Characterization of Biocrude Produced by Hydrothermal Liquefaction of Municipal Sewage Sludge in a 500 mL Batch Reactor

This paper investigates the production of biocrude via hydrothermal liquefaction (HTL) of sewage sludge. Results on the effect of the separation protocol of the oily/aqueous phases highlighted the superior performance of inertial separation over liquid–liquid extraction, with a biocrude yield of 43% attained at a HTL temperature of 350 °C. Experimental results revealed that processing time exerts a nonmonotonic effect on the biocrude yield with maximum yields of 35 and 43% after 30 and 10 min processing times at 300 and 350 °C, respectively. The best results in terms of energy recovery (about 59%) and reduced content of S in the biocrude (about 1%) are obtained at 350 °C. A minimum yield of water-soluble compounds (29%) is achieved at 10 min of processing time at 350 °C, whereas the maximum yield (49%) is attained after 30 min of processing time at the same temperature. From both NMR and LC-MS analyses, the biocrude obtained proved to be a complex mixture mainly constituted by aliphatic compounds, with small amounts of aromatic compounds (including PAHs) and olefins. Finally, inorganic elements present in the sludge report entirely to the solid phase.