Dependence on fossil fuels? NC State
scientists don't cotton to that

Editor’s note: The following feature is the first in an occasional series examining NC State’s work on alternative fuel research.

How would you like to drive a car fueled by cotton stalks?

Although most people think of corn when they think about ethanol – and rightly so, since corn is the most common crop used in ethanol production – there are other ways to produce the alternative fuel.

Photo by Roger Winstead

Converting cotton stalks to ethanol, says Dr. Ratna Sharma (above), will benefit North Carolina’s environment and its farmers by adding value to the otherwise troublesome agricultural waste.

North Carolina devotes 1 million acres of farmland to cotton production, making us the sixth largest producer nationally. That’s a lot of cotton, and after harvest time, that’s a lot of leftover “junk” – plant stalks and leaves that remain in the fields after the fluffy cotton bolls are taken – for farmers to deal with.

Part of the challenge of growing cotton is dealing effectively with its waste, because the leftover stalks don’t break down quickly enough to use as mulch, and they have the additional drawback of being very attractive to boll weevils and other pests who could damage future harvests.

Fortunately, NC State scientists are working on a way to utilize those cotton stalks that will be beneficial to both cotton farmers and the environment: by turning that agricultural residue into ethanol.

Dr. Ratna Sharma is an assistant professor of bioprocessing in the biological and agricultural engineering department, and one of many NC State scientists working on converting different types of biomass – such as cotton stalks – into fuel.

“Many biological resources can be converted into ethanol,” Sharma says. “The resources can come from agriculture, forestry or food industry residue – it doesn’t have to be corn, though corn is one of the easier resources to convert into ethanol.

“Getting ethanol from biomass such as cotton stalks isn’t terribly difficult, but if the idea is not just to provide an alternative to fossil fuels but to add value to farmers’ crops, it’s important that we keep the process as streamlined as possible,” Sharma says. “And our goals become ‘How do we do this in a cost-effective way without harming the environment?’”

Traditionally, corn has been used in ethanol production because it contains easily accessible simple sugars, which are important to making ethanol. With cotton stalks, the process of getting to the sugars is more complicated than with corn. Cotton stalks contain lignin, a fibrous material that gives the stalks their structure. When the lignin breaks down, cellulose is released. Enzymes can then be used to convert the cellulose to sugars, which are fermented and distilled just as corn-based ethanol is.

What this means is that making ethanol from cotton stalks requires some additional work, which translates into additional production costs. The lignin has to be pre-treated with acids to break it down, which has environmental implications – how do you safely dispose of the leftover acid? Additionally, the cotton stalks release a variety of sugars, not just one, and these sugars require specially designed varieties of yeast for fermentation.

Sharma believes that it’s possible to address both the added cost and environmental impact of producing ethanol from cotton stalks – and she’s looking at some innovative ways to do it. White rot fungus, a naturally occurring fungus that feeds on wood, could be used to break down the lignin in the stalks instead of acid, saving both the cost of the acid and the cost to the environment.

As for the distillation process, which can be energy-intensive, Sharma is collaborating with colleagues in the College of Textiles to engineer a selective nano-membrane that would act as a filter, only allowing particular molecules – like ethanol – to pass through it.

Is it worth the effort? Sharma thinks so, especially if her work can help alleviate U.S. dependence on oil.

“I remember looking at this cartoon graph, with a peak representing where our civilization was right now, and a little guy with a bow and arrow standing on either side, illustrating how our civilization would fail if we depended solely on fossil fuels,” Sharma says. “It was a pretty scary realization.”

Posted July 21, 2006

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