Effect of ultrasound pretreatment of municipal sewage sludge on characteristics of bio-oil from hydrothermal liquefaction process

Abstract Hydrothermal liquefaction experiments with a moist (water content 80.64 wt%) municipal sewage sludge (SS) were conducted at 280, 300, 320, 340 and 360 °C for raw and ultrasound pretreated (energy input 4500 kJ/kg of solid matter) sludge. The main aim of the experiments was to explore the effects of ultrasounds on yields and composition of bio-oil and liquefaction by-products (gas and char). The bio-oils obtained were viscous, dark brown liquids with a specific odor and heating values in the range 31.39–37.35 MJ/kg. A temperature of 320 °C was found to be the optimum for the maximum yields of the SS-derived bio-oil. During the tests a significant effect of ultrasounds on bio-oil yields was proven. A positive effect of ultrasound energy input was observed at each of the liquefaction temperatures and the maximum effect (increase of 19 wt% in bio-oil yield) was achieved at 320 °C. The GC–MS analysis revealed that the bio-oils were complex mixtures of many chemical compounds belonging to different chemical classes: aliphatic hydrocarbons, fatty alcohols, N-aliphatics, phenols, aromatic esters, aldehydes, ketones, alcohols and carboxylic acids. It was found that influence of ultrasounds on the chemical composition of the bio-oils was negligible.