The integration of low temperature supercritical water gasification with continuous in situ nano-catalyst synthesis for hydrogen generation from biomass wastewater

Supercritical water gasification mechanism, illustrating the interplay between the oxidant, iron oxide catalyst and the gasification reactions. [Display omitted] •A continuous flow catalytic supercritical water gasification at 430 °C and 20 secs.•H2 and CH4 yields at 17 and 12 mol/(kg biomass) generated from bio-wastewater.•Continuous in situ nano-catalyst formation prevents any loss of gasification performance.•Chemical oxygen demand (COD) lowered during the SCWG with 77 % efficiency.•Gasification efficiency of 72 % achieved. A continuous hydrothermal process is demonstrated, for the first time, that can operate at low gasification temperature (430 °C) and residence time (20 s) by combining supercritical water gasification (SCWG) and partial oxidation with in situ synthesis of virgin metal oxide nano-catalyst. Using olive wastewater as the feedstock, this gasification study experimentally investigated the impact of multiple variables: (1) COD feed concentration, (2) the in situ synthesis of different metal oxide nano-catalysts, (3) the partial oxidation coefficient (ɳ) and (4) the nano-catalyst precursor solution concentration. The optimum conditions for the generation of hydrogen and methane from olive wastewater were a feed COD of 38.6 g/L, ɳ = 0.8, and 60 mM precursor concentration for the in situ synthesis of Fe2O3 nano-catalyst. These optimised conditions were further investigated using spent lees and stillage. The efficiency of hydrogen and methane yields and COD reduction were in the order of stillage > spent lees > olive wastewater. The highest hydrogen molar selectivity, hydrogen and methane yields at 18.8 %, 17 and 11.4 mol/(kg biomass) respectively were obtained with stillage feedstock. Gasification, COD and TOC reduction efficiencies were 68.8–71.7 %, 72.6–76.5 % and 53.9–55.7 % respectively, with this process. Importantly, this novel gasification approach prevents any performance drop or catalyst deactivation during continuous operation. This study exemplifies that the co-generation of catalyst during SCWG is a promising and economically feasible direction for large-scale continuous generation of hydrogen and methane from different types of biomass wastewater at