Effect of coal properties on CO2 sorption capacity under supercritical conditions

Relationships between coal properties and CO2 sorption capacity have been investigated for 30 coals from various sources in Australia and elsewhere, selected to cover a wide range of rank and maceral composition. Samples were characterised by their CO2 sorption capacity, porosity, density and chemical and petrographic composition.Carbon dioxide adsorption isotherms were measured on dry coal at 53 °C and at pressures up 16 MPa using a gravimetric apparatus. Isotherms were fitted to a modified Dubinin–Radushkevich (DR) model using gas density rather than pressure to enable the model to be applied to supercritical conditions. A term related to Henry's Law to account for gas absorbed by the coal is also included in the model. This model provided excellent fits to the experimental data; in most cases the difference between the predicted and measured value was less than 1% of the sorption capacity over the entire pressure range.Sorption capacities of the coals investigated were found to vary by over a factor of 2, even when comparing on a dry-ash-free basis. Carbon dioxide sorption capacity tended to reduce with rank to reach a minimum that corresponded to a vitrinite reflectance of about 1.2% after which it began to increase. Increasing hydrogen content tended to result in a decrease in sorption capacity. Two of the coals showed significantly higher sorption capacity than any of the other samples examined. In both of these cases, the coals had been naturally weathered and exhibited high porosity relative to the other samples. In general, however, the correlations observed were quite weak with considerable scatter in the data. This implies that CO2 sorption capacity, required for assessing sequestration potential, cannot be reliably inferred from the coal properties investigated here.