Catalytic supercritical water gasification of sugar beet vinasse for production of gas and water-soluble by-products

• 92.9 % of sugar beet vinasse energy content has been recovered by SCWG at 600 °C. • Highest CE of 77.7 wt% was obtained at 600 °C without catalyst. • Highest CH 4 yield of 11.02 mol/kg at 450 °C with a catalyst loading of 0.98 g cat. /g. • Organic acids and amines formation have negative impact on gas production. • Sulfur and other inorganics monitoring allowed to highlight their influence in SCWG. The efficient valorization of biomass and waste is one of the main concerns for the production of non-fossil materials and energy. This study focuses on the conversion of a high-volume industrial effluent (sugar beet vinasse) into an energy-rich gas using Supercritical Water Gasification (SCWG). The valorization of this effluent aims contributing to the circular carbon economy by recovering the energy content and the inorganics of the vinasse, without compromising its current use as potassium fertilizer. The impacts of reaction temperature (350, 450 and 600 °C) and catalyst presence on gasification efficiency (GE), carbon gasification efficiency (CE), gas yields and aqueous phase composition were discussed. Depending on the reaction conditions, the interpretation of these results allowed to distinguish different mechanistical paths towards gaseous species formation. Comparative experiments with and without Raney nickel catalyst at 450 °C have shown enhanced gas production with GE and CE reaching respectively 59.3 and 69.6 wt%, compared to 29.9 and 28.7 wt% at 450 °C without catalyst. Also, an important increase in methane yield from 1.69 to 11.02 mol/kg of dry matter feedstock was observed, thanks to catalyst utilization. The monitoring of organic acids, as well as sulfur, ammonia and inorganic content provided better insight on the fate of these species after vinasse SCWG by both high temperature and catalytic routes. Water-soluble inorganics from vinasse were recovered in the aqueous phase such as potassium (∼20 g/L), ammonia (∼9 g/L at highest conversion) and sodium (5.5 g/L), allowing their further utilization as intended.