Biodiesel-containing mixtures have higher cloud points and pour points (the temperature at which the fuel has gelled so it no longer flows) than pure hydrocarbon diesels. Therefore, biodiesel usually is blended with petroleum-based diesel for final use. Because of its higher oxygen content, FAME has 10 percent lower energy content than hydrocarbon diesel; hence, it has reduced vehicle mileage per gallon in use. FAME biodegrades with long-term storage because of its chemical activity, and exposure to air and water accelerates the degradation (NRC, 2011). However, FAME can be made with relative efficiency at small scales so that algal processing and finished fuel production can occur at the same site. It also has low sulfur content and aromatics, and therefore results in low particulate emissions when the fuel is combusted. Coproduct glycerol also is produced in this pathway, but glycerol has a low market value.
Previously described processes for algal biofuel production have focused on openpond systems for algae cultivation, and most analyses indicate that photobioreactor systems are cost prohibitive for the production of fuels (Williams and Laurens, 2010). At present, photobioreactor systems are used to produce algal biomass for high-value products, such as nutraceuticals and cosmetic ingredients (BioProcess Algae, 2011; Boussiba, 2011; Photon8, 2011; Thomas, 2011). The next pathway described assumes that a marine species of algae or cyanobacteria directly produce a valuable fuel product (Figure 3-12). Direct synthesis of fuel components virtually requires that the algae or cyanobacteria be cultivated in
FIGURE 3-12 The Algenol direct synthesis pathway uses a closed reactor with an organism that directly produces alcohols.