Exxon Mobil and genome expert Craig Venter hope to strike it green with oilgae, but a few obstacles remain on the path to commercialization of biofuel from algae.

Once the microalgae are cultivated, biofuel manufacturers are faced with two major technical hurdles: harvesting and dewatering. Microalgae cultures can be 80%-90% water, so cells must be collected by settling, which is time-consuming, although this can be hastened with flocculating agents that cause cells to clump and precipitate. More high-tech methods like centrifugation and filtering are faster, but are more costly in both dollars and energy.

Once harvested, cells may be air- or sun-dried, requiring a large surface area and significant time, or they can be dried using heat or a vacuum, again increasing the cost and reducing energy efficiency.

Finally, extracting the oils is another challenge. Options include extraction with solvents like hexane, enzymatic digestion of cell walls, or physical disruption with ultrasonic sound waves or microwaves.

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.”