Sustainable hydrogen production via subcritical and supercritical water gasification of food waste: An optimization and reaction pathway study

Abstract

Hydrogen is recognized as a sustainable source of fuel. In this work, hydrogen production from food waste was explored using subcritical and supercritical water gasification. A representative food waste mixture (rice, orange peel, chicken meat, and lettuce) was gasified in a batch reactor. The influence of temperature (350–400 °C), biomass concentration (5–15 wt%), and reaction time (30–60 min) on hydrogen generation was examined. To evaluate the process and determine the conditions that maximize hydrogen generation, a response surface methodology was employed. Key operating parameters' independent and combined effects on total gas yield and hydrogen mole fraction in final gases were determined. Under optimal conditions at 400 °C, 5 wt% feedstock, and 60 min, the maximum total gas yield (8.3 mmol/g), hydrogen yield (2.44 mmol/g), H₂ mole fraction (29.5 %), and hydrogen selectivity (41.84 %) were obtained. Temperature exhibited the strongest influence, while feed concentration and residence time had comparatively lesser effects. The catalytic effect of Co₃O₄ and MnO₂ at different loadings was evaluated at optimal conditions. Co₃O₄ exhibited a superior performance, enhancing H₂ content, hydrogen yield, and hydrogen selectivity to 36.1 %, 3.36 mmol/g, and 56.49 %, respectively. Finally, a comprehensive study on the reaction mechanism of food waste was proposed to explain its conversion route into valuable products.