthracite to supercritical CH₄is twice that of gas coal;the absolute adsorption capacity showed a sharp increase at first,then tended to be stable,and then increased rapidly again and tended to be saturated.The volume of the adsorbed phase showed the same trend as the absolute adsorption capacity,and the density of the adsorbed phase first increased sharply to 0.8 g/cm³and then slowly increased.The liquid phase density of CO₂;the absolute adsorption capacity of the mixed gas is greater than that of CH₄and less than that of CO₂,and the absolute adsorption capacity increases stepwise with the increase of CO₂volume fraction.（3）The maximum absolute adsorption capacity of CH₄-CO₂mixed gas has a good linear relationship with CO₂volume fraction,and the maximum absolute adsorption capacity of unknown CO₂volume fraction mixture can be predicted by using the maximum adsorption capacity of three or more groups of CH₄-CO₂mixed gas;the average fractal there is a good linear relationship between the dimension and the maximum absolute adsorption capacity of the mixed gas,which can be used as a method to predict the maximum absolute adsorption capacity of the mixed gas for coals with different metamorphic degrees.（4）The absolute adsorption amount of a single component in the mixed gas shows that CO₂is greater than CH₄when the CO₂component is greater than or equal to the CH₄component.The higher the proportion of CO₂,the greater the adsorption amount of the CO₂component,and the CO₂component is 20%.The absolute adsorption amount of CO₂component is less than the absolute adsorption amount of CH₄;the selectivity coefficient of mixed gas indicates that CO₂has preferential adsorption in mixed gas.（5）Discussion on adsorption analysis layer combined with competitive adsorption mechanism:CH₄-CO₂mixed gas has multilayer adsorption from the beginning of adsorption,which has nothing to do with CO₂volume fraction;the increase of CO₂volume fraction in the same coal sample increases the adsorption molecular layer continuously,CO₂in CH₄-CO₂mixed gas is in the dominant position of competitive adsorption.