Supercritical water gasification of lipid-extracted hydrochar to recover energy and nutrients

This article reports experiments where lipid-extracted algal hydrochar (LEH) was gasified in supercritical water to recover energy (in the form of fuel gases) for use within a biorefinery and to recycle nitrogen (in the form of ammonium) for algae growth. Supercritical water gasification (SCWG) of LEH at 450-600 °C produced H₂, CO₂, CH₄, CO, C₂H₄, and C₂H₆, and converted the organic-bound nitrogen in the LEH into ammonium in the aqueous phase. Increased gasification severity increases the energy and nitrogen recovery. An energy recovery of 75% and complete nitrogen recovery as ammonium were achieved after SCWG at 600 °C and 6 h. These findings demonstrate for the first time the conversion of the byproduct LEH to materials that can be used within an algal biorefinery. Recovering energy and nutrients in this manner may improve the environmental and economic sustainability of making nutraceuticals and biodiesel from microalgal lipids via an approach that uses hydrothermal carbonization of the biomass followed by solvent extraction of the lipids from the hydrochar.