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Zusammenfassung

Underground coal gasification is currently being considered as an economically and environmentally sustainable option for development and utilization of coal deposits not mineable by conventional methods. This emerging technology in combination with carbon capture and sorptive CO2 storage on the residual coke as well as free-gas CO2 storage in the cavities generated in the coal seams after gasification could provide a relevant contribution to the development of Clean Coal Technologies. Three hard coals of different rank from German mining districts were gasified in a laboratory-scale reactor (200 g of coal at 800 °C subjected to 10 L/min air for 200 min). High-pressure CO2 excess sorption isotherms determined before and after gasification revealed an increase of sorption capacity by up to 42%. Thus, physical sorption represents a feasible option for CO2 storage in underground gasification cavities. ’INTRODUCTION Deep coal deposits that cannot be economically developed using conventional methods are being currently considered as carbon dioxide sinks in combination with enhanced coal bed methane recovery.1-3 Carbon dioxide storage capacities of these deep deposits mainly depend on their accessibility to carbon dioxide with respect to pore space for free gas storage and surface area for storage by sorption. However, unworked deep coal seams are relatively inaccessible to pore gases, as porosity and permeability of coals tend to decrease with increasing overburden pressure.4,5 Underground coal gasification (UCG) is discussed as an option to circumvent the limitations resulting from low accessibilities by generation of energy in combination with carbon capture and its geological storage (CCS). The general UCG-CCS concept is discussed by many authors, though studies generally have not considered experimental or field data taking into account carbon dioxide sorption capacities.5-12 However, there is an agreement on the suitability of gasified coal seams for carbon dioxide storage (reactor zone carbon storage). Even though this concept has been widely discussed,5,7,13 qualified data on carbon storage capacities in the former reactor zone are not yet available. Sorption capacities of the coal gasification residues and the reactor faces are required for quantitative assessment of total carbon storage potential in coal gasification cavities. The aim of the present study was the determination of carbon dioxide storage capacities of coal gasification residues. For that purpose, representative hard coals were retrieved from deep German hard coal deposits and gasified using a laboratory gasification reactor. Coal properties and carbon dioxide sorption capacities were determined before and after the gasification process to assess the impacts of gasification. As illustrated in Figure 1, the UCG reactor is represented by different zones: the void space (also involving accumulated ash and rubble) and the surrounding coal (char and tars) thermally influenced by the UCG process. By the methodology presented within this study, we investigated CO2 sorption capacities of the coal present close to the reactor wall and the virgin coal (raw coal) present outside the area influenced by the UCG process. These data can be applied for the assessment of CO2 storage potentials in coal seams gasified by the UCG method. ’EXPERIMENTAL SECTION Samples. Samples were taken from three German mines producing Upper Carboniferous coals (mainly of Westphalian age) in the paralic “foredeep” of the Variscan foldbelt. This sequence contains approximately 200 seams within a total sediment thickness of more than 3000 m of clastic rocks (sand-, silt-, and mudstone). The highest percentage of coal within this stratigraphical sequence is reached in the Lower Westphalian B with almost 5%.14 Due to the thickness of the stratigraphic column, the lateral extension, and the complex burial and tectonic history, the coal shows a wide range of ranks from high-volatile bituminous coal to anthracite (vitrinite reflectance from 0.95 to 3.22%). The Westphalian B strata of the Prosper-Haniel mine deliver high-volatile bituminous coal used for power generation (33-38% volatile matter).Here, seams are mined in workings down to 1200 m, while the samples were retrieved at a depth of 915 m. In the Lippe mine;abandoned in 2008; Westphalian C and D with up to 40% volatile matter (high-volatile Received: August