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Argonne LCA finds renewable diesel from algae fractionation has 63-68% lower GHG than petroleum diesel

A new analysis from Argonne National Laboratory, funded by the US Department of Energy’s Bioenergy Technologies Office (BETO), shows the potential of an algae fractionation process to produce renewable diesel fuel with 63%–68% lower greenhouse gas (GHG) emissions than conventional diesel. The study is published in the journal Algal Research.

In some algal biofuel production methods, lipids are extracted from algae and converted to renewable diesel, while the non-lipid components of the algae are converted to biogas. The biogas is used for renewable heat and electricity to power the conversion process of the lipids to renewable diesel.

In the algae fractionation process, algal biomass is separated into carbohydrate, lipid, and protein-rich fractions that can be converted into fuels and co-products. The carbohydrate-rich stream, produced via acid hydrolysis, is fermented to ethanol. Lipids are extracted from the lipid rich fermentation stillage and upgraded to renewable diesel (RD), which accounts for most of the produced fuel. The protein-rich residue left after lipid extraction is converted to biogas by anaerobic digestion (AD); this biogas was used for co-generation of heat and power on-site.

Fractionation
Fractionation process flow diagram. Biomass production is in open ponds, harvested via settling, and dewatered via dissolved air flotation and centrifugation. Pegallapati and Frank. (2016) Click to enlarge.

Early research on the fractionation process conducted at the National Renewable Energy Laboratory (NREL) demonstrated encouraging results with high yields (~65%) for hydrolysis of algal carbohydrates to monomeric sugars, and high yields (~80%) of fermentable sugars to ethanol as well as recovery of lipids via a wet extraction process at roughly 80 wt% moisture content.

Other work has shown that the fractionation pathway is attractive economically; fractionation could achieve a selling price of $4.35 per gallon of gasoline equivalent (GGE) in 2011 dollars for a high-lipid feedstock.

The fractionation pathway is attractive economically, but it redirects biomass away from on-site renewable heat and power generation, so it is possible that GHG and energy use performance may be impaired. Performance is often assessed via a metric of emissions or energy use per unit of produced fuel. The question addressed by this paper is whether the fuel yield increase in the fractionation process compensates for the decrease in on-site heat and power production and compensates for the increase in process energy demand, e.g., for ethanol distillation.

This study presents an LCA of energy use and greenhouse gas emissions for algal biofuels produced via the fractionation process… The LCA considered all operations in the fuel pathway, including upstream material and energy provisioning operations, cultivation, and final fuel use.

—Pegallapati and Frank (2016)

The Argonne team used life-cycle analysis to analyze whether the energy required for this process would still result in lower GHG emissions relative to conventional fuel. Their analysis showed lower GHG emissions from algae fractionation than those associated with conventional low-sulfur petroleum diesel—showing that the biofuel pathway could still reduce GHG emissions compared to petroleum fuels that are used today.

… the fractionation pathway could provide a method to increase the total fuel yield per mass of algae feedstock while achieving good GHG and energy performance if it achieves the performance targets assumed in Davis et al. Future work should consider feeds with lower lipid levels, should be better tied to observed productivities and compositions, and should develop additional experimental work to further validate the empirical basis for key data such as yields, energy use, mass balances, recycle rates, and generation of waste streams requiring treatment.

—Pegallapati and Frank (2016)

This project is part of BETO’s Strategic Analysis and Sustainability Program, which conducts research and analysis to understand and enhance the environmental, social, and economic benefits of bioenergy production while mitigating concerns. BETO also conducts research, development, and demonstration projects to lower the cost of algal biofuel production.

Resources

  • Ambica K. Pegallapati, Edward D. Frank (2016) “Energy use and greenhouse gas emissions from an algae fractionation process for producing renewable diesel,” Algal Research, Volume 18, Pages 235-240 doi: 10.1016/j.algal.2016.06.019

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