Catalytic hydrothermal gasification of algae for hydrogen production: Composition of reaction products and potential for nutrient recycling

Chlorella vulgaris, Spirulina platensis and Saccharina latissima were processed under supercritical water gasification conditions at 500 °C, 36 MPa in an Inconel batch reactor for 30 min in the presence/absence of NaOH and/or Ni–Al2O3. Hydrogen gas yields were more than two times higher in the presence of NaOH than in its absence and tar yields were reduced by up to 71%. Saccharina, a carbohydrate-rich macro-alga, gave the highest hydrogen gas yields of 15.1 mol/kg. The tars from all three algae contained aromatic compounds, including phenols, alkyl benzenes and polycyclic aromatic hydrocarbons as well as heterocyclic nitrogen compounds. Tars from Chlorella and Spirulina contained high yields of pyridines, pyrroles, indoles and pyrimidines. Up to 97% TOC removal were achieved in the process waters from the gasification of the algae. Analyses for specific nutrients in the process waters indicated that the process waters from Saccharina could potentially be used for microalgae cultivation.

► Hydrogen gas production from algae samples increased in the presence of sodium hydroxide. ► Tar yields declined significantly in the presence of the alkali. ► Nickel catalyst and sodium hydroxide led to significant reduction in phenols and tars. ► Tar was mainly composed of stable aromatic compounds and heterocyclic nitrogen compounds. ► Process waters from hydrothermal gasification of Saccharina could be used for microalgae cultivation.