Cellulosic ethanol from plants is becoming a high priority

Second generation biofuels, including cellulosic ethanol, are making progress.

Cellulosic ethanol is a common term to some, but to others it sounds like a complex product. To make it clearer, let’s define this term. Cellulose is a polymer of a simple plant sugar that is produced in every living plant, making it “the most abundant biological molecule in the world,” according to Purdue Extension Bulletin ID335, “Cellulosic Ethanol – Biofuel Beyond Corn.” Ethanol is an alcohol that is being used as a renewable replacement for gasoline. One nontechnical definition is “Cellulosic ethanol is a biofuel produced from wood, grasses, or the non-edible parts of plants.”

Traditionally, cellulose has been used to make paper, wood products and fabrics. These uses benefit from the structural strength of cellulose. However, ethanol production does not use the structural strength rather it uses the fact that it is a polymer of sugar in conversion to an alcohol. According to “Cellulosic Ethanol – Biofuel Beyond Corn,” if cellulose chains are broken down into the individual “links,” the released sugar can be used to make ethanol. The production of ethanol from grain and cellulose is similar except for the front-end processing.

Cellulose can be resistant to degradation. This can be seen in nature by finding intact corn stalks for more than one year after they have matured and died. The same is true for the slow decay of tree parts like leaves, twigs and stumps. Plant cell walls contain cellulose, hemicellulose and lignin. Cellulose and hemicellulose are made up of polymers of five and six carbon sugars. Lignin is the protector of cellulose and hemicellulose. It acts like a bubble wrap, protecting them from degradation. The biggest challenge – and expense – with making cellulosic ethanol is unwrapping this protective structure (lignin) so that enzymes can access the long chains (polymers) of sugar in cellulose and hemicellulose.

Pretreatment of the cellulose is necessary to help break up plant parts and make them accessible to microbes that will break them down. Michigan State University Chemical Engineering and Materials Science expert Dr. Bruce Dale has developed a pretreatment process called AFEXTM. The pretreatment aids in the next step: hydrolysis. Hydrolysis utilizes enzymes to break down the cellulose into simple sugars (glucose and xylose) that can then be fermented into ethanol. Fermentation is the process of yeast and bacteria consuming sugars and producing ethanol.

Recent progress in biotechnology has increased the yield of ethanol per ton of cellulose feedstock from improvements made to enzymes, yeasts and bacteria. This progress along with advances in pretreatment technology has driven down the final cost of ethanol production per gallon to near competitive levels with grain ethanol and gasoline. According to Michigan State University Extension, when the cost becomes competitive with petroleum, the cellulosic ethanol industry will see great growth and utilize an abundant feedstock throughout much of the country.

Many experts agree that the U.S. ethanol industry has nearly maximized the production of ethanol from corn grain. This makes the advancement of cellulosic ethanol a high priority and its continued development is supported by the U.S. Renewable Fuel Standard. Ethanol from sustainable and abundant plant material is encouraging to many American policy makers and citizens as they look for sustainable replacements for petroleum-based liquid fuels.

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