Biofuel potential of liquid products from protein- and lipid-rich biomass hydrothermal liquefaction

Hydrothermal liquefaction (HTL) of oleaginous microorganisms is an economical method with which to transform all compounds of wet biomass into competitive biofuels without the need for pretreatments such as feedstock drying and lipid extraction. In the present work, liquefaction of microalgae or yeasts in subcritical water is investigated in a self-built HTL system. Fractional yield distribution of the end-products and characterization of biocrudes from algae HTL are studied by considering the effects of the reaction temperature and residence time. The identified compounds in the biocrudes are compared to those in yeast-derived HTL biocrudes and the reaction scheme is reported. The present work reveals that no significant quality change of algae-derived HTL biocrudes is observed in a moderate temperature range thus the N-containing compounds vary complicatedly at different residence times. Compared to the pyrolysis biocrudes of algae and yeast, the collected HTL biocrudes do not perform the advantage, in terms of biofuel potential of a high portion of N-containing compounds, especially heterocyclic compounds. It should be noted that the low protein content in the feedstocks cannot definitely be proven to have led to the reduced heterocyclic compounds in HTL biocrudes. If lipids had been present, this would have enhanced the formation of N-containing heterocyclic compounds. Instead, amino acid degradation was accelerated by the high level of carbohydrates, forming amides. This paper presents a promising effort to develop an efficient route to transform oleaginous microorganisms into biofuels. • Reaction time showed strong effect on HTL biocrude quality. • Yeast as HTL feedstock has obvious advantages compared with algae. • N-heterocyclic compounds formation in HTL biocrude is enhanced by lipids. • Removal of proteins inhibits the N-heterocyclic compounds formation in pyrolysis.