Evolution pathway of nitrogen in hydrothermal liquefaction polygeneration of Spirulina as the typical high-protein microalgae

Abstract

Hydrothermal liquefaction (HTL) is a promising thermochemical technology for converting microalgae growing in aquatic habitats into fuels as well as high-value chemicals. Microalgae HTL polygeneration was carried out in order to reveal the evolution pathway of nitrogen during the HTL process. In this study, Spirulina with high protein content was subjected to hydrothermal treatment at 200–340 °C. The results show that 65.80–76.34 % of the nitrogen enters into the aqueous phase, while 12.96–34.00 % of the nitrogen is transferred to bio-oil. Ammonium ion is the main nitrogen-containing species in the aqueous phase, and the conversion of nitrogen in Spirulina reaches 11.72–51.18 %. The main nitrogen-containing compounds in bio-oil are cyclodipeptides and fatty acid amides respectively at 200–240 °C and 260–340 °C. Due to incomplete decomposition of Spirulina at 200–240 °C, 0.20–17.12 % of nitrogen is retained in the solid residue in the form of pyrrole-N, protein-N and quaternary-N. Understanding the evolution pathway of nitrogen during microalgae HTL polygeneration is conducive to achieving high-value utilization of microalgae.