Thermodynamic analysis and life cycle assessment of the preferred supercritical water gasification coupled system for energy self-sufficiency: From food waste to hydrogen

Supercritical water gasification (SCWG) technology can result in the resourceful and harmless conversion of organic waste into hydrogen-rich gas due to the homogeneous reaction environment of heat absorption and reduction. However, currently designed uncoupled SCWG systems supply the heat required for gasification heat absorption by elevating the supercritical water enthalpy, resulting in harsh heat transfer environments and energy losses. This paper developed a coupled system integrating the gasification reactor with the oxidation reactor through a bushing to reduce the temperature of the oxidation reactor. A comprehensive comparison of the thermodynamic performance and environmental burden of coupled and uncoupled systems for treating food waste was conducted. The coupled system directly transfers the heat of 727.80 kW–1491.75 kW from the oxidation reactor to the gasification reactor through the bushing, reducing oxidation temperature by 162 °C–368 °C to save combustible gas consumption. The energy efficiency of the coupled system was improved by 5.25 %–13.73 % over the uncoupled system under different conditions. Life cycle assessment (LCA) revealed that employing carbon capture and storage (CCS) technology can reduce the GWP of the coupled system to 0.31 kg CO₂-eq/kg H₂, which was only 35.23 % of that of the uncoupled system. Exergy flow analysis further indicated that the oxidation reactor, heat exchanger, and cooler are the largest sources of energy loss in the system. The coupled system can control the oxidation reactor temperature with direct heat transfer, resulting in a decrease in 795.91 kW exergy loss of these units. This work would be of great value for the optimal design of the SCWG system.