Measuring and Modeling the Competitive Adsorption of CO₂, CH₄, and N₂ on a Dry Coal

Abstract

Data on the adsorption behavior of CO 2, CH 4, and N 2 on coal are needed to develop enhanced coalbed methane (ECBM) recovery processes, a technology where the recovery of CH 4 is enhanced by injection of a gas stream consisting of either pure CO 2, pure N 2, or a mixture of both. The pure, binary, and ternary adsorption of these gases on a dry coal from the Sulcis Coal Province in Italy has been measured at pressures up to 180 bar and temperatures of 45 and 70 degrees C for the pure gases and of 45 degrees C for the mixtures. The experiments were performed in a system consisting of a magnetic suspension balance using a gravimetric-chromatographic technique. The excess adsorption isotherms are successfully described using a lattice density functional theory model based on the Ono-Kondo equations exploiting information about the structure of the coal, the adsorbed gases, and the interaction between them. The results clearly show preferential adsorption of CO 2 over CH 4 and N 2, which therefore indicate that ECBM may be a viable option for the permanent storage of CO 2.

Prediction of competitive adsorption on coal by a lattice DFT model

“…For the sake of clarity, the most important equations are summarized in the following for the case of a gas mixture (Ottiger et al 2008b).…”

“…In other words, one has to go from the dimensionless excess adsorption based on lattice occupancies Γ lat i , to the excess adsorption defined in typical adsorption units, such as moles of adsorbate per unit mass adsorbent. To achieve this some issues need to be addressed, which are summarized in the following (Ottiger et al 2008b).…”

“…In a previous publication (Ottiger et al 2008b), competitive high-pressure adsorption data of CO 2 , CH 4 and N 2 on an Italian coal have been described using the same lattice DFT model presented in Sect. 2.1.…”

“…| in | = √ | ii || nn |. The values of the fluid-solid interaction energies U is have been obtained by fitting the experimental adsorption data (Ottiger et al 2008b). Values for the pure components physical properties as well as for the energetic parameters are summarized in Tables 1 and 2, respectively.…”

“…In the past, this model has been used to investigate the behavior of supercritical CO 2 on different adsorbents with different pore size distributions (Aranovich and Donohue 1996;Hocker et al 2003). More recently, we have extended the lattice DFT model to mixtures and we have applied it to coal, where experimentally obtained competitive adsorption isotherms of CO 2 , CH 4 and N 2 on an Italian coal have been successfully described (Ottiger et al 2008a(Ottiger et al , 2008b.…”