An experimental approach to adsorption of CO2 + CH4 gas mixtures onto coal (European RFCS CARBOLAB research project)

Abstract : The main goal of the CARBOLAB research project was to fill the gap concerning the lack of data and comprehension of the physicochemical mechanisms that exist between laboratory experiments on gas adsorption properties of coal and full-scale CO2 injection tests from the surface. Gathering Spanish (HUNOSA, AITEMIN), French (BRGM, INERIS) and Polish (GIG) partners, the project contained three main actions : laboratory tests, numerical modelling and in-situ injection at a seam scale. The injection site was within an existing underground infrastructure, a crosscut of a coal mine located in Asturias (North of Spain). The objectives of laboratory tests performed in the CARBOLAB project were to determine the intrinsic parameters of coal samples to help to forecast the performance of the targeted coal seam concerning coal bed methane recovery and CO2 storage. Coal samples were collected in situ from the wall of the crosscut. Adsorption isotherms have been determined through gravimetric experiments to assess quantities of gas that can be adsorbed or desorbed. Kinetics of exchanges between adsorbed and free gas phases have been calculated using the linear driving force model. Adsorption experiments have been determined for pure gas phases (CO2 and CH4) and binary gas mixtures (5 types of CO2 + CH4 mixtures). All experiments have been performed at 298 K (25°C). Here, we will see that CO2 and CH4 adsorption capacities determined with pure gas phases cannot be added to determine the total adsorption capacity of a gas mixture ; and that both CO2 and CH4 adsorption capacities for experiments performed with gas mixtures are lower than those determined for experiments with pure gases. These results confirm that both gas compete to adsorb onto coal. We will also see that the extended Langmuir model that is often used to model sorption of binary gas mixtures does not fit our experimental data because the extended Langmuir model underestimates the adsorption capacities of CO2 for experiments performed with binary gas mixtures having a CO2 concentration <= 50 vol. %; and underestimates the adsorption capacities of CH4 for experiments performed with binary gas mixtures having a CO2 concentration < 50 vol. %. We will propose a new equation that fit our experimental results and that can be used in numerical codes.