Hydrotreatment of solvent-extracted biocrude from hydrothermal liquefaction of municipal sewage sludge

Abstract

The objective of this research was to investigate the influence of four solvents on hydrotreatment of hydrothermal liquefaction (HTL) biocrude derived from municipal sewage sludge. HTL of sludge was performed at 350 °C in a 1.8 L batch reactor. The biocrude was then extracted from the reaction mixture using dichloromethane (DCM), hexane, toluene, and acetone as solvents. Solvent extraction using DCM, hexane, toluene, and acetone resulted in 38%, 12%, 18%, and 10% biocrude yields (dry-ash-free basis), respectively. Hydrotreatment of four biocrude samples was studied using Ni/SiO2-Al2O3 catalyst at 350 °C in a 100 ml batch reactor. Physicochemical properties of biocrude samples and hydrotreated oils were determined using 2D-NMR, GC–MS, bomb calorimeter, viscometer, TAN and CHNS-O analyzers, and simulated distillation. The hydrotreatment of DCM-extracted biocrude was quite challenging due to catalyst deactivation, polymerization of biocrude, and equipment corrosion. The challenge was mainly attributed to high chloride content in DCM-extracted biocrude. However, the upgraded oil was produced at 86%, 60%, and 47% liquid yield from hexane-, toluene-, and acetone-extracted biocrudes. Most nitrogen-containing compounds were recovered using acetone. Among these three solvents, hexane produced the highest quality oil in terms of fuel properties such as acid number, heating value, boiling point distribution, and elemental composition.