Optimization of synthesis gas heating values and tar by-product yield in underground coal gasification

Increasing global energy demand is a central challenge of the 21st century. Apart from renewable energy concepts, sustainable bridging and buffering technologies are needed which simultaneously offer long-term energy supply guarantees. Underground coal gasification (UCG) shows a promising potential to meet these requirements of future energy markets by in situ conversion of coal to a valuable synthesis gas that can be used in various industrial applications. Currently conducted in more than 14 countries worldwide, a main focus of international UCG efforts lies in improved technical control to reach the desired high gas qualities and simultaneously reduce the tar by-product yield as a potential source of groundwater contamination. Referring to an innovative thermodynamic UCG model parameterized with exemplary base-case data from the UCG field trials Hanna-I, Centralia-Partial Seam CRIP (PSC) and Pricetown, we investigated pyrolysis temperature, operating pressure, water influx and gasification agent injection, optimizing synthesis gas heating values and reduced tar production. General best-fit scenarios were performed assuming an idealized UCG reactor. Besides these best-fit simulations, selected worst-case scenarios considering gas losses were tested. Overall, the model results indicate that operational process improvement potentials vary over wide ranges in the order of few percent to more than 50% compared to the examined base cases. In view of selected available literature data, near-zero tar production rates and simultaneous Lower Heating Value (LHV) improvements in the range of similar to 2-40% are feasible.