Two-stage liquefaction of sewage sludge in methanol-water mixed solvents with low-medium temperature

Abstract

Hydrothermal liquefaction could be a promising technology to convert sewage sludge into bio-oil for energy recovery. In this study, in order to obtain the maximum yield of bio-oil and improve the energy recovery of sludge, the influence of temperature and residence time on the multi-temperature continuously liquefaction of sludge in methanol-water mixed solvents was investigated. The results showed that the two-stage liquefaction significantly affected the yield, elemental composition, higher heating value, energy recovery and chemical composition of bio-oil. By optimizing the reaction conditions of hydrothermal liquefaction, the maximum utilization of sludge resources was realized. The maximum yield (42.2 %) and ester content (41.8 %) of bio-oil were obtained when the reaction conditions were 260 °C, 40 min for the first stage and 340 °C, 40 min for the second stage. The migration and transformation of nitrogen indicated that nitrogen elements were mainly concentrated in the liquid product and the N-heterocyclic compounds are the main forms of nitrogen in bio-oils.