Multi-layer adsorption behavior and absolute adsorption of CO2 in coal: Insights from the Ono-Kondo lattice model

Adsorption is the main mechanism controlling CO 2 sequestration in coal seams, and understanding the adsorption behavior of CO 2 in coal is crucial for assessing storage capacity. In this study, the Ono-Kondo (OK) lattice model, which incorporates multilayer adsorption, is applied to describe CO 2 adsorption in coal and compared with the Langmuir and SDR models to assess the effects of different adsorption assumptions. The applicability and parameter ranges of the OK model for coal are also determined. Results show that the OK model accurately reproduces CO 2 excess adsorption isotherms irrespective of the assumed number of adsorption layers, while the prediction of absolute adsorption is highly sensitive to the multilayer assumption due to changes in adsorption-phase volume. When the pressure exceeds the CO 2 critical pressure, the absolute adsorption predicted by the OK model is approximately 1.3 and 1.7 times that of the Langmuir model under two-layer and three-layer assumptions, respectively. Analysis of adsorption-layer density and isosteric heat indicates that only the first two layers exhibit adsorption characteristics: the first layer shows strong adsorption and dominates excess adsorption, whereas the second layer contributes modestly but increases absolute adsorption; the third layer behaves nearly as bulk CO 2 . Considering adsorption mechanism, thermodynamic behavior, and computational efficiency, a two-layer OK model is recommended for characterizing CO 2 adsorption in coal. This model provides a more reliable approach for estimating absolute adsorption and improves the evaluation of CO 2 storage potential in coal seams.