Further application of the D-A model to evaluate the isothermal CO₂ ad/ desorption characteristics of coal

The CO2 ad/desorption characteristics is essential for evaluating the effectiveness and suitability of CO2 injection into coal-bed to storage and enhances CH4 recovery (CO2-ECBM). We performed the isothermal CO2 ad/desorption experiments on four different ranks of coal by using the self-developed coal seam high-pressure gas ad/desorption test system in this study. It was found that the isothermal CO2 ad/desorption behavior can be adequately explained by the D-A model, as its parameters can reflect the adsorption capacity, the desorption hysteresis characteristic, and the micropore structure in coal samples. The saturation CO2 adsorption amount (V-max) for the four coal samples is CC > YW > XLH > HG, showing a "decreasing-increasing-decreasing" trend as the coal rank growth, which is in general consistent with the relation between the total specific surface area, the total micropore volume and the micropore content in coal samples. The difference value in the characteristic adsorption energy (Delta E) in the D-A model can be used as a parameter to quantitatively characterize the hysteresis in the CO2 desorption processes compared to the adsorption processes in coal samples, and the larger Delta E implies the more significant desorption hysteresis. The desorption hysteresis of four coal samples is XLH approximate to HG > YW > CC in order, showing a trend of decreasing first and then increasing as the coal rank growth. The coefficient of structural heterogeneity n reflects the distribution of micropore structure, the larger value of n indicates a higher micropore volume and micropore content in coal. The pore structure is changed by the effect of CO2 ad/desorption, which increases the micropores content in coal, resulting in a larger structural heterogeneity coefficient in the desorption process.