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Cheap Ethanol from Tires and Trash

GM teams with a startup aiming to produce low-cost biofuels.
January 14, 2008

Yesterday at the North American International Auto Show in Detroit, General Motors announced a partnership with Coskata of Warrenville, IL, a new company that claims it can make ethanol from wood chips, grass, and trash–including old tires–for a dollar a gallon. That’s significantly less than it costs to make the biofuel from corn grain, which is the source of almost all the ethanol made in the United States.

Fuel fibers: A bundle of hollow fibers is the heart of a new process for making ethanol from sources other than corn. Organic materials are heated up to form a mixture of hydrogen, carbon dioxide, and carbon monoxide. When the pictured bioreactor is in operation, the gases flow through the center of the fibers and feed bacteria growing on the outside. The bacteria convert the gases into ethanol.

Coskata executives, who until the announcement had kept the company’s existence and technology under wraps, say they have developed a hybrid approach involving both thermochemical and biological processes for making ethanol. Until now, most researchers have focused on developing either thermochemical or biological methods. Coskata says that besides being cheaper than other ethanol production processes under development, its technology uses less energy and water.

GM will give financial, technical, and marketing support to Coskata to help it scale up its process, which so far has been demonstrated only at the lab scale. Coskata is completing a pilot-scale ethanol production facility and will announce locations for a 40,000-gallon-per-year facility and a 100-million-gallon-per-year commercial-scale plant later this year.

Coskata joins a number of other companies looking for ways to make biofuels from alternative sources. A new federal mandate, signed into law late last month, calls for 36 billion gallons of biofuels to be produced by 2022; of that, 21 billion gallons is to come from sources other than corn grain. But so far technology for making ethanol from such feedstocks has not been proved commercially.

The Coskata process begins with gasification, a well-known technology that involves heating up a wide range of organic materials until their components disassociate and form synthesis gas, a mixture of hydrogen, carbon monoxide, and carbon dioxide. Then, instead of using chemical catalysts to convert the syngas into various alcohols as is done in conventional processes (see “Breaking Ground on Cellulosic Ethanol”), Coskata uses new strains of bacteria to convert it into ethanol. Since ethanol is the only product, the technique produces a better overall yield than catalytic processes. Bacteria are also easier to work with than catalysts in some ways. For example, they’re not as particular about the ratio of gases in the syngas. “It is theoretically possible to feed our organism exclusively carbon monoxide and it will make ethanol from that,” says Richard Tobey, vice president of R&D and engineering at Coskata. “You can’t do that with the catalytic approaches.”

The hybrid system makes it practical to use an alternative to the conventional distillation step used in ethanol production; the Coskata version uses only half as much energy. In this alternative process, called vapor permeation, water and ethanol vapor pass through a tubelike membrane. By the end, almost all the water has been removed, leaving behind ethanol that’s 99.7 percent pure. Ordinary fermentation processes produce a broth of water and ethanol full of processed biomass that would clog up such a membrane.

At least one other company has tried a hybrid approach to making ethanol: the biofuels company BRI Energy found similar bacteria that can process syngas. But Andy Aden, a senior researcher investigating cellulosic ethanol at the National Renewable Energy Laboratory in Golden, CO, says one problem with such approaches is that it’s been difficult to make the syngas accessible to the bacteria, since syngas doesn’t dissolve easily in water. Coskata has tackled this problem with a new bioreactor design in which bacteria grow in dense biofilms on the outside of hollow fibers. Syngas is pumped through the inside of these fibers and diffuses through them directly to the biofilm. Aden says the biofilm approach sounds promising, although he cautions that such systems have been difficult to scale up to the commercial scale.

While Coskata says its process can work with a very wide range of feedstocks, in practice it might be best suited for specific materials. “I think that it will work very well for woody materials and maybe almost uniquely well for municipal solid waste and some of these other high-carbon wastes, like tires,” says Bruce Dale, a professor of chemical engineering and materials science at Michigan State University. But he says biological approaches could work better with feedstocks such as switchgrass.

So far the company makes ethanol only a few drips at a time. The economics of the process at the commercial scale will depend on a number of factors, including how much the feedstock costs and whether the system works well in larger bioreactors.

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