Cellulose is the fiber contained in leaves, stems, and stalks of plants and trees – the most abundant organic compound on earth. Unlike corn and sugar – the plants now used to make most ethanol – cellulose is not used for food, and it can be grown in all parts of the world. Cellulosic ethanol is expected to be less expensive and more energy-efficient than today’s ethanol because it can be made from low-cost feedstocks, including sawdust, forest thinnings, waste paper, grasses, and farm residues (e.g., corn stalks, wheat straw, and rice straw). Switchgrass and other perennial grasses, in particular, are considered to be promising sources of cellulosic ethanol. Perennial grasses are less expensive to produce because they don’t have to be replanted each year. Fast-growing woody crops, such as poplar and willow, are also attractive options because of harvesting and storage advantages.

Ethanol can be made from cellulose much as it is today from corn – once the very tightly bound sugars in the plant fiber are broken down by enzymes. Accomplishing this task at low cost has been the principal obstacle to commercial development.

The enzymes needed to break cellulose down into fermentable sugars are genetically improved natural organisms. One such fungus, Trichoderma reesei, plagued U.S. troops with jungle rot that “ate” their clothing in the South Pacific during World War II. Another promising source of enzymes is termite guts. Termites, after all, sustain themselves by converting woody biomass to sugars. Thanks to biotechnology, the cost of such enzymes is dropping rapidly, down 30-fold in the last five years – to 10-18 cents per gallon of ethanol produced.

• Termites are one of the planet’s most abundant creatures. There are more than 2,500 species. The termites’ digestion process is fast and efficient – typically achieving 95% conversion in 24 hours or less.

• U.S. Patent No. 5,000,000 was awarded to University of Florida Professor Lonnie Ingram in 1991 for using an E. coli bacterium to metabolize multiple sugars and make ethanol.

Acid can also be used to break down cellulose, or, alternatively, cellulose can be heated and turned into a gas that can be converted into biofuels – even bio-gasoline.

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The success of cellulosic ethanol will depend on how fast the technology is commercialized and how much it costs compared to the alternatives. These are some of the promising recent developments:

• U.S. Secretary of Energy Samuel Bodman has set a goal of making ethanol a practical and cost-competitive alternative by 2012 (at $1.07 a gallon) and displacing 30% of gasoline (60 billion gallons) by 2030.

• A Canadian company, Iogen, which specializes in industrial enzyme production, is operating a small pilot plant in Ottawa and is planning to build a commercial plant in Idaho, both operating on wheat straw.

• U.S. ethanol producer Broin intends to convert one of the six corn-to-ethanol plants that it currently operates in Iowa into a biorefinery that will use both corn grain and stover – the stalk, leaves, and cobs that come with the grain. DuPont and Novozymes are partners in the project.

• A Spanish company, Abengoa, is building a cellulosic ethanol plant in Spain, using wheat straw, and is planning another unit for York, Neb., using corn stover and distillers grains.

• An independent analysis prepared for the National Commission on Energy Policy predicts, “Advanced biofuels production facilities could produce gasoline alternatives at costs equal to between $0.59 and $0.91 per gallon of gasoline by around 2015.”

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The chief advantage of cellulose is its abundance. Indeed, cellulose is estimated to make up half of all the organic carbon on the planet. In the U.S., ethanol production from corn is expected to hit a limit of 15 to 20 billion gallons per year. Additional feedstocks will be needed to replace a larger share of gasoline demand, now running at 140 billion gallons per year.

A second advantage to processing cellulosic biomass is that it also contains lignin, a natural fiber. Lignin can not be converted to ethanol but can serve as an energy-rich boiler fuel. There is enough lignin in plants to provide all the energy needs of an ethanol production facility, with electricity left over for sale to the power grid.

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Switchgrass (Panicum virgatum), shown at right, is a prairie grass native to the United States known for its hardiness and rapid growth. It was once part of the tall-grass prairie that covered most of the Great Plains and also grew in Alabama and Mississippi. Because it is native, switchgrass is resistant to many pests and plant diseases and i
s capable of producing high yields with relatively low applications of fertilizer and other agricultural chemicals. It is also tolerant of poor soils, flooding, and drought. Half the plant’s carbon is stored in its root system, improving soil quality and preventing erosion. It is an approved cover crop for land protected under the federal Conservation Reserve Program.

President Bush praised switchgrass in his 2006 State of the Union address for its potential as a biofuel source. It can yield 6 to 8 tons per acre, compared to 4 tons per acre for corn, and progressive breeding could double that yield over time.

The predicted distribution of switchgrass cultivation at a market price of $40 per ton.

Other perennial grasses may be even more productive than switchgrass, depending on the climate. Miscanthus, shown at right, is one such example. Tests in Illinois suggest that it could yield 11 to 17 tons per acre. Recent research also suggests that mixed prairie grasses may be more productive than monocultures.

In addition to grasses, some fast-growing trees are prodigious producers of biomass, including poplar, willow, sweetgum, and cottonwood.

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