Underground in situ coal thermal treatment for synthetic fuels production

Underground coal thermal treatment (UCTT) is a promising concept that was recently proposed for extracting high-value hydrocarbon fuels from deep coal seams, which are economically unattractive for mining. UCTT is essentially an in situ pyrolysis process that converts underground coals into synthetic liquid and gaseous fuels, while leaving most of the carbon underground as a char matrix. The produced synthetic fuels have higher H/C ratios than coals. The remaining char matrix is an ideal reservoir for CO2 sequestration because pyrolysis significantly increases the surface area of the char. The UCTT concept is relatively new, and there is little research in this area. However, underground oil shale retorting, which is also an in-situ hydrocarbon fuels conversion process, shares key features with UCTT and has gained momentum in demonstration and commercial development. As such, there is a large body of literature available in this area. A review of the studies on underground oil shale retorting that are closely related to UCTT will shed light on the UCTT process. This paper presents a review of the recent literature on underground oil shale retorting that are most relevant to UCTT process. The review provides a background to the reader by comparing the properties of coal with oil shale, with an emphasis on the feasibility of applying oil shale retorting techniques to UCTT process. The review further discusses the coal and oil shale conversion issues and uses the knowledge of the latter as guidance for the development of UCTT. Published data on pyrolysis of large coal blocks at conditions relevant to UCTT process is scarce. Therefore, literature on conventional coal pyrolysis is reviewed for optimization of the UCTT process. Despite the abundant studies on pulverized coal pyrolysis, there are still many open questions on whether they can be directly applied to UCTT. A comparison of the unique environment of UCTT with conditions of conventional pulverized coal pyrolysis clearly shows there are knowledge gaps. Future research needs are then proposed to close these gaps.