Supercritical water upgrading of water-insoluble and water-soluble biocrudes from hydrothermal liquefaction of Nannochloropsis microalgae

Treatment of wet algal biomass in hot compressed water produces crude bio-oil that spontaneously separates from the aqueous phase upon cooling. Additional bio-oil (water-soluble biocrude) can be obtained via solvent extraction of the aqueous phase coproduct. The present study is the first to examine the supercritical water upgrading (both with and without catalysts) of these two biocrude types independently. Of the catalysts tested, Pt/C had the best effect on upgrading the water-insoluble biocrude from algae liquefaction. It provided the highest C (80.9 wt%) and H content (10.6 wt%) and heating value (41.6 MJ/kg), and the lowest N (2.47 wt%), S (0.26 wt%), and O contents (4.80 wt%). The use of process water from liquefaction for supercritical water upgrading leads to higher quality upgraded oils. Hydrothermal upgrading of the water-soluble biocrude from algae liquefaction significantly improved its quality as the C content increased from 61.0 to 75.1 wt%, heating value increased from 29.3 to 35.4 MJ/kg, and the combined N, S and O content decreased from 30 wt% to 16 wt%. The material produced by upgrading the water-soluble biocrude from algae HTL contained aromatic hydrocarbons.