The effect of coal composition, moisture content, and pore volume distribution upon single and binary gas equilibrium and nonequilibrium adsorption : implications for gas content determination

Clarkson, Christopher Raymond

An experimental and modeling study was undertaken to determine the effect of coal composition, moisture content, and pore structure, upon natural gas adsorption and matrix transport. A volumetric high-pressure (up to 17 MPa) adsorption apparatus was constructed for the collection of single and multicomponent adsorption equilibrium and non-equilibrium adsorption data. A variety of equilibrium and non-equilibrium adsorption models were applied to determine which provided the best fits to the data. Coals selected for study included medium volatile bituminous coals of the Gates Formation, Northeastern B.C. Canada, and a suite of coals from the Sydney and Bowen Basins of Australia. Coal composition affects pore volume distribution, which in turn dictates the equilibrium and non-equilibrium adsorption characteristics of coals. Bright and banded bright coals have a greater amount of microporosity than dull coals, and hence have larger methane and carbon dioxide adsorption capacities. Dull coals have less microporosity but a greater amount of mesoporosity. Pore volume distributions in turn affect the adsorption rate behaviour of coals; bright coals have a uniform microporous structure and are adequately modeled using unipore diffusion models whereas dull and banded coals require models that account for a multimodal pore volume distribution. Coal composition also affects binary gas total adsorption isotherms, but has little effect upon carbon dioxide gas selectivity over methane. Coal moisture content appears to have a greater effect upon selective adsorption, but this requires further investigation. New numerical models, which account for bimodal pore volume distributions and non-linear adsorption characteristics, provide an adequate fit to adsorption rate data of the Gates coals. A bidisperse analytical model also provides excellent fits to the data, but does not account for non-linear adsorption. Models that do not account for non-linear adsorption yield optimized methane diffusivities that increase with pressure. The numerical model diffusivities decrease with an increase in pressure, possibly reflecting a bulk gaseous diffusion mechanism. Carbon dioxide diffusivities obtained from all models are larger than methane diffusivities. Methane diffusivities obtained using moisture-equilibrated coal data are smaller than those determined for dry coal. The Dubinin-Astakhov and Dubinin-Radushkevich isotherm equations provide better fits than the Langmuir equation to equilibrium methane and carbon dioxide adsorption data. The Dubinin models, which are based upon pore volume filling/adsorption potential theory, also have general validity in their application to supercritical methane-coal systems. Binary gas equilibrium predictions vary depending on whether the IAS or extended Langmuir model is used. The IAS theory, used in conjunction with the Dubinin-Astakhov equation, provides the best fit to CH₄/CO₂ adsorption data collected during this study.


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The effect of coal composition, moisture content, and pore volume distribution upon single and binary gas equilibrium and nonequilibrium adsorption : implications for gas content determination