"You aren't going to start raising animals for energy," Holmgren says.
Better options are non-food crops, such as jatropha, camelina and algae, she says. Fuel from jatropha and camelina will be on the market within three to five years, she says. Fuel from algae is eight to 10 years away because research on it began later.
There's not likely to be a single winner among the various alternative fuels that will be adopted for use by all the world's airlines, says Sebastien Remy, head of alternative fuel research for European airplane maker Airbus. "But there will be one specification for the quality of the fuels to come from these various alternative sources," he says. "The engines should not be able to tell the difference between conventional jet fuel and whatever alternative fuels airlines choose to use in the future."
Which fuel is best for an airline may depend on location, he says. In Asia, camelina is abundant. In Australia, Mexico and parts of South America, where conditions for growing jatropha are ideal, it likely will be the primary source of alternative fuels, Remy says. In North America, synthetic fuel may make more sense because coal and natural gas are abundant.
Longer term, algae may be what fuels the engines in U.S. airline jets. It can be produced in high volume, and the USA has plenty of space to grow it.
"Jatropha and other grains will be on the market sooner, but only in the tens or hundreds of millions of gallons," says UOP's Holmgren. "Algae will be produced in the billions of gallons a little bit further down the road."
Algae is among the fastest-growing organisms on Earth. It takes up little space relative to its production capacity. Some strains can go from incubator to harvest in 14 days. And it grows best in brackish water, either in ponds, in a high-tech greenhouse environment known as a bioreactor, or on "algae farms," where nutrient-filled water flows through miles of tubes winding around a few acres of land.
"You get an awful lot of productivity out of 1 acre of algae when compared with 1 acre of just about any other crop," says William Thurmond, a bio-energy researcher affiliated with the National Algae Association.
That kind of large-scale production is necessary to get the cost of algae fuel down enough to compete with conventional fuels. But it's doable, says Cohen of the National Algae Association.
Three years ago, a gallon of algae fuel cost about $300. Today, it's $10 to $20 a gallon. Cohen says the cost of producing unrefined algae crude could be less than $1 a gallon in a few years as technical problems are solved and as production is ramped up to industrial levels. But that will require significant investment.
Cohen, citing government estimates, says about $66 billion will be needed to meet demand by 2025. However, he says, that's far less than the $1 trillion invested in U.S. oil production, refining and distribution infrastructure.
Tim Zenk, vice president at Sapphire Energy, the San Diego start-up that produced the algae fuel used in the Continental flight, says his company's investors are motivated in part by environmental concerns.
But that's not the only reason. Investors led by Microsoft co-founder Bill Gates' Cascade Investments and by the Rockefeller Foundation's venture-capital arm funded Sapphire to the tune of $100 million because they also think algae production will be a great investment.
"We think we'll get 3,000 gallons (of biocrude) a year per acre," Zenk says. "You're going to see very large scales of production."