Element and chemical compounds transfer in bio-crude from hydrothermal liquefaction of microalgae

• Fatty acid amide contents of bio-crude increased with temperature rising.
• Two types of N-heterocyclic compounds performed differently with temperature rising.
• High HTL temperature enhanced the heteroatom compounds in bio-crude.

In this study, hydrothermal liquefaction (HTL) experiments of Nannochloropsis and Spirulina were carried out at different temperatures (220–300 °C) to explore the effects of temperature on bio-crude yield and properties. The optimal temperature for bio-crude yield was around 260–280 °C. Transfers of element and chemical compounds in bio-crude were discussed in detail to deduce the reaction mechanism. The hydrogen and carbon recoveries were consistent with the results of bio-crude yields at every temperature point. The relative percentage of fatty acid in bio-crude decreased and the amine and amide increased for both microalgae with temperature rising. The N-heterocyclic compounds in bio-crude increased with temperature rising for Nannochloropsis, while decreased when temperature increased from 220 °C to 280 °C for Spirulina. Bio-crude gained at higher temperature or from microalgae with high protein content may contain high heteroatom compounds.