High pressure CO2 adsorption onto Malaysian Mukah-Balingian coals: Adsorption isotherms, thermodynamic and kinetic investigations

Abstract

CO₂ sequestration into coalbed seams is one of the practical routes for mitigating CO₂ emissions. The adsorption mechanisms of CO₂ onto Malaysian coals, however, are not yet investigated. In this research CO₂ adsorption isotherms were first performed on dry and wet Mukah-Balingian coal samples at temperatures ranging from 300 to 348 K and pressures up to 6 MPa using volumetric technique. The dry S1 coal showed the highest CO₂ adsorption capacity of 1.3 mmol g^-1, at 300 K and 6 MPa among the other coal samples. The experimental results of CO₂ adsorption were investigated using adsorption isotherms, thermodynamics, and kinetic models. Nonlinear analysis has been employed to investigate the data of CO₂ adsorption onto coal samples via three parameter isotherm equilibrium models, namely Redlich Peterson, Koble Corrigan, Toth, Sips, and Hill, and four parameter equilibrium model, namely Jensen Seaton. The results of adsorption isotherm suggested that the Jensen Seaton model described the experimental data well. Gibb's free energy change values are negative, suggesting that CO₂ adsorption onto the coal occurred randomly. Enthalpy change values in the negative range established that CO₂ adsorption onto coal is an exothermic mechanism. Webber's pore-diffusion model, in particular, demonstrated that pore-diffusion was the main controlling stage in CO₂ adsorption onto coal matrix. The activation energy of the coals was calculated to be below -13 kJ mol^-1, indicating that adsorption of CO₂ onto coals occurred through physisorption. The results demonstrate that CO₂ adsorption onto coal matrix is favorable, spontaneous, and the adsorbed CO₂ molecules accumulate more onto coal matrix. The observations of this investigation have significant implications for a more accurate measurement of CO₂ injection into Malaysian coalbed seams.

Keywords: CO(2) adsorption; Entropy loss; Gibbs free energy change; Jensen Seaton model; Malaysian coal.