CO2 storage and gas diffusivity properties of coals from Sydney Basin, Australia

Measurements of CO2 adsorption and diffusion properties of coals are reported for various coalfields within Sydney Basin, New South Wales (NSW), Australia. Adsorption measurements were undertaken using a gravimetric method. Measurements carried out on 27 coals show that Sydney Basin coals at CO2 sub-critical conditions, namely gas pressures below 6 MPa and temperatures below 39 degrees C, can adsorb a maximum volume (Langmuir volume) of 40 to 80 m(3) of CO2 per tonne of coal on a dry ash free basis (dat). The coals used in this study are of sub-bituminous to bituminous rank, ranging from 0.66 to 1.45% mean maximum vitrinite reflectance, and are from depths ranging from about 27 m to 723 m. The highest adsorption capacity applies to the highest rank coal, which is also the deepest coal. The standard deviation between Langmuir modeled and measured values is less than 1.5 m(3)/t, corresponding to a relative error of less than 2.7% for all except one coal. Based on adsorption isotherms, the CO2 storage capacity for in-situ seam pressure conditions range from about 6 to 51 m(3)/t. CO2 diffusion properties of 15 of these coals, determined using a newly developed system capable of accurately measuring diffusivity of gases in solid coal indicate that CO2 diffusivity (diffusion coefficient) in the Sydney Basin coals varies from 1.2 x 10(-6) to 10.2 x 10(-6) cm(2)/S. The diffusivity does not show any discernable trend with the variation in depth and rank. Porosity measured by a mercury injection method varies from 4 to 10% and decreases with increase in coal depth and rank. For some of the coal samples adsorption measurements for pure CH4, CO2 and N-2 indicate that the Sydney Basin coals can store twice as much CO2 as CH4 and six times more CO2 than N-2 (volume basis). Also, measurement of diffusivity in solid coal samples shows that CO2 diffuses twice as quickly as CH4. The data obtained from this study and the estimated coal resources in the state of New South Wales, allow CO2 sequestration potentials to be calculated. (c) 2006 Elsevier B.V. All rights reserved.