Effect of Process Variables on Food Waste Valorization via Hydrothermal Liquefaction

We examined hydrothermal liquefaction (HTL) of simulated food waste over a wide range of temperatures (200-600 degrees C), pressures (10.2-35.7 MPa), biomass loadings (2-20 wt %), and times (1-33 min). These conditions included water as vapor, saturated liquid, compressed liquid, and supercritical fluid and explored both isothermal and fast HTL. The highest biocrude yields (similar to 30 wt %) were from HTL near the critical temperature. The most severe reaction conditions (600 degrees C, 35.3 MPa, 30 min) gave biocrude with the largest heating value (36.5 MJ/kg) and transfer of up to 50% of the nitrogen and 68% of the phosphorus in the food mixture into the aqueous phase. Energy recovery in the biocrude exceeded 65% under multiple reaction conditions. Saturated fatty acids were the most abundant compounds in the light biocrude fraction under all the reaction conditions. Isothermal HTL gave a higher fraction of heavy compounds than fast HTL. A kinetic model for HTL of microalgae predicted 2/3 of the experimental biocrude yields from HTL of food waste to within +/- 5 wt %, and nearly 90% to within +/- 10 wt %. This predictive ability supports the hypothesis that biochemical composition of the feedstock is important input for a predictive HTL model.