Preparation of biocrude based on fish proteins and lipids as feedstock by subcritical hydrothermal liquefaction

The fact that aquatic product waste causes environmental pollution and is difficult to handle has become a major challenge globally. Hydrothermal liquefaction (HTL) has provided new ideas for addressing this challenge. In this study, small yellow croaker was used as the raw material for HTL. It was found that the transition temperature point R from P1 (liquefied but without biocrude production) to P2 (biocrude production) lies between 200 and 250 °C. The impact of the pyrolysis temperature on the physical properties and composition of the produced biocrude was examined. The results showed that as the temperature increased, the acidity and density of the biocrude decreased. Furthermore, gas chromatography–mass spectrometry (GC-MS) analysis identified the components, revealing that changes in the physical properties were closely related to changes in the chemical composition. Although the composition remained similar with increasing temperature, the content varied significantly: the content of fatty acids and their derivatives decreased markedly, whereas that of aromatic compounds increased significantly. At 300 °C, the carbon chain distribution of the compounds was notably lower, with most being light oils, resulting in a reduced viscosity and density of the biocrude. Additionally, GC-MS analysis explored the effect of residence time on biocrude quality and found that reaction conditions of 300 °C for 125 min yielded high-quality biocrude and showed great potential for quality improvement with the addition of catalysts. Moreover, by altering the temperature and residence time, the content of the products can be controlled to produce high-value-added products.