Estimation of Gas Adsorption Capacity in Coal: A Review and an Analytical Study

Adsorption plays an important role in carbon dioxide sequestration and methane recovery processes in deep coal seams. If the effects of coal physical properties on its gas adsorption capacity are considered, it increases with vitrinite content and pressure and reduces with liptinite, mineral matter, moisture contents, and temperature and follows a U-shaped variation with carbon content. Furthermore, CO2 has higher adsorption capacity compared to other gases. There are two main methods to estimate the amount of gas adsorbed in a coal seam, which are called direct and indirect methods. The latter method is more common for coal. In the direct method, the volume of gas released from a coal mass into a sealed desorption canister is measured, and under the indirect method, adsorption isotherms and empirical relations are used. However, any of those methods are unable to count the effect of swelling-induced strain and, therefore, fail in measuring the absolute adsorption in coal. Moreover, among various adsorption models, Langmuir and Dubnin-Radushkevich (D-R) equations are the most widely using models. However, since gas can be absorbed through both adsorption and absorption processes for coal, it is important to have an additional term to count the missing absorption mechanism in both the Langmuir and D-R models. Finally, a new descriptive model for gas adsorption capacity of coal as a function of effective factors is proposed. The new model is based on the existing D-R equation, which was modified by inserting a new expression for the term of micropore capacity. Two types (CO2 and N-2) of gas adsorption data for coal from five different locations (British Colombia and Alberta in Canada and Victoria, Sydney and Bowen in Australia) at three different temperatures (273, 296.5, and 318 K) were considered for the model development. According to the model results, new gas adsorption equations can fairly well accurately predict the adsorption capacity in coal.