Effect of pressure and temperature on diffusion of CO2 and CH4 into coal from the Lorraine basin (France)
Résumé
Adsorption kinetics of CO2 and CH4 from a high volatile bituminous coal of the Lorraine basin (north-east of France) were studied at 0.1 MPa and 5.0 MPa in a range of temperatures from about 283.15 K to 333.15 K. Kinetic measurements were performed using a gravimetric experimental set-up, which allows measurement of gas density. Sorption equilibrium is reached faster for CO2 than CH4. The differences are mainly due to the physico-chemical parameters of both gas molecules and their coal-interactions. The pressure-dependence of diffusion parameters in coals was shown. Increasing gas pressure decreases the equilibrium time. For example, the CO2 equilibrium time is about 15 h at 5 MPa and about 40 h for 0.1 MPa. The study of kinetic sorption at different temperatures suggested that the diffusion of gases into coal is activated for CO2 and CH4. The effective diffusion, De, and the diffusion coefficient, D, estimated from modelling experimental data by assuming Fickian and unipore diffusion, increased with increasing temperature. The diffusion coefficient was estimated at about 10- 12 m2 s- 1. Assuming Arrhenius' law, the activation energy of CO2 adsorption was rather low, about 18 kJ mol- 1 compared to the one of CH4, about 34 kJ mol- 1.