Algae, Renewable Diesel and Aviation Fuels

The following is an excerpt from an September 15, 2009 article "Algae Biofuels: From Pond Scum to Jet Fuel"

Powering Trucks and Jets

Oil obtained from microalgae can be used as a straight vegetable oil fuel, but this requires a modified engine. Dr. Eric Jarvis, a scientist at the National Renewable Energy Laboratory (NREL), said that while the home hobbyist might enjoy modifying engines to use algae biofuel, "no one wants to do it at the commercial level."

Biodiesel can be used in existing diesel engines and is produced by straightforward and established transesterification technology. This chemical reaction starts with simple triglyceride lipids, which are fats and oils from plants, waste foods or algae. The triacylglycerols are chemically reacted with alcohol, with the help of enzymatic or chemical catalysts. The resulting biodiesel has the characteristics of petroleum diesel and can be used alone or in a blend.

The big pay-off in algae biofuels will be as drop-in replacements for gasoline or jet fuel. Successful test flights have already been run on mixtures of petroleum and algal-based jet fuels. Chisti says, "generally, only a portion of the crude algal oil is suitable for making biodiesel, but all of it can be used to make gasoline and jet fuel." For this, the fatty acids in the algal oils are refined by hydrogenation and hydrocracking.

NREL’s Jarvis believes the refinery pathway has the most flexibility, in part because the techniques are already established for petroleum. He says that "oil chemists know how to do the cracking and hydrogenation, so they can change the fatty acids into what they need." Also, refining is necessary "to get the energy-dense targets like jet fuels. You can't use ethanol on airplanes." In addition, less refined products have problems with gelling, which Jarvis cautions, "you don't want happening at 30,000 feet."

Even with the proven potential of algal biofuels, cost-effectiveness is an issue. Biofuels currently compete with petrochemical fuels, which have economy of scale. A 2007 analysis of the economics of algal biofuels by Chisti suggested that a five-fold reduction in production costs was needed to compete with plant- or petroleum-based diesel. Now, Chisti says, "issues relating to climate change may leave us with no choice but to replace petroleum fuels with renewable, carbon-neutral algal fuels, despite a somewhat higher cost."