Experimental investigation on gas-liquid–solid tri-phase product distributions during the simulated in-situ pyrolysis of tar-rich coal

•Underground in-situ pyrolysis of tar-rich coal was experimentally simulated.•The distribution of gas–liquid-solid three-phase products was reported.•The optimal tar production temperature is 500 °C, with a yield of 12.13 %.•The tar yield and reservoir pressure are negatively correlated.•The CO2 promotes the production of pyrolysis gas to some extent. The in-situ pyrolysis of tar-rich coal is a new technology for converting coal into tar and gas underground, which involves unique environmental conditions of high temperature, high pressure, large-sized coal and extremely low heating rate. Due to the different research purposes and application scenarios, the unique experimental environment of the in-situ pyrolysis is significantly different from that of conventional pyrolysis. Existing pyrolysis studies cannot be directly applied to the field of the in-situ pyrolysis of tar-rich coal, and the specialized research is indispensable. The product distribution exerts important impacts on the subsequent processing, hence an essential evaluation criterion for in-situ pyrolysis characteristics is the yield of gas-tar-char three-phase products. However, there have been few reports on the distribution of tri-phase products during in-situ pyrolysis of tar-rich coal. High temperature, high pressure, and extremely slow heating under multi-factor control were the environment of the in-situ pyrolysis which were experimentally simulated by gas pressurization in the autoclave, and the impacts of temperature, heating rate, and atmosphere at various reservoir pressures were studied. The optimal tar production temperature is 500 °C, with a yield of 12.13 %. With the increase of temperature, the aromatic hydrocarbons content in the tar product increases, the aliphatic hydrocarbons content gradually decreases and reaches 0 at 500 °C, and the phenols content first climbs and subsequently falls. There is a negative relationship between the tar yield and reservoir pressure. The impact of heating rate and atmosphere upon pyrolysis products distributions is insignificant. The present work is favorable to effective tar extraction by controlling pyrolysis parameters, and also provides better understanding for the development of large-scale tar-rich coal in-situ pyrolysis technology.