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

Abstract

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.