Images of algal cultures grown in the lab for use at the Illini Algae Project (TOP). Flue gas from energy production is a primary contributor to glogal warming (MIDDLE) which has led to an increase in average global temperatures (BOTTOM).
The use of algae as an input sourse for the production of biodiesel has received renewed interest recently due to dwindling fossil fuel reserves, concerns of national energy security, and the realization of the the human contribution to global warming. Algae offer a carbon neutral source of energy since algae consume two pounds of carbon dioxide for every one pound of biomass produced. This stored carbon is re-released during combustion, but is directly offset by the carbon required to grow the algae. Additionally, algae provide a source of alternative renewable fuel that is compatible with current diesel engines and our fuel distribution infrastructure.
Algae's Potential for Biofuel
Algae have several key advantages over terrestrial crops for biodiesel production including: higher growth rates than other plant species, the ability to grow on marginal lands, the ability to consume excess nutrients in eutrophic waters, and high oil content in certain species. Some algae species have been shown to be sufficiently high in oil content that less than 20 million hectares (10% of arable US land) could potentially produce the entire oil consumption of the US transportation sector (approximately 35 quads per year) . Thus, algae based biofuels could effectively replace liquid petroleum fuels without significantly compromising the availability of land for food production - a critical limitation of current bioenergy scenarios. Furthermore, because algae can be grown on degraded lands and water bodies that are not suitable for agriculture, the competition between food and fuel can be fully mitigated.
Lipid Extraction and Purifcation
In order to realize the potential of algae as a biodiesel feedstock the intracellular lipids must be extracted and refined for conversion into biodiesel (primarly of fatty acide method esters or FAMEs). Algae store their lipids in various ways depending on the algal species and environmental growth conditions. Lipids can be predominantly bound in cell vacuoles as triacylglycerides (TAGs) or within the cell membranes as glyco or phospho lipids (lipid structure with a sugar or phosphorus functional group).
Neurtal Storage Lipids
TAGs are highly desirable from the algae extract since they contain fatty acids that are ester linked to a glycerol molecule. The fatty are small carbon chain molecules that can be removed and converted through transesterfication into a suitable fuel that burns in current diesel engines. Small carbon chain molecules are preferred over long chain carbon molecules since they give the fuel the necessary viscosity to smoothly flow across engine components, especially at colder temperatures.
Various methods have been proposed to extract lipids with organic solvent-based applications receiving a large amount of attention due to extensive use in vegetable oil extractions. Algals extraction sovlent work in a two-part fashion to first rupture the cell walls and break apart chemical bonds to free the lipds, and second to solubilize the lipids so that they can be removed from the remaining cellular material. Storage lipids can be solubilized into nonpolar solvents since they are neutral molecules and "like dissolves like." Once the lipids are dissolved into the solvent, the solvent can then be distilled to leave the lipids behind and recover and reuse the solvent again.
Biodiesel Conversion and ASTM Testing
After the lipids have been extracted from the algae they can then be converted into fatty acid methyl esters (FAMEs). Lipid extracts must be converted into FAMEs so that the fuel will perform consistently in engines that burn traditional diesel fuel. Several conversion methods exist depending on the quality of the crude extract. Typically, oils that have a low fatty acid content can be converted in a one-step transesterfication process using a base catalyst and methanol reagent.
During the transesterfication process, fatty acids bound to glycerol molecules are ripped off and readjoined to the methanol reagent. This process consumes the methanol reagent and leaves behind free free glycerol molecules which can be converted into soap or hydraulic fluids. The converted fuel must then be washed to remove any remaining purities.
Before the converted biodiesel can be deemed suitable for use, the fuel must undergo extensive testing put forth by the American Society for Testing and Materials (ASTM). Their testing requirements include the specifc gravity, viscosity, heating value, cloud point, flash point, sulfur content, acid number, phosphate, and several others. This seal of approval provides added assurance to customers, as well as engine manufacturers, that the biodiesel meets the ASTM standards and that fuel suppliers will stand behind its products. Most major engine companies have stated formally that diesel fuels blended with up to 20% certified biodiesel will not void their parts and workmanship warranties.