Co-liquefaction of municipal sludge and tobacco stems: a novel strategy for crafting bio-oil enriched with high value-added nitrogen-containing compounds and amide entities

Nitrogen-containing compounds are an important organic compound. High Value-Added Nitrogen-containing Compounds (NCCs) can be obtained through co-hydrothermal liquefaction (co-HTL) of sludge (SD) and tobacco stems (TS) with high nitrogen content. The co-HTL process is studied and optimized to obtain a maximum bio-oil yields (11.5%), corresponding to the reaction condition of 340℃, 5 MPa, SD:TS = 1:2, and t = 30 min. The characterization and analysis show that such co-HTL not only reduces the acidic components in bio-oil, thereby decreasing the propensity for vessel corrosion, but also increases the nitrogen compounds (NCCs) as well as effectively enriches the amide entities in bio-oil. When SD:TS = 2:1, the bio-oil displays a NCC concentration of up to 40.73%, while the proportion of amide entities peaks at 7.3%. The effects of adding different additives on the co-HTL are investigated as well, showing that the additives reduce the bio-oil yield, inhibit from producing NCCs, while elevate the CH₄ and H₂ content of hydrothermal gas. Specifically, KOH behaves the best to remove NCCs as well as enhance the hydrocarbon formation. The potential reaction mechanism of co-hydrothermal liquefaction of SD and TS was also deduced. Such co-HTL and addition of alkaline additives behave promising for a wide range of applications in preparing high value-added nitrogen-containing compounds.