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More Energy-Efficient Ethanol

A process used in wastewater treatment may increase efficiency in ethanol plants.
August 18, 2008

Making corn ethanol is an energy-intensive process, requiring fossil fuels to grow and harvest corn and to power the production plant. To make the process more energy efficient, researchers at Washington University are proposing to borrow a process used in breweries and wastewater treatment facilities: oxygen-less vats of bacteria that naturally feed on organic waste produced from the fermentation process.

As bacteria break down waste, it releases methane, which can be funneled back through the system to help power a plant. The process requires little additional energy to run, and can further cut down on energy costs by producing its own power. Largus Angenent, a professor of chemical engineering, and his team at Washington University have tested anaerobic digestion on waste from ethanol plants and found that the process could cut down an ethanol facility’s use of natural gas by 50 percent. The team has published the results in the recent issue of the journal Environmental Science and Technology.

Angenent says that the process would serve as a short-term solution until more-efficient biofuel, such as cellulosic ethanol, is commercially viable. “Rather than have hope for new technology that comes to fruition in 10 or 20 years, we need technology we can implement now,” says Angenent, who recently became an assistant professor of biological and environmental engineering at Cornell University. “This is an interim process, and it’s off the shelf.”

Nearly all ethanol biofuel in the United States is made from corn. Typically, the ethanol production yields organic waste that is then consolidated into two parts: a dry, cake-like substance and a syrupy solution, called thin stillage, that’s layered on top. The concoction is used as animal feed. Angenent says that a large portion of this feed, particularly thin stillage, which is laden with salts, provides low nutritional value but may have high energy potential for powering a plant when broken down via anaerobic digestion.

To test this theory, the researchers cultivated thermophilic bacteria from a wastewater treatment plant in two small, five-liter anaerobic digesters. Angenent and his colleagues then slowly began feeding waste samples into the digesters, which were kept at 55 °C to maximize the bacteria’s activity. As the digesters ran, the team measured the amount of methane released.

However, initial tests found that the process produced very little methane. Angenent guessed that the system might be missing an essential ingredient but was unsure as to what that might be. So the team dug into the scientific literature and found that methane-producing bacteria require certain trace elements to jump-start the process–particularly cobalt.

When Angenent added cobalt to the mix, he recalls, “it was unbelievable. Overnight, the process was recovered.” In lab tests, the average output yielded a quarter of a liter of methane per gram of waste fed into the digester. Angenent calculates that this number, scaled up to industrial production rates, would decrease the amount of natural gas needed to power an ethanol plant by 50 percent.

In a 2006 study, researchers at the University of Minnesota calculated the total amount of energy used in the production of ethanol, from how much it costs to build and run tractors to how much it costs to power a biofuel plant. They found that ethanol provides a scant 26 percent more energy than is used to produce it.

When Angenent plugged results of his process into the Minnesota model, he found that energy output was bumped up to 70 percent, meaning that anaerobic digestion significantly boosts the energy value of ethanol biofuel. Angenent says that percentage may change slightly in a real-life scenario if ethanol plants choose to install anaerobic digesters.

“If you put in a digester, you have a lot of liquid that needs to be recycled back into the system, and that would create changes throughout a plant,” says Angenent. “So someone will have to do a study to find out what that net energy balance really is.”

Douglas Tiffany, a research fellow at the University of Minnesota and a coauthor on the 2006 study, says that operating anaerobic digesters in ethanol plants may be a challenge, since it requires expertise to maintain a stable bacterial community at high temperatures and avoid system crashes. However, if these problems are sorted out, Tiffany says, the process may improve ethanol’s energy and environmental potential.

“We can improve these existing corn-ethanol plants dramatically and reduce greenhouse gases far more than they do today,” says Tiffany. “This process is attractive because it’s a low-energy, low-capital approach. It will take some [ethanol producers] to stick their necks out to try it, but once it’s happening in a number of plants, it should work out pretty well.”

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