High-pressure adsorption of pure coalbed gases on dry coal matrices

As part of the efforts to model enhanced coalbed methane recovery and CO₂ sequestration in coalbeds, the pure adsorption isotherm behavior of five dry coals, i.e., Illinois No. 6, Wyodak sub-bituminous, Pocahontas, Beulah Zap, and Upper Freeport coal was studied. Using a volumetric technique, isotherms of pure CH₄, N₂, CO₂, and ethane were measured at 328.15 K and 13.8 MPa. The average expected experimental uncertainties of the pure-gas adsorption data was ∼ 7%. Uncertainties were smallest for nitrogen and largest for ethane. At low pressures, an amount of adsorbed gas increased in the order: N2, CH₄, C₂H₆, and CO₂. C₂H₆ isotherms had excess adsorption maximums between 4 to 6 MPa and CO₂ isotherms had excess adsorption maximums between 6 to 8.5 MPa. The excess adsorption isotherm of CH₄ on Upper Freeport and Pocahontas was flat at > 10 MPa. CH₄ and N₂ showed far less variation in adsorption amounts than either CO₂ or ethane. The ratio of CO₂ to ethane adsorption varied among the coals. Little variation in isotherm reproducibility was shown for methane after the coal was subjected to CO₂ and ethane gas adsorption. The Ono-Kondo model was capable of representing precisely each isotherm within the experimental uncertainties. This is an abstract of a paper presented at the AIChE 2004 Annual Meeting (Austin, TX 11/7-12/2004).