Hydrothermal Conversion of Lipid-Extracted Microalgae Hydrolysate in the Presence of Isopropanol and Steel Furnace Residues

Abstract

Purpose: Microalgae have a high potential as a feedstock for the production of biofuels, either indirectly, through the extraction of lipids, which can be transformed into biodiesel, or directly via whole cell conversion using hydrothermal liquefaction (HTL). Both approaches have disadvantages, due to the high cost of cultivating microalgae with sufficient lipid content (>40%), while the whole cell conversion produces low quality oils, which require significant further upgrading. This work investigated the possibility of realising the benefits of both processes, by studying the liquefaction reaction of a lipid-extracted algae hydrolysate. Methods: In order to enhance oil yields, the reaction was conducted in the presence of varying loadings of iso-propyl alcohol (IPA) and applied two waste steel furnace residues as potential liquefaction catalysts. Results: Primarily, The lipid extraction process needs to be optimized to reduce the amount of acid contaminant within the liquefaction medium. For the HTL process, the addition of 50 vol% IPA resulted in remarkably high oil yields of up to 60.2 wt% on an organic basis, whereas the two furnace residues had no positive effect on the product distribution, and instead favoured the formation of solid reaction products. Nevertheless, the results suggested that the presence of iron potentially reduced the nitrogen and oxygen content of the bio-oil. Conclusions: As such, HTL is a suitable method for valorising lipid-extracted algal biomass, where the bio-oil yields can be enhanced substantially by using IPA in conjunction with the water.