Adding a pretreatment step would allow cellulosic ethanol producers to get more of it from switchgrass harvested in the fall, according to a Purdue University study.
Michael Ladisch, an agricultural and biological engineer, and research scientist Youngmi Kim compared switchgrass based on growing location, harvest time and whether it was given a pretreatment step. They found that location wasn't important, but the other two factors could significantly increase the amount of ethanol obtained from the feedstock.
"Switchgrass harvested in the spring had more cellulose, but also more lignin," says Kim. "You do not get the advantage of the increased cellulose content because it's more difficult to extract those sugars because of the lignin."
Lignin is one of the most significant problems with cellulosic ethanol production. Besides harvest timing, a pretreatment step – cooking switchgrass in hot water under pressure for about 10 minutes – would also help work around lignin.
Before pretreatment, Kim says about 10% of cellulose is converted to glucose, the yeast-fermentable sugar that produces ethanol. After pretreatment, that number jumps to as much as 90% percent. The pretreatment dissolves hemicellulose, which bonds cellulose and lignin in the plant. Once it is gone, there is more access to the sugars contained in the cellulose.
"There is more surface area for the enzymes to digest cellulose," says Kim.
Ladisch says advancements in techniques to work around lignin could make spring switchgrass more attractive. But fall switchgrass given a pretreatment and fermentation with special yeast shows potential to give as much as 800-1,000 gallons of ethanol per acre per year compared with 150-250 gallons per year without pretreatment. Corn ethanol from grain produces about 500-600 gallons per acre per year.
"This shows that we can improve the processes and increase the amount of ethanol we get from switchgrass," says Ladisch.
He’s chief technology officer at Mascoma, a renewable fuels company based in New Hampshire. This research is part of a concerted research effort on pretreatments by a consortium made up of the University of California Riverside, Auburn University, Texas A&M University, Michigan State University, Genencor and the National Renewable Energy Laboratory together with Purdue University.