Biofuels from Algae Boosted by NREL Refinery Process

A new biorefinery process developed by scientists at the National Renewable Energy Laboratory (NREL) has proven to be significantly more effective at producing ethanol from algae than previous research.

Economic production of algal biofuel is a major challenge, and the U.S. Department of Energy has made reducing the costs of both algae production and conversion of algal intermediates to fuels a priority. In traditional processes, the algae produce lipids that are converted into fuels. However, simply increasing the amount of lipids in algae isn't expected to bring down costs far enough.

NREL determined that further progress could be made by more completely using all algal cellular components instead of just relying on the lipids. By applying specific processing techniques, microalgal biomass can produce carbohydrates and proteins in addition to lipids, and all of these can be converted into co-products.

The process, dubbed combined algal processing (CAP), follows work previously done at NREL to determine the applicability of two promising algal strains, Chlorella and Scenedesmus, as biofuel and bioproduct producers. That work concluded that Scenedesmus performed better in this process, with impressive demonstrated total fuel yields of 97 gallons of gasoline equivalents (GGE) per ton of biomass.

In their previous work, NREL researchers determined that through the use of a solid-liquid separation process, the carbohydrates can be converted to fermentable sugars, which can then be used to produce ethanol. However, as much as 37% of the sugars were lost during that process. Those trapped sugars cannot be used for fermentation without a costly washing step, resulting in an overall loss of fuel yield.

In their most recent work, NREL researchers hypothesized the amount of ethanol could be significantly increased by simplifying the processing. By skipping the solid-liquid separation process and exposing all algae components directly to fermentation conditions, both ethanol (from the carbohydrate fraction) and lipids can be recovered simultaneously.

Using Scenedesmus and the CAP, and after upgrading the lipids to renewable fuels, scientists were now able to produce a total fuel yield estimated at 126 GGE per ton. That amounts to 88% of the theoretical maximum yield and is 32% more than the yield from lipids alone.

The NREL researchers also were able to recover 82-87% of the lipids from the CAP, even after ethanol fermentation and distillation, indicating that the initial fermentation of sugars in the pretreated biomass slurry doesn't significantly impede lipid recovery. These results led to the conclusion that the novel CAP process is capable of reducing the cost of algal biofuel production by nearly $10/GGE compared to a "lipids only" process—taking the modeled cost down to $9.91/GGE.

While this is not low enough to compete with petroleum, NREL says this approach can be combined with reduced costs for biomass production to provide a path forward to achieve that goal.