Monday, August 18, 2008

More Energy-Efficient Ethanol

A process used in wastewater treatment may increase efficiency in ethanol plants.

By Jennifer Chu

Credit: NREL

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.