Exergy analysis of underground coal gasification using supercritical water/carbon dioxide mixture as combined gasifying agent

Underground coal gasification plays a critical role in enabling efficient development and low-carbon utilization of deep coal resources (>2,200 m), characterized by gasifying agents operating under supercritical conditions. This study proposes a novel underground coal gasification system incorporating supercritical hydrothermal combustion, supercritical H₂O/CO₂ gasification and high-pressure pyrolysis. The thermodynamic performance evaluation and sensitivity analysis of the system are carried out. The results show that the largest exergy loss in the system occurred in the reduction zone, accounting for 27.9–35.2 % of the total exergy loss; increasing the gasification temperature could intensify gasification reactions, thereby raising the system's energy efficiency and exergy efficiency to 43.3 % and 35.2 %, respectively, but it would lead to the CO mole fraction in product gas rapidly increasing to 41.8 %; the increase of steam amount will increase the gasification efficiency on the one hand, and increase the heat loss on the other hand, so the energy efficiency and exergy efficiency of the system reach the maximum at 70 %wt.%; at 800 °C, CO₂ inhibits coal gasification and increases gas purification and compression energy consumption, thereby reducing the system's energy efficiency and exergy efficiency by 25 % and 20.4 %, respectively.