Hydrothermal liquefaction of sewage sludge to produce bio-oil: Effect of co-pretreatment with subcritical water and mixed surfactants

Sludge contains numerous organic matters that benefit the production of fuels. In this study, the effects of different pretreatments on the hydrothermal liquefaction of sludge were investigated. The results showed that the release of bound water from sludge promoted liquefaction reactions at lower temperatures and inhibited the macromolecular chain-breaking into low-molecular compounds at higher temperatures. Compared with bio-oil from the raw sludge, the nitrogen content in the bio-oil was reduced from 6.1% to 2.9% after subcritical water (SCW) pretreatment. The calorific value of the bio-oil increased from 32.2 MJ/kg to 35.2 MJ/kg after cetyl trimethyl ammonium bromide (CTAB) pretreatment. The alcohol content in the bio-oil increased eight fold, and the contents of hydrocarbons and their derivatives increased by 11.5 times after fatty alcohol polyoxyethylene ether (AEO(9)) pretreatment. CTAB-AEO(9) could promote various reactions (e.g., nucleophilic, elimination, and oxidation), but the transesterification reaction dominated. CTAB-AEO(9)-SCW pretreatment increased the oil yield one fold and reduced the acid content by 95.2%.