The influence of lipids on the fate of nitrogen during hydrothermal liquefaction of protein-containing biomass

Abstract

Hydrothermal liquefaction (HTL) of carbohydrate (lactose), protein (lysine), and lipid (palmitic acid) surrogates was carried out in batch reactors for 20 min in a temperature range of 250–350 °C in order to understand interactions in hydrothermal reactions. A binary mixture of lysine and lactose leads to a higher bio-oil yield when compared to experiments made with isolated compounds, indicating a relevant contribution by Maillard reactions (MRs). Emulsification happens to proteins and lipids during HTL, making it difficult to separate and collect bio-oil products at low temperatures. When testing the ternary mixture, the nitrogen content of the bio-oil is negligibly affected by the changes of temperature, presenting a constant value of 5.3 wt. %. However, adding lipids dramatically changes the bio-oil composition. This is indicated by an increasing amount of fatty acid amides and a decrease in Maillard reaction products, suggesting a strong competition between amidation and MRs. A reaction scheme was proposed based on the final products and assumed reaction pathways. These findings here can contribute to a better understanding of the HTL of biomasses with complex organic compositions.