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Added step helps boost ethanol from switchgrass

Thursday, September 1, 2011

WEST LAFAYETTE -- Adding a pretreatment step would allow producers to get more ethanol from switchgrass harvested in the fall, according to a Purdue University study.

Michael Ladisch, a professor of agricultural and biological engineering, and Youngmi Kim, a research scientist, 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," Kim said, whose findings were published in an early online version of the journal Bioresource Technology. "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, a rigid substance found in plant cell walls, is one of the most significant problems with cellulosic ethanol production. Besides the harvest time, a pretreatment step -- cooking switchgrass in hot water under pressure for about 10 minutes -- would also help work around lignin.

Before pretreatment, Kim said about 10 percent of cellulose was converted to glucose, the yeast-fermentable sugar that produces ethanol.

After pretreatment, that number jumped 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," Kim said.

Ladisch said advancements in techniques to work around lignin could make spring switchgrass more attractive. But he said that 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 to 150-250 gallons per year without pretreatment.

Ladisch said 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," Ladisch said.



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