Jet Fuel from Plants

Researchers at a startup in Colorado have turned plant scraps into jet fuel, an important demonstration that high-energy fuels can be made efficiently from renewable and abundant biomass.

The company, Gevo, has engineered a yeast that helps transform the cellulose found in wood chips and plant stalks into butanol, an ingredient of gasoline. The researchers can then modify the butanol into jet fuel.

Butanol has 30 percent more energy than an equal amount of a conventional biofuel such as ethanol. Because of that appeal, such companies as Cobalt Biofuels, Gevo, and DuPont have been developing ways to cheaply and efficiently produce butanol from renewable sources. One method starts with the sugars in the starch of corn and sugarcane. Another way to do it is with the cellulose found in plant stalks and wood chips. It has been easier to design yeasts and bacteria to ferment starch-based sugars into butanol, but the abundance of natural cellulose makes it a better raw material for biofuel production, says Mike Cleary, director of the National Bioenergy Center at the National Renewable Energy Laboratory.

“Cellulose is the biggest source of sugars on the planet,” Cleary says. “The difficulty is, it’s harder to get at that cellulose and get at those sugars than it is to get the sugars from corn kernels.”

To make a biofuel, bacteria or yeast digest plant starch into sugars, which are then fermented. In 2005, when Gevo launched, it announced that it had created a highly efficient method of converting corn-based sugars into butanol by rewiring the enzyme pathways in yeast. Now the researchers have inserted their butanol fermentation pathway into a yeast that has been customized to go to work on a mixed sugar slurry formed from cellulose in plant stalks.

Because it packs more energy than first-generation biofuels like ethanol, butanol burns more efficiently–in other words, it could deliver more miles per gallon. Another advantage: while ethanol can be blended into gasoline in limited amounts, there is no corresponding limit for butanol, as it is already a key part of gasoline. The molecular structure of butanol allows it to be readily converted into chemical products that refineries make from petroleum fuels. And butanol could be easier to use and transport than ethanol–it does not absorb water like ethanol does, making it easier to carry in pipelines.

“We wanted to fundamentally change the way biofuels were made and the sort of biofuels we could make,” says Peter Meinhold, cofounder and director of research at Gevo. “We wanted to develop a drop-in biofuel–something that you could use directly without having to change the gasoline supply and the gasoline infrastructure.”

Gevo has also developed a unique separation technology to increase the efficiency of its production process, which makes a variant of butanol known as isobutanol. It’s a challenge to extract the biofuel from the fermentation slurry of sugars and microbes before it is toxic to the organisms. Gevo’s technology quickly collects the isobutanol as it is produced, enabling it to be made in large quantities.

But Gevo’s technology has yet to be demonstrated at commercial scale. That means it remains to be seen whether Gevo’s product can make it into the market at a cost that is competitive with ethanol and gasoline.

Founded by Caltech professor Frances Arnold, Meinhold, and others, Gevo is supported by such investors as Khosla Ventures. While commercializing its technology, Gevo will have to compete with such companies as LS9, which is developing a way to produce diesel from cellulose in a single-step process. Also, DuPont and BP have partnered to develop their own organisms to produce butanol from various sugar sources.