Adsorption isotherms of CO2 - CH4 binary mixture using IAST for optimized ECBM recovery from sub-bituminous coals of Jharia coalfield: an experimental and modeling approach

Prediction of adsorption equilibria of methane (CH4)/carbon dioxide (CO2) on coal at various feed gas composition is essential for the commercial development of primary and enhanced coalbed methane (ECBM) recovery by CO2 sequestration from coal reservoirs. In this article, an attempt is made to determine the adsorption equilibria of CH4 and CO2 mixture over a wide range of pressure and free gas compositions with the help of developed computational approach using the Ideal Adsorption Solution theory (IAST) for multicomponent adsorption. The advantage of the model is that it can predict the adsorption equilibrium of the mixture from the pure gas equilibrium data. Considering the complex adsorption during gas transportation and efficient gas injection modes at constant pressure or constant composition, the two different computational approaches were developed to solve the iterative calculation of IAST, i.e. constant mole fraction with variable pressure (P = 0 to 1000 psi) and constant pressure with mole fraction as a variable (y = 0 to 1). Individual adsorption isotherm of CH4 and CO2 for the two different sub-bituminous coal samples was determined up to a pressures of 1250 psi. Pure gas isotherms reveal that volume wise CO2 has 2.00 to 2.63 times more adsorption capacity than CH4 for the samples respectively. Computational model developed using ideal solution theory can predict the adsorbed gas volume at any pressure on the experimentally studied samples collected from Indian CBM fields. The developed computational code using MATLAB (c) software was validated using the binary adsorption data for the two coal samples from global CBM fields published in the literatures, with calculated error lying within the permissible limit.