Cost of algal biofuel production is still a major challenge and the Energy Department has made reducing the costs of both algae production and conversion of algal intermediates to fuels significant goals.

In traditional processes, the algae produce lipids that get converted into fuels. However, simply increasing the amount of lipids in algae isn’t expected to bring costs down enough. NREL determined further progress could be made by more completely using all algal cellular components instead of just relying on the lipids. By applying certain processing techniques, microalgal biomass can produce carbohydrates and proteins in addition to lipids, and all of these can be converted into co-products.

Process flow diagrams for Parallel Algal Processing (PAP) and Combined Algal Processing (CAP) (A): PAP and (B): CAP. Source: Dong et al. Click to enlarge.

In their initial work, the NREL researchers determined that through the use of a solid-liquid separation (SLS) process, the carbohydrates can be converted to fermentable sugars, which can then be used to produce ethanol. However, process modeling has revealed that approximately 37% of the soluble sugars were lost in the solid cake after the SLS.

Those trapped sugars “cannot be used for fermentation without a costly washing step, resulting in a loss of overall fuel yield,” according to the Algal Research report.

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—88% of the theoretical maximum yield and 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.

The techno-economic analysis (TEA) indicated that the CAP can reduce microalgal biofuel cost by $0.95 per gallon gasoline equivalent (GGE) to $9.91/GGE from $10.86/GGE—a 9% reduction compared to the previous biorefinery scenario. While this is not nearly low enough to compete with petroleum, this approach can be combined with reduced costs for biomass production to provide a path forward to achieve that goal.

This work was funded by DOE’s Office of Energy Efficiency and Renewable Energy’s (EERE) Bioenergy Technologies Office.

Resources

• Tao Dong, Eric P. Knoshaug, Ryan Davis, Lieve M.L. Laurens, Stefanie Van Wychen, Philip T. Pienkos, Nick Nagle (2016) “Combined algal processing: A novel integrated biorefinery process to produce algal biofuels and bioproducts,” Algal Research doi: 10.1016/j.algal.2015.12.021