Adsorption modeling of coalbed gases and the effects of water on their adsorption behavior

Abstract

Scope and Method of Study: The simplified local-density/Peng-Robinson (SLD-PR) adsorption model was utilized to investigate the adsorption behavior of coalbed gases on coals of varying rank. The model parameters were generalized in terms of readily-accessible coal properties such as the ultimate and proximate analyses of the coals. Further, the effects of water present in coals on the gas adsorption behavior were studied. In particular, the SLD-PR model was used to investigate this effect wherein water was treated as a separate adsorbed component in a binary mixture. To conduct this study, new high-pressure gas adsorption measurements were acquired for CO2 on wet Argonne coals and for methane, nitrogen and CO2 on dry and wet activated carbon using a volumetric technique.

Findings and Conclusions: The generalized SLD-PR model was found to be capable of accurate predictions of the adsorption of coalbed gases and their mixtures on dry and wet coals. Specifically, the generalized model was capable of (a) predicting the pure-gas isotherms for methane, nitrogen and CO2 on coals within two times the expected experimental uncertainties and (b) predicting, a priori, the adsorption of mixtures formed by these gases within three times the expected experimental uncertainties, on average. The generalized model was validated with an external data set which comprised of CO2 adsorption isotherms on 27 diverse coals.

CO2-water binary mixed gas adsorption modeling results on wet coals indicated that the SLD-PR model is capable of representing the adsorption of this highly asymmetric mixture within the experimental uncertainties, on average. The model parameterization used and the molecular interactions accounted for in describing water adsorption behavior on coals illustrated a viable method to obtain precise representations of this adsorbed mixture. The phase-check analysis of the same mixture indicated that there is a potential for the formation of an aqueous phase in these systems for coals that contain large amounts of moisture, with the exception of Beulah Zap lignite coal.