COALSWAD - a European project for the determination of the carbon dioxide storage capacity in coal

CO2 storage in appropriate geological reservoirs is considered as one possibility to decrease the amount of CO2 in the atmosphere. COALSWAD (www.coalswad.eu) deals with the storage of CO2 in coal seams. Our project is focussed on a comprehensive investigation of the adsorption behaviour of coal taking into consideration the influence of swelling behaviour. Coal samples of comparable rank but different source (Germany, Czech Republic and Spain) are examined. All samples are dried at 353 K for 36h under vacuum before the gravimetric adsorption measurements with He, CO2, CH4 and H2O at 313 K are started. The received adsorption data correspond to so-called excess-data, which are close to the absolute data in the case of gas-phase adsorption. Depending on the situation in coal seams carbon dioxide may exist under supercritical conditions. Adsorption experiments under supercritical conditions will give excess data, which differ strongly from the absolute data. They are not directly applicable for evaluating the carbon dioxide storage capacity of the coal seam. In order to determine the absolute data we need to know on the one hand either the density or the volume of the adsorbed phase. With this knowledge the received excess data have to be corrected to get “absolute” adsorbed amounts. To achieve this several approaches are described in literature. Any of these approaches face limitations caused by the heterogeneous microporous structure of natural coal. In consequence some results seem unrealistic, e.g. the adsorption capacity goes through a maximum with increasing carbon dioxide pressure. On the other hand these adsorption data are influenced by the swelling process occurring in coal, because volume changes possibly influence the buoyancy of the sample. Therefore it is essential to get detailed knowledge of swelling behaviour. By using SAXS- and NMR-techniques a detailed picture of structural changes in coal structure is gained. As structural heterogeneity can be characterised by means of fractal geometry it is investigated whether it is possible to calculate absolute adsorption data. In the end the calculated adsorption data and the corrected data are compared. The comprehensive analysis (experimental and theoretical) of the adsorption processes, together with the swelling mechanism proceeding in natural coal provides a deeper insight into the impacts of the injection of carbon dioxide into coal seams. These results can be included in existing reservoir simulation programmes. Therefore it will be possible to pre-estimate the economic feasibility of carbon dioxide storage under given physical and chemical conditions