Significance China boasts abundant tar-rich coal resources, which prove to be potential coal-based oil and gas resources capable of ensuring China's energy security. Extracting hydrogen while preserving carbon through the in-situ pyrolysis of tar-rich coals has developed into a trend in the clean and low-carbon utilization of coal resources. Downhole heaters serve as critical equipment for the efficient in-situ pyrolysis of tar-rich coals.

Advances Based on investigations and the systematic collection of relevant information and data, this study offers a brief introduction of the currently used methods for in-situ convection heating, focusing on the present situation of the research and development (R&D) of heaters for in-situ convection heating. Finally, this study briefly explores the application of downhole electric heaters, as well as providing future prospects for making breakthroughs. Relatively mature surface heaters are only suitable for shallow strata and strata requiring low heating temperatures (< 350 ℃). Downhole combustion heaters face challenges in effectively enhancing the stability of combustion reactions and the reliability of secondary ignition under extreme working conditions. Downhole electric heaters, which combine an enhanced heat transfer structure with electric heating rods, allow for reducing the surface temperatures of electric heating rods while decreasing the sizes of heaters, thereby effectively enhancing the heating efficiency and prolonging the service life. Additionally, employing seal structures can effectively improve the adaptability of downhole electric heaters to complex working conditions. Using a self-developed downhole electric heater with a closed double-shell structure, Shaanxi Coal Geology Group Co., Ltd. has successfully extracted the world's first barrel of coal tar from tar-rich coals through the in-situ pyrolysis in the Dabaodang mine field of the Yushen mining area in the Jurassic coalfield in northern Shaanxi.

Prospects To boost the heating rate and efficiency of tar-rich coal seams, subsequent research on downhole electric heaters can center around high-power downhole electric heaters and composite heating. However, composite heating suffers certain heat loss during medium transport, especially with an increase in heating well number and coal seam depth. Hence, it is recommended that high-power downhole electric heaters should be highlighted. This study is expected to provide technical support for the R&D and industrial application of the in-situ pyrolysis technology for tar-rich coals in China.