Effect of Aging in Nitrogen and Air on the Properties of Biocrude Produced by Hydrothermal Liquefaction of Spirulina

Hydrothermal liquefaction (HTL) allows direct conversion of wet biomass into biocrude oil, which has comparable energy content to petroleum. The stability of biocrude oil has significant effects on the downstream oil-refining and field applications. In this work, we investigated the physiochemical properties of HTL biocrude oil converted from Spirulina at different storage times and environment conditions: temperature (15 and 35 °C), headspace environment gas (air or N₂), and storage duration (up to 12 weeks). A layer of “oxidation shell” was formed on top of the biocrude oil during the storage. The rigidity of the “oxidation shell” and the viscosity of the inner oil increased significantly when the temperature shifted from 15 to 35 °C. The total acid number of the biocrude oil decreased by 22.6–24% in an N₂ environment but increased by 9.1–10.1% in air, regardless of the temperature, after long-term storage. Gas chromatography–mass spectrometry analysis revealed significant differences among chemical compositions in biocrude oil. Seven aging pathways were proposed for the physiochemical property transformation of algal biocrude oil during different storage scenarios. The formation of the “oxidation shell” was mainly due to the oxidation and polymerization of phenols and nitrogen-containing compounds, whereas the aging of the inner biocrude oil was caused by esterification and polymerization of esters, phenols, ketones, and nitrogen-containing compounds.