Fast hydrothermal co-liquefaction of high-ash sludge and Chlorella for biocrude production

Fast hydrothermal liquefaction (HTL) shows great potential for producing biocrude. This research examined the influences of mixing ratios of sludge and Chlorella during both isothermal (300 °C, 1800 s) and fast (500 °C, 20 s) co-HTL. Adding Chlorella could efficiently retard repolymerization reaction and increase the biocrude production. The highest co-liquefaction effect was achieved from a sludge to Chlorella ratio of 2:6 by fast HTL, producing a biocrude yield of 29.65 wt%, closely approaching the calculated yield of 29.39 wt% and demonstrating an additive effect. However, for the high ash content of sludge, all isothermal and other fast HTL conditions presented an antagonistic effect on biocrude production. Meanwhile, co-liquefaction also exhibited a slight antagonistic effect on the heating value and energy recovery of biocrude, with experimental values reaching 32.73 MJ·kg⁻¹ and 52.74 %, respectively. FT-IR and maturity analyses indicated that compared to isothermal co-HTL, fast co-HTL biocrude was more favorable for the conversion into gasoline/diesel due to its lower paleo-temperature. GC–MS analysis identified amides (isothermal co-HTL) and nitrogen heterocycles (fast co-HTL) as the dominant components, suggesting that the holding time significantly influenced the competition of Maillard and amidation reactions. Besides biocrude, the major composition of aqueous phase products (APs) was also nitrogen heterocycles. Notably, fast co-HTL induced a substantial decrease in COD, NH₃-N, and TN contents of APs, reducing the discharge challenge of the by-products.