New technique improves efficiency of biofuel production

"This technique makes the process more efficient and less expensive," says Dr. Ratna Sharma-Shivappa, associate professor of biological and agricultural engineering at NC State and co-author of the research. "The technique could open the door to making lignin-rich plant matter a commercially viable feedstock for biofuels, curtailing biofuel's reliance on staple food crops."

Traditionally, to make ethanol, butanol or other biofuels, producers have used corn, beets or other plant matter that is high in starches or simple sugars. However, since those crops are also significant staple foods, biofuels are competing with people for those crops.

However, other forms of biomass -- such as switchgrass or inedible corn stalks -- can also be used to make biofuels. But these other crops pose their own problem: their energy potential is locked away inside the plant's lignin -- the woody, protective material that provides each plant's structural support. Breaking down that lignin to reach the plant's component carbohydrates is an essential first step toward making biofuels.

At present, researchers exploring how to create biofuels from this so-called "woody" material treat the plant matter with harsh chemicals that break it down into a carbohydrate-rich substance and a liquid waste stream. These carbohydrates are then exposed to enzymes that turn the carbohydrates into sugars that can be fermented to make ethanol or butanol.

This technique often results in a significant portion of the plant's carbohydrates being siphoned off with the liquid waste stream. Researchers must either incorporate additional processes to retrieve those carbohydrates, or lose them altogether.

But now researchers from NC State have developed a new way to free the carbohydrates from the lignin. By exposing the plant matter to gaseous ozone, with very little moisture, they are able to produce a carbohydrate-rich solid with no solid or liquid waste.

"This is more efficient because it degrades the lignin very effectively and there is little or no loss of the plant's carbohydrates," Sharma-Shivappa says. "The solid can then go directly to the enzymes to produce the sugars necessary for biofuel production."

Sharma notes that the process itself is more expensive than using a bath of harsh chemicals to free the carbohydrates, but is ultimately more cost-effective because it makes more efficient use of the plant matter.

The researchers have recently received a grant from the Center for Bioenergy Research and Development to fine-tune the process for use with switchgrass and miscanthus grass. "Our eventual goal is to use this technique for any type of feedstock, to produce any biofuel or biochemical that can use these sugars," Sharma-Shivappa says.

The research, "Effect of ozonolysis on bioconversion of miscanthus to bioethanol," was co-authored by Sharma-Shivappa, NC State Ph.D. student Anushadevi Panneerselvam, Dr. Praveen Kolar, an assistant professor of biological and agricultural engineering at NC State, Dr. Thomas Ranney, a professor of horticultural science at NC State, and Dr. Steve Peretti, an associate professor of chemical and biomolecular engineering at NC State. The research is partially funded by the Biofuels Center of North Carolina and was presented June 23 at the 2010 Annual International Meeting of the American Society for Agricultural and Biological Engineers in Pittsburgh, PA.

NC State's Department of Biological and Agricultural Engineering is a joint department of the university's College of Engineering and College of Agriculture and Life Sciences.