Supercritical water gasification of sewage sludge and combined cycle for H₂ and power production – A thermodynamic study

An integrated system of supercritical water gasification (SCWG) and combined cycle has been developed for H₂ production and power generation. Sewage sludge and lignite coal were selected as raw material in this simulation. The effects of feed concentration (10–30 wt%) and lignite coal addition (0–50 wt%) on syngas yield and H₂ yield were also investigated in the temperature range of 500 °C–700 °C. Several heat exchangers were considered in the proposed integrated system to minimize energy loss. High pressure syngas was expanded by using turbo-expander to produce electricity, resulting in the improvement of the total efficiency. The results showed that the minimum feed concentrations of 14.25 wt%, 18.75 wt%, and 25.50 wt% were required to achieve self-sufficient energy at 500 °C, 600 °C, and 700 °C, respectively. However, the lower feed concentration and higher temperature were preferable for syngas production. The highest syngas and H₂ yield were obtained at 700 °C and 10 wt% feed concentration. The SCWG could produce 178.08 kg syngas from 100 kg feed and 9.06 kg H₂ were obtained after H₂ separation. The total power generation from turbo-expander and combined cycle module was 48.37 kW. By combining SCWG and combined cycle, the total efficiency could reach 63.48%. It worth mentioning that the addition of lignite coal could help reduce the minimum feed concentration to achieve autothermal condition, but did not have significant improvement on H₂ production.