Recovery of Energy and Nitrogen via Two-Stage Valorization of Food Waste

We carbonized simulated food waste (stage I) and then liquefied the biochar produced (stage II) with the goals of producing bio-oil and recovering nitrogen. Both stages used hydrothermal and pyrolytic approaches, so the influence of water during the treatments could be discerned. Pyrolysis produced biochars in the greatest yield (57 wt %) from the biomass feedstock, and it produced biocrudes with the greatest HHV (39.4 MJ/kg) via the liquefaction of biochar from hydrothermal carbonization. Pyrolysis of biochar for stage II gave negligible aqueous-phase product yields, however, which limited the nitrogen recovery with this approach solely to that recovered in the initial carbonization step. The highest N recovery (75%) in the aqueous-phase products occurred with hydrothermal treatment for both carbonization and liquefaction. This N recovery greatly exceeded those (<10%) for single-step hydrothermal liquefaction of this same feedstock. Energy recovery in the biocrude oil produced from this two-step process exceeded 50% in several runs. This two-step approach for food-waste valorization provides an opportunity for comparable energy recovery and much greater N recovery than are available from single-step hydrothermal liquefaction.