The challenge of nitrogen compounds in hydrothermal liquefaction of algae

There is a real interest in developing biofuels from microalgae. Their high productivity, lipid content and their capacity to capture CO2 make microalgae a competitive biomass for energy purposes and environmental con-cerns. Among various valorisation pathways of such biomass, hydrothermal liquefaction (HTL) process attracts interest. This process converts microalgae between 270 and 350 degrees C, in hot compressed water to produce a biocrude oil, which is then upgraded to produce biofuel. Biocrude oil is mainly composed of carboxylic acids and oxygenated compounds but also contains some cyclic nitrogen molecules or amides which are hard to upgrade and thus detrimental for biofuel quality. Therefore, in addition to the usual problem of oxygenated compounds in biocrude oils, the use of algae introduces another difficulty with the nitrogen compounds. The present review attempts to summarize the nature of the nitrogen compounds in algae, and their transformation during HTL conversion.