High pressure supercritical carbon dioxide adsorption in coal: Adsorption model and thermodynamic characteristics

• A dual-site Langmuir model can simulate high pressure scCO2 adsorption in coal.
• The crossovers of adsorption isotherms under different temperatures are interpreted.
• Adsorption isotherms beyond tested temperature range can be predicted.
• The isosteric heat of adsorption depends on both temperature and adsorption uptake.

This work uses a dual-site Langmuir adsorption model to describe supercritical carbon dioxide (scCO2) adsorption in coal up to 20 MPa and 253 K. The isosteric heat of adsorption for scCO2 in coal is calculated analytically by considering both the real gas behavior and the adsorbed phase, which are ignored in the classic Clausius-Clapeyron approximation. It was found that the proposed model can not only reasonably interpret observed test phenomena but also has the intrinsic ability to extrapolate adsorption isotherms under different temperatures beyond test data. The crossovers of the observed adsorption isotherms under different temperatures can be simply attributed to the pressure-volume-temperature (PVT) behavior of the bulk gas during the adsorption process. Both the temperature dependence and adsorption uptake dependence of isosteric heat of adsorption are revealed; the higher the temperature and the adsorption uptake, the lower the isosteric heat of adsorption. The ideal gas law overestimates the isosteric heat of adsorption for scCO2 in coal. The application of the dual-site Langmuir model allows for estimating the CO2 storage capacity of deep unmineable coal seams, modeling CO2 transport behavior via differentiating the true ratio between bulk phase and adsorbed phase, and investigating the true thermodynamic characteristics of CO2 in coal under high pressures.