requirements, GHG emissions, and nutrient requirements depending on what the highquality coproduct is. Coproducts also affect economic viability.

If the coproduct is an animal feedstuff, then a coproduct credit could be assigned to the LCA of nutrient and energy requirements, and to GHG emissions for the animal feedstuff that is substituted by the coproduct of algal biofuel. Safety would have to be considered if the coproduct is to be fed to animals or used to fertilize food crops. Algae potentially can accumulate toxic compounds (for example, mercury can accumulate in cultivated algal cells if unscrubbed flue gas is used as a source of supplemental carbon dioxide [CO2]). Toxicants accumulated in cultivated algae can be bioaccumulated if fed to animals or taken up by crop plants from fertilizers, or can inhibit anaerobic digestion if lipid-extracted algae are to be used for electricity generation. Other than safety, the nutritional quality of the feedstuff and the effect of the feedstuff on the quality of food animal (for example, meat quality) would have to be assessed to determine its suitability as a primary feedstuff or a supplement. A feedstuff coproduct can contribute to offsetting costs of algal biofuel production if there is a large enough market for the sale of the coproduct. If the feedstuff is only suitable for certain animals and has a limited market, then saturating the market with large quantities of the coproduct could lower its market price and utilization options.

If the coproduct is electricity, then market saturation will not be a concern. The energy requirement and GHG emissions could be lower compared to the reference pathway, and the cost of energy input into the algal biofuel production pathway could be reduced.

6.1.3 Alternative Pathway #2–Raceway Pond Producing FAME

The key difference between this and the reference pathway is the fuel produced, with this scenario assuming the fuel product to be fatty-acid methyl esters (FAME). With most processes along the supply chain being equal, the ability to meet various sustainability goals and the potential concerns for this pathway are similar to the reference case. However, FAME’s poor cold-flow properties could affect their marketability and hence their economic viability. In northern-tier states, FAME might have to be stored in heated tanks in winter to keep the fuel fluid. In fact, many of the biodiesel refineries producing FAME from soybean in the United States are idle. In 2011, the production capacity of biodiesel in the United States was about 2 billion gallons per year, but only 1 million gallons were produced (EIA, 2010).

6.1.4 Alternative Pathway #3–Photobioreactors with Direct Synthesis of Ethanol

Growing microalgae in photobioreactors can avoid a number of the sustainability concerns associated with open-pond cultivation but may require substantial energy input for pumping and mixing water and for temperature control. Incidents of contamination by algae and other microorganisms and evaporative loss of water likely would be reduced. Other than using a different cultivation system from the other pathways discussed above, this pathway does not require harvesting, drying, and rupturing the algal cells to extract algal oil because the cyanobacteria secrete alcohol into the medium continuously. The direct synthesis of ethanol reduces downstream processing and could result in substantial energy savings and associated cost savings. In addition, some members of the public might find cultivation of genetically modified algae in enclosed reactors more acceptable than in open ponds.

A key barrier to sustainable development of algal biofuels using such systems is the potentially high capital cost (Tredici, 2007; Davis et al., 2011). Another disadvantage of this

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