Wednesday, September 1, 2010

Cellulosic Ethanol—Biofuel Beyond Corn. TheBioenergySite.com

Fuel ethanol production in the US is expected to exceed 7.5 billion gallons before 2012, writes Nathan S. Mosier from the Department of Agricultural and Biological Engineering at Purdue University.
This represents a doubling of ethanol production from 2004, which consumed approximately 10% of the corn produced in the U.S. in that year. Increased demands for domestically produced liquid fuel is increasing competition between animal feed and fuel production uses of corn.



Cellulosic feedstocks (wheat straw, corn stover, switch grass, etc.) can also be converted to ethanol. Overcoming the technological and economic hurdles for using cellulose to produce liquid fuel will allow the U.S. to meet both food and fuel needs.

Cellulose as Ethanol Feedstock

Cellulose is a polymer of sugar. Polymers are large molecules made up of simpler molecules bound together much like links in a chain. Common, everyday biological polymers include cellulose (in paper, cotton, and wood) and starch (in food). Cellulose is a polymer of glucose, a simple sugar that is easily consumed by yeast to produce ethanol (Mosier and Illeleji, 2006).

Cellulose is produced by every living plant on the earth, from single-celled algae in the oceans to giant redwood trees. This means that cellulose is the most abundant biological molecule in the world.

A study completed by the USDA and the U.S. Department of Energy concluded that at least 1 billion tons of cellulose in the form of straw, corn stover, other forages and residues, and wood wastes could be sustainably collected and processed in the U.S. each year. This resource represents an equivalent of 67 billion gallons of ethanol, replacing 30% of gasoline consumption in the U.S (U.S. Department of Energy Biofuels: 30% by 2030 Website).

Plants use cellulose as a strengthening material, much like a skeleton that allows plants to stand upright and grow toward the sun, withstand environmental stresses, and block pests. People have used cellulose for centuries in paper, wood, and textiles (cotton and linen).

Figure 1. Major Challenges in Producing Cellulosic Ethanol
If cellulose chains are broken down into the individual “links,” the released sugar can be used to make ethanol. This ethanol can then be purified using the same technology as corn-based ethanol production (Mosier and Illeleji, 2006). A number of technological advances are current under development to make this approach to biofuels economical.

Challenges in Cellulosic Ethanol

Using technology available today, cellulose can be converted into ethanol. The major difference between cellulosic ethanol and grain ethanol is the technology at the front end of the process. The technology for fermentation, distillation, and recovery of the ethanol are the same (Mosier and Illeleji, 2006).

The major challenges (Figure 1, page 2) are linked to reducing the costs associated with production, harvest, transportation, and up-front processing in order to make cellulosic ethanol competitive with grain-based fuel ethanol and gasoline (Eggeman and Elander, 2005). The major processing challenges are linked to the biology and chemistry of the processing steps. Advances in biotechnology and engineering will likely make significant impacts toward achieving the goals of improving efficiency and yields in processing plant material to ethanol.

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