Marked Improvement in Bio-Oil Yield and Compositional Quality Using Two-Step Supercritical Water Liquefaction of Different Biomass Types

High Resolution Image Download MS PowerPoint Slide Supercritical water liquefaction of a range of biomass types with different compositions was investigated under two process scenarios. A one-step supercritical water liquefaction process, in which biomass–water reactants are directly heated to supercritical conditions, was compared with a two-step process consisting of (i) subcritical water treatment with a 30 min holding time, followed by (ii) supercritical water liquefaction. In this work, the two-step supercritical water liquefaction process was tested using various biomass types grouped according to their chemical composition (lipid-rich, carbohydrate-rich, and protein-rich). A total of seven biomass feedstocks were examined, including Jatropha curcas seed and leaf, Calophyllum inophyllum seed, Ceiba pentandra seed, corn cob, water hyacinth, and oil palm empty fruit bunches, to evaluate the hypothesis that the two-step process enhances product yield and improves bio-oil quality. The results show that the two-step process increased bio-oil yield by more than 50% on average. In addition, bio-oil quality improved significantly, with reductions of 65% in nitrogen-containing compounds and complete removal of sulfur-containing compounds. The proportion of oxygenated compounds in the bio-oil produced via the two-step process was lower than that from the one-step process, due to enhanced formation of hydrocarbons, heteroatom-containing compounds, and cyclic structures. Furthermore, the biopolymer composition of the biomass strongly influenced both product distribution and composition. For instance, the highest bio-oil yield (65.98 wt %) was obtained from lipid-rich biomass. In contrast, protein- and carbohydrate-rich biomass produced higher amounts of biochar and gaseous products.