IAST Modelling of Competitive Adsorption, Diffusion and Thermodynamics for CO2-ECBM Process

Abstract

The CO2 emission is one of the main causes for the global warming and it may be controlled by sequestrating CO2 into the geological formation. The coalbed formation provides a dual advantage for CO2 sequestration as CO2 may be stored in coal forever with enhancing the coalbed methane recovery. Thus, the cost of CO2 sequestration may be offset completely or partially. The main objective of the paper was to comprehend the CO2-ECBM displacement using the three concepts viz. competitive adsorption, diffusion, and thermodynamic modelling of coal In this paper, the pure gas isotherm on coal for CH4, and CO2 was evaluated using manometric method. The binary gas isotherm or competitive adsorption was studied using IAST modelling. MATLAB code was developed for the solution of IAST model and Newton Raphson approach was followed. The IAST modelling was done by taking 50%/%50 mole fraction of CH4/CO2. By analyzing the binary gas isotherm, the optimum injection pressure was evaluated. On the same injection pressure, co adsorption isotherm was drawn at different mole fraction of CO2 in gas phase. Separation factor was calculated by taking ratio of CO2 and CH4 in the gas and adsorbed phase respectively. Furthermore, adsorption data was used for discussing the sorption kinetics in coal and diffusion coefficient was evaluated. Furthermore, the thermodynamic parameters were also calculated and integrated with above noted parameters for the methane displacement in CO2-ECBM process. The CO2-ECBM displacement is very much dependent on the competitive adsorption and diffusion process in coal. The surface potential and Henry constant are important parameters for defining the CO2-ECBM displacement. The coadsorption isotherm was drawn at the optimum injection pressure and it shows that methane displacement would be the optimum by taking 11 %/89% mole fraction of CO2 and CH4 for two temperatures i.e., 288 K, 308 K. It is identified through diffusion regime that diffusion coefficient for the binary gas isotherm is the average of the diffusion coefficients of pure CO2 and CH4. This is the first kind of study which provides the completely integrated approach for describing the methane displacement in CO2-ECBM process. This novel study promotes our understanding of the complex mechanisms of CO2-ECBM displacement process.