March/April 2008

Ethanol from Garbage and Old Tires

A versatile new process for making biofuels could slash their cost.

By Kevin Bullis

Ethanol Factory: Coskata vice president Richard Tobey (above) stands before bales of hay, a feedstock that his company’s new technology can efficiently convert into ethanol. He’s holding the centerpiece of that technology, a bioreactor. Credit: Thomas Chadwick

Multimedia •  William Roe, Coskata's president and CEO, and Vinod Khosla, one of the company's main investors, describe the benefits of its technology. •  View the process for making biofuels.

As he leads a tour of the labs at Coskata, a startup based in Warrenville, IL, Richard Tobey, the company's vice president of research and development, pauses in front of a pair of clear plastic tubes packed with bundles of white fibers. The tubes are the core of a bioreactor, which is itself the heart of a new tech­nology that Coskata claims can make etha­nol out of wood chips, household garbage, grass, and old tires--indeed, just about any organic material. The bioreactor, Tobey explains, allows the company to combine thermochemical and biological approaches to synthesizing ethanol. Taking advantage of both, he says, makes Coskata's process cheaper and more versatile than either the technologies widely used today to make ethanol from corn or the experimental processes designed to work with sources other than corn.

Tobey's tour begins at the far end of the laboratory in two small rooms full of pipes, throbbing pumps, and pressurized tanks--all used to process synthesis gas (also known as syngas), a mixture of carbon dioxide, carbon monoxide, and hydrogen. This is the thermo­chemical part of Coskata's process: in a well-known technique called gasi­­fication, a series of chemical reactions carried out at high temperatures can produce syngas from almost any organic material. Ordi­narily, chemical catalysts are then used to convert the syngas into a mixture of alcohols that includes ethanol. But making such a mixture is intrinsically inefficient: the carbon, hydrogen, and oxygen that go into the other alcohols could, in principle, have gone into ethanol instead. So this is where Coskata turns from chemistry to biology, using microbes to convert the syngas to ethanol more efficiently.

Down the hall from the syngas-­processing equipment, Tobey shows off the petri dishes, flasks, and sealed hoods used to develop species of bacteria that eat syngas. The bioreactors sit at the far end of the room. Inside the bioreactors' tubes, syngas is fed directly to the bacteria, which produce a steady stream of ethanol.

Coskata's technology could be a big deal. Today, almost all ethanol made in the United States comes from corn grain; because cultivating corn requires a lot of land, water, and energy, corn-derived ethanol does little to reduce greenhouse-gas emissions and can actually cause other environmental damage, such as water pollution. Alternative etha­nol sources, such as switchgrass, wood chips, and municipal waste, would require far fewer resources. But so far, technology for processing such materials has proved very expensive. That's why Coskata's low-cost technique has caught the attention of major investors, including General Motors, which earlier this year announced a partnership with the startup to help deploy its technology on the commercial scale worldwide.

Sipping Ethanol
Combining thermochemical and biological approaches in a hybrid system can make ethanol processing cheaper by increasing yields and allowing the use of inexpensive feedstocks. But Coskata's process has another advantage, too: it's fast. Though others have also developed syngas-fed bioreactors, Tobey says, they have been too slow. That's because the bacteria are suspended in an aqueous culture, and syngas doesn't dissolve easily in water. Coskata's new bioreactor, however, delivers the syngas to the bacteria directly.

The thin fibers packed into the bioreactor serve two functions. First, they act as scaffolding: the bacteria grow in biofilms on the outside of the fibers. Second, they serve as a delivery mechanism for the syngas. Even though each fiber is not much bigger than a human hair, Tobey says, it acts like a tiny plastic straw. The researchers pump syngas down the bores of the hollow fibers, and it diffuses through the fiber walls to reach the bacteria. Water flows around the outside of the fibers, delivering vitamins and amino acids to the bacteria and carrying away the ethanol the bacteria produce. But the water and the syngas, Tobey says, never meet.