Exergy and Energy Analysis of Coal Gasification in Supercritical Water with External Recycle System

Supercritical water gasification (SCWG) is a novel and clean technology for lignite translating into hydrogen-rich gas. Previous experimental researches show that the use of external recycle system of liquid residual can improve the energy efficiency, but there is not a theoretical model to figure out the component of which exergy lost most and to provide guidance for further optimization of the existing system. In this paper, the thermodynamic model of liquid residual external recycle system was established, based on which energy and exergy balance of the system was evaluated and the exergy efficiency of the main equipment was calculated. Moreover, the influence of recycle flow ratio (0–37.5 %), gasification temperature (550 °C–650 °C), gasification pressure (23–25 MPa) and slurry concentration (2.73–4.15 %) on the exergy and energy efficiency were analyzed. The results showed that the exergy destruction rate of reactor was the highest, which reached 5.52 kW. Both energy and exergy efficiency increased as recycle flow ratio, gasification temperature and pressure increased. The energy and exergy efficiency of the system reached 70.26 % and 56.86 % respectively at the condition of recycle flow ratio of 30 %, gasification temperature of 650 °C, pressure of 25 MPa and slurry concentration of about 2.93 %.