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Using the remaining coal seams of closed coal mines for CO 2  geological storage can effectively slow down the greenhouse effect and realize the redevelopment and utilization of closed coal mines. The burial depth of coal seams in closed coal mines in China covers an extensive range, and CO 2  and CH 4  could exist in various phases in the coal. The existing experimental objects for CO 2  adsorption in coal are mainly associated with the supercritical and gaseous phases, and a research gap comparison between the CO 2  adsorption characteristics in the liquid phase and those of other phases is very obvious. Herein, the isotherm adsorption experiments of CO 2  and CH 4  in different phases on different rank coal samples are methodically conducted in the pressure range of 0-12MPa. The experimental results indicate that the excess adsorption capacity of gaseous CO 2  and CH 4  (gas pressure &lt;4MPa) rapidly raises with the growth of the adsorption pressure; however, a significant difference for the case of the medium- and high-pressure stage (&gt;4MPa) is detectable. With the growth of the adsorption pressure, the excess adsorption capacity of the supercritical CH 4  gradually reaches the maximum value and then remains stable, where the maximum experimental value is 1.009 mmol/g. While the excess adsorption capacity of CO 2  near the critical pressure of liquid and supercritical state rapidly reduces and then tends to be stable. The maximum adsorption capacity is capable of touching 2.096 mmol/g, which is stable around 0.5 mmol/g. The analysis indicates that the change in the excess adsorption amount of CO 2  near the critical pressure corresponds to the change in its density. The absolute adsorption capacity of CO 2  and CH 4  monotonically grows in the pressure range of 0-12MPa, and the absolute adsorption capacity of the same coal sample follows the following order: liquid CO 2  &gt; supercritical CO 2  &gt; CH 4 . The maximum adsorption capacity values for these substances in order are 2.493, 2.345, and 1.296 mmol/g. With the growth of the coal rank, the Gibbs free energy, specific surface area, and adsorption capacity of the understudied coal samples exhibit a U-shape relationship as a function of their coal ranks.

Isothermal adsorption characteristics of various phases of CO 2 and CH 4 in different rank coals