Researchers at the National Renewable Energy Laboratory (NREL) have demonstrated a novel integrated conversion process for algal biomass to produce fuel and chemical products of sufficient quality to be blend-ready feedstocks for further processing. A paper on their work is published in the RSC journal Green Chemistry.
Biomass from the green alga Scenedesmus acutus was acid-pretreated; charged polyamide-induced flocculation and vacuum filtration separated the resulting slurry into its solid and liquor components. Continuous fermentation of the algal liquor with Actinobacillus succinogenes 130Z delivered maximum productivity, process conversion yield, and titer of 1.1 g L−1 h−1, 0.7 g g−1 total sugars, and 30.5 g L−1, respectively.
Succinic acid was recovered from the fermentation media with a yield of 60% at 98.4% purity while lipids were recovered from the flocculated cake at 83% yield with subsequent conversion through deoxygenation and hydroisomerization to a renewable diesel blendstock.
Though algae-based feedstocks offer a greater theoretical con- version yield in terms of gallons of gasoline equivalent per ton of biomass over terrestrial-based feedstocks, reducing the costs for algal feedstocks remains a challenge for renewable fuel production. One strategy to offset these costs is to improve the net profitability of biorefineries through co-production of higher value chemicals in tandem with commodity fuels. Succinic acid, produced via fermentation of storage carbohydrates present in algal biomass, can serve as a higher-value chemical and as a feedstock for other chemicals such as maleic anhydride, 1,4-butanediol, tetrahydrofuran, and poly-butyl succinate.
These down-stream chemical products can supply multiple markets and thus disperse the potentially large succinic acid supply when commercial scale algal biofuels come online. Conversion of algal biomass to succinic acid using a biological route is particularly advantageous from a sustainability viewpoint as renewable algae biomass has a higher productivity than terrestrial feedstocks and biological routes to succinic acid consume CO2 rather than releasing it and reduce the need for conversion of dangerous chemicals such as benzene or butane.
… Here we demonstrate the complete PAP-SA [parallel algal processing for succinic acid] from microalgal biomass through to finished RDBS [renewable diesel blendstock] and a purified, high-value, chemical co-product (succinic acid). Significantly, this is the first integrated demonstration of each of the units of operation for establishing an algae feedstock-based biorefinery. This demonstration is notable for employing flocculation to separate the pretreated algae solids from the liquor with subsequent RDBS and continuous succinic acid production from the recovered solids and liquor respectively.—Knoshaug et al.
|Process flow diagram for PAP-SA including acid pretreatment, hydrolysate flocculation, liquor fermentation to succinic acid and purification, algal oil extraction and upgrading, and residual solids digestion. Knoshaug et al. Click to enlarge.|
The algae was harvested at the mid-cultivation stage for high-carbohydrate content. The biomass was pretreated at 155 ˚C for 15 minutes with sulfuric acid and water prior to flocculation and filtration.
Data from the demonstration was used as inputs for the NREL technoeconomic model previously used to evaluate other process configurations that employed fermentation of algal sugars to ethanol. The substitution of the succinic acid for the low-value biofuel ethanol resulted in a significant improvement in the overall process economics, despite sub-optimal yields and complex succinic acid purification scheme.
The NREL team will focus future work on optimization of the various unit operations to further reduce costs, as well as an incorporation of this process into a harmonized techno-economic, life-cycle, and resource assessment evaluation.
Eric P. Knoshaug, Ali Mohagheghi, Nick J. Nagle, Jonathan J. Stickel, Tao Dong, Eric M. Karp, Jacob S. Kruger, David G. Brandner, Lorenz P. Manker, Nick A. Rorrer, Deb A. Hyman, Earl D. Christensen and Philip T. Pienkos (2017) “Demonstration of parallel algal processing: production of renewable diesel blendstock and a high-value chemical intermediate” Green Chemistry doi: 10.1039/C7GC02295F