Development, validation and scenario analyses of two Underground Coal Gasification (UCG) model approaches focussing on gas quality-tar production control and tar pollutant-water solubility risk prognoses

Increasing global energy demand marks a central problem of the 21st century, requesting sustainable future technologies which simultaneously offer long-term supply guarantees. In view of these requirements in-situ conversion of coal, known as Underground Coal Gasification (UCG), is a promising technology, recently conducted in more than 14 countries worldwide. The key goal of this technique is the production of a high calorific syngas for multiple industrial applications under simultaneous minimization of environmentally relevant by-products. In view of undesired by-products many problems have already been solved, e.g. carbon dioxide emissions, which can effectively be reduced by coupling to Carbon Capture and Storage (CCS) under post-operative CO2 sequestration in converted coal seams. Major remaining challenges especially refer to gas quality-related tar production control and gas loss affiliated single tar pollutant-water solubility risk estimation for elevated p/T conditions in nearby groundwater systems. Focussing these problems within the scope of the present thesis two new thermodynamic equilibrium models were developed using the commercial simulator Aspen Plus ®. Upon development these models were utilized to investigate specific operating parameter sensitivities, optimization measures and decision support advice in the frame of literature-based scenario analyses. Besides diverse coal type-based differences these analyses especially revealed that case-specifically a quite substantial optimization of gas quality and participating tar yields can be reached by gasification agent regulation when other operating parameters are simultaneously kept at pre-optimized operating points. Total tar and single tar compound quantities were merely controlled by temperature within different model zones. Reactor temperature can thus be varied by different factors (water influx, gasification agent). With focus on tar spectrum little single operating parameter effects were found, generally the spectrum was merely controlled by coal type chemistry. In a summarized view the analyses confirmed typical single tar pollutant trends known from previous literature data. In terms of key findings within introductory p/T-dependent tar pollutant-water solubility prognoses pressure was found to be of minor quantitative significance, main quantitative solubility changes were controlled by temperature. Within the scope of first results for enhanced risk-related low-molecular weight tar pollutants thus potential deep future UCG applications, considering elevated solubility risks, should be provisionally limited to depths of maximum 2000 m