Stalking a Better Fuel

by Randy Weckman

Two hundred years ago Elijah Craig, a minister from Bourbon county, made Kentucky 's art of distilling legend. A team of University of Kentucky scientists may be retooling the art of producing alcohol into a science that will allow us to run our cars and tractors on corn stalks, wood shavings, and cardboard boxes. They are developing a process to convert cellulose into ethyl alcohol efficiently, using a soil bacteria and a still that looks and operates a lot like a pressure cooker.

Bacteria? Bacteria have a bad reputation and only some of it is deserved. True, some types can cause all manner of sickness in animals, including humans. On the other hand, however, most of their activities are beneficial and absolutely stupefying. Some bacteria have the ability to eat cellulose and metabolize it into ethanol (alcohol) and other chemicals which can be used for a variety of industrial purposes, including as a gasoline substitute in fuel.

Sue Nokes, from Biosystems and Agricultural Engineering, along with a team of food microbiologists (Herb Strobel and Karl Dawson) and chemical engineer Barbara Knutson, is researching ways to harness the activities of an anaerobic soil bacterium called Clostridium thermocellum to produce ethanol. (The Clostridia group of bacteria includes members that cause botulism, tetanus, and blackleg, but many other clostridia have potential uses in industrial processes.)

The C. thermocellum bacteria being used by the researchers feast on cellulose and metabolize it into ethanol quite readily, to a point. When the concentration of ethanol accumulates past 5 percent in the growing medium, the bacteria die due to the toxic effects of ethanol, in the same way that yeast die when they are used to ferment wine (usually at about 12 percent alcohol).

But the research team members are developing a process that permits them to remove the ethanol from the fermentation broth, allowing the bacteria to continue metabolizing cellulose into ethanol.

The team uses an apparatus something like a pressure cooker. Cellulose, bacteria, nutrients, and water are placed in the bottom of the pressure cooker. Then, instead of air above the brew, the team introduces a carrier gas (either carbon dioxide, nitrogen, or ethane) to help “absorb” the ethanol produced by the bacteria. These gases have special properties at high pressure. They become supercritical, which helps in the removal of ethanol.

The vessel is then pressurized and heated to 60 degrees Celsius. In the heated broth, the bacteria thrive (remember they are C. thermocellum ) and produce ethanol. Every so often, a mixture of ethanol and the carrier gas is permitted to escape to another container. At room temperature and pressure, the ethanol separates from the carrier and becomes a liquid at the bottom of the container. The carrier gas is then returned to the pressurized fermentation vessel to absorb more ethanol.

While the possibility of using bacteria to manufacture ethanol is well known, and supercritical carbon dioxide has been used in the fermentation process with some small successes since the mid 1990s, it is this research team's efforts that may allow much more efficient fermentation of cellulose for ethanol.

“To our knowledge, we're the first group of researchers to investigate supercritical fluids other than carbon dioxide to extract ethanol,” Nokes said.

Picture - Sue Nokes and Barbara Knutson

In their experiments, the team compared the amount of ethanol generated from the fermentation vessel in the presence of three supercritical fluids (carbon dioxide, nitrogen, and ethane). In the batch using carbon dioxide, the rate of ethanol production was one-seventh that of the rate in the batch which used nitrogen (the control gas). However, fermentation in the presence of supercritical ethane resulted in ethanol values which were only one-fifth those of the control values. Clearly, the team's work was successful.

How did such success happen?

According to Nokes it took the combined talent of the team.

“Individually, none of us on the team had the knowledge to put all of this together. I was interested in ethanol production; the microbiolgists knew about how the bacteria metabolize the cellulose; and the chemical engineer knew about the behavior of the supercritical fluids. As a team, we were able to put the pieces together,” Nokes said.

Currently, ethanol is widely used as a fuel additive. Some 1.5 billion gallons of ethanol are added to gasoline in the U.S. each year to improve vehicle performance and to reduce air pollution. In some areas of the country, ethanol is added at a rate of 10 percent to gasoline. It also can be used at an 85 percent blend or even a 100 percent blend, Nokes said.

If the team's work can be commercially applied, it would help reduce the cost of ethanol production, which right now is subsidized by the federal government.

Currently, the production of ethanol for addition to gasoline is by a complex process that uses corn grain, which is treated with enzymes to release the sugars. The sugars are then fermented using yeast; finally, the yield is distilled in the same manner as if you were distilling whiskey.

“Our research shows that it may be possible to produce ethanol in a one-step process. In addition, because this process can use cellulose and a very specific bacteria to metabolize it into ethanol, our process may allow the use of a variety of cellulose sources, which traditionally have been wasted in agricultural production,” Nokes said.

Think about it. Instead of using corn grain to make ethanol fuel, you could use the corn stalks, which have little value currently. Conceivably, Nokes said, recycled newspaper and cardboard boxes, or sawdust for that matter, could be a source of cellulose for the reactor.

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