Energy Environ. Sci. http://doi.org/bfjc (2016)

Producing biofuels from macroalgae is attractive, compared with land-based feedstocks, due to the relative ease of harvest and the decreased possibility for resource conflict with edible crops. However, macroalgae typically contain high concentrations of nitrogen, which must be substantially reduced to produce useable biofuel. To address this issue, Brian Haynes, Moti Herskowitz, Thomas Maschmeyer, Rocky de Nys and colleagues in Australia and Israel demonstrate how the nitrogen content can be minimized at multiple stages of an integrated process to yield a high-quality product for use in transportation fuels.

Credit: NANCY NEHRING / E+ / GETTY

The researchers grew filamentous green algae under nitrogen starvation conditions to minimize the initial nitrogen content of the biomass. Then, using a hydrothermal liquefaction process, coupled with in situfractionation and the appropriate solvent mix, they were able to produce a biocrude from the biomass with very low nitrogen and sulphur levels. The biocrude was combined with liquid hydrocarbons catalytically synthesized from a mixture of CO2 and H2, which could also be produced from biomass. Subjecting the blend to hydrotreatment using a nickel phosphide catalyst removed oxygen and further reduced the nitrogen content. This stage was critical to the success of the final step of the process, hydrocracking and isomerization, which uses catalysts intolerant to high nitrogen concentrations. The final product contained 277 ppm nitrogen and 0.12% oxygen, making it a suitable renewable fuel blend-stock.