Hydrothermal co-liquefaction of chlorella vulgaris with food processing residues, green waste and sewage sludge

Hydrothermal liquefaction (HTL) is a promising technology for the production of renewable biocrude from a variety of biomass. Experiments with different biogenic wastes such as sewage sludge, food and green waste and grease residue were performed to assess their suitability for hydrothermal liquefaction either as individual material or in physical mixtures with microalgae biomass (= hydrothermal co-liquefaction). The experiments were carried out in a high pressure batch autoclave at temperatures of 350 degrees C, a holding time of 15 min in nitrogen-atmosphere and a dry matter content of 10%. Mass balances revealed the yields of the four HTL-phases (biocrude, gaseous, solid and water phase). Additional analyses comprise Soxhlet-extraction to determine the lipid content of the raw materials, FTIR spectroscopy for HTL-gas phase composition as well as determination of calorific values and elemental compositions for raw material and respective biocrude samples. The following biocrude yields were achieved for the individual raw materials: green waste - 4.4%; sewage sludge - 12.0%; food waste - 18.2%; grease residue - 76.3%; chlorella vulgaris - 18.3%. Co-liquefaction effects for binary physical mixtures (50:50) of the biogenic residues with chlorella vulgaris could not be observed on a statistically significant basis. Hence, for the investigated samples, co-liquefaction is an option to reduce logistics costs and ensure sufficient and constant raw material mixtures for processing rather than increasing the biocrude yields by synergistic effects during Co-HTL.