Effect of Adsorbed Phase Density on Excess Adsorption in Coal: An Experimental Study Based on High-Pressure Adsorption

Coalbed methane adsorption isotherms are crucial tools for characterizing the gas adsorption capacity, playing a crucial role in accurately predicting actual methane adsorption in coalbed. Pore information on coal samples was acquired through low-pressure N 2 /CO 2 adsorption measurements. By employing adsorption models grounded in micropore filling and surface coverage coupled with the adsorbed phase volume method, the adsorbed phase density was calculated. Ultimately, the results were validated through high-pressure adsorption experiments. In order to investigate the influence of adsorbed phase density on excess adsorption, the methane adsorption data are subjected to corrected analysis. Results show that the calculated adsorbed phase density is 0.36 g/cm 3 . In methane adsorption experiments at 6 MPa, the variation in the adsorption amount between the corrected absolute adsorption and excess adsorption for four coal sample groups ranged from 0.013 to 3.641 cm 3 /g, indicating significant differences between the two. In adsorption experiments at 20 MPa, the Langmuir volumes obtained from the absolute adsorption data in four different pressure intervals were 39.83–48.51 cm 3 /g, 20.06–29.08 cm 3 /g, 21.85–27.15 cm 3 /g, and 21.32–24.48 cm 3 /g, respectively, indicating that selecting various pressure intervals significantly influences the Langmuir volume. From the slopes of the adsorption curves, the slope of the adsorption points for AN, HV, and JD coal samples increased at pressures of 17.06, 15.63, and 12.04 MPa, respectively, while DT coal samples exhibited an almost constant slope at 14.05 MPa, deviating from the inherent adsorption characteristics of the adsorbent. Therefore, only the absolute adsorption data from the continuously decreasing segment of the slope were chosen for fitting, resulting in Langmuir volumes ranging from 23.73 to 44.16 cm 3 /g. This study provides an accurate method for determining adsorbed phase density values and criteria for selecting pressure ranges for absolute adsorption calculations, which is a significant theoretical guidance for reserve estimation.