Quantitative analysis of difference in CH4 and CO2 adsorption capacity in coal based on adsorption model

Understanding the difference between CH4 and CO2 adsorption mechanisms is essential for better implemen-tation of CO2 Enhanced Coal Bed Methane Recovery (CO2-ECBM). In this paper, with the help of low-pressure physisorption of N2 at 77 K and CO2 at 273 K, an adsorption model based on microporous filling and surface covering has been used to estimate the gas adsorption capacity. The model was validated by CH4 and CO2 high-pressure adsorption experiments at 303 K and gas pressure up to 6 MPa using four coal samples with different ranks. Then the model-based adsorption space and bulk density were used to determine the factors affecting the differences between CH4 and CO2 adsorption. Results show that the correlation between the model calculated adsorption capacity and the experimental adsorption capacity is 0.952 for CH4 and 0.967 for CO2, which supports the reliability of the model. The microporous filling is found to account for 85.06-93.41% and 91.87-97.15% of the total gas adsorption capacity for CH4 and CO2, respectively, meaning that should be the main adsorption behavior in coal. In terms of adsorption space, it should also be noted that for these four samples, 2.85-13.88% of the CO2 is adsorbed within 0.33-0.38 nm which is inaccessible to CH4. Within different pore sizes at the microporous filling stage, the bulk densities of CH4 and CO2 are 19.737-31.171 mol/L and 30.197-47.885 mol/L, respectively; the bulk density of CH4 is 1.5 times bigger than that of CO2 for the same pore size. Based on these findings, a method for calculating adsorption phase density has been proposed.