Efficient Hydrogen and Methane Generation from Biomass Wastewater Via the Integration of Low Temperature Supercritical Water Gasification with Continuous Hydrothermal Synthesis of in Situ Nano-Catalyst

Abstract

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 and partial oxidation with in situ synthesis of virgin metal oxide nano-catalyst. Using olive wastewater as the feedstock, this study experimentally investigated the impact on gasification 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 Fe 2 O 3 nano-catalyst. These optimised conditions were further investigated using spent lees and stillage from Warner’s Distillery and Sedamyl UK Ltd industries respectively. The efficiency of hydrogen and methane yields and COD reduction were in the order of stillage > spent lees > olive wastewater. Importantly, this novel gasification approach prevents any performance drop or catalyst deactivation during continuous operation. This study exemplifies that co-generation of catalyst during supercritical water gasification is a promising and economically feasible direction for large scale continuous generation of hydrogen and methane from wastewater at < 450 °C. The process can generate hydrogen and methane rich syngas from different types of biomass wastewater, whilst lowering its COD and TOC. Furthermore, the metal oxide nano-catalyst can be recovered as a secondary product.

Keywords: supercritical water gasification, biomass wastewater, partial oxidation, metal oxide nano-catalyst, continuous hydrothermal synthesis, Hydrogen