Ethanol from Garbage and Old Tires
A versatile new process for making biofuels could slash their cost.
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.
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.
Coskata has also improved the last steps of the process, in which the ethanol is separated from the water. Ordinarily, this is done using distillation, which is expensive and consumes 30 percent as much energy as burning the ethanol will release. Coskata instead uses a modified version of an existing technology called vapor permeation. Vapor permeation uses hydrophilic membranes to draw off the water, leaving pure ethanol behind. It also consumes half as much energy as distillation per gallon of fuel. Vapor permeation is difficult to use with most biological manufacturing processes, Tobey says, because biomass fed to the microörganisms washes out with the water and can clog up the system. But in Coskata’s process, the bacteria feed only on syngas, not on biomass. So no extra filtration is required to make vapor permeation work.
Coskata continues working on its bacteria, trying to increase the amount of ethanol they can produce. The company now uses varieties of Clostridium, a genus that includes a species that make botulism toxin and another that processes manure on farms. Coskata has started building an automated system for screening new strains of Clostridium according to their ability to make ethanol. Along the way, it has had to develop techniques for protecting its bacteria from being exposed to oxygen; the bacteria are anaerobic, and oxygen kills them at about the same concentrations at which carbon monoxide kills humans. The automated system should allow the company to sort through 150,000 new strains a year, up from a few thousand now.
The researchers can go only so far by sorting through random variations, however. Eventually, Tobey hopes to begin manipulating the microbes’ genes directly, activating only those that improve ethanol production. Such engineering is fairly common now, but the Clostridium bacteria that Coskata uses haven’t been studied much. So although Tobey knows what chemical steps the bacteria use to transform syngas into ethanol, he doesn’t yet know the details of how genes regulate this process, and what role these genes play in the general processes that keep the bacteria alive. What’s more, effective ways of manipulating the genes in these particular bacteria haven’t yet been developed.
Even as Coskata continues to improve its microbes, it is planning to move the fuel production process out of the lab and scale it up to the commercial level. With the help of GM and other partners, the company will build a facility that’s able to produce 40,000 gallons of ethanol per year. Coskata representatives say construction will begin within the year. The company’s bioreactors should make it easy to adapt the technology to a larger scale, Tobey says; they can simply be lined up in parallel to achieve the needed output volumes. The next two or three years will reveal whether Coskata’s process can start to replace significant amounts of gasoline with cheap ethanol.
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