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Brewing a Better Ethanol
Researchers at the National Renewable Energy Laboratory seek ways to make cellulosic ethanol commercially viable.

Research technician Monica Silva in a switchgrass test plot in Prosser, Washington.
Dr. Hall Collins/USDA-ARS

Curious to see how cellulosic ethanol is produced? Click here for a VIDEO tour of the National Renewable Energy Laboratory.


Cellulosic ethanol may be the great green hope of the coming era of renewable fuel, but it is not yet commercially viable. That could change in the next five or so years, as the federal government pumps hundreds of millions of dollars into research and development intended to make this biofuel competitive in the marketplace. 

“People talk about it requiring the equivalent of a moon shot to make a big dent in our oil consumption with biofuels. It won’t,” says Thomas Foust, technology manager for biofuels research at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, near Denver. Even so, he acknowledges that it will take hard work and a sustained effort. Data from an NREL biofuels pilot plant suggest that if anyone were actually producing cellulosic ethanol for sale today, a gallon would cost about $2.60.

That's considerably more than the $1.50 a gallon it costs to make corn ethanol today.

That’s because corn kernels give up their sugar for fermentation easily, whereas the fibrous sources such as switchgrass used to make cellulosic ethanol lock up their sugars in long chains of cellulose and hemicellulose molecules. These chains form a tough matrix of fibers that allow plants to stand up straight. Breaking those fibers to release the sugars for fermentation into ethanol is difficult.

But not impossible. Help came more than 60 years ago from, of all things, jungle rot in the Pacific during World War II. A voracious fungus was devouring soldiers’ tents and clothes, aided by a cellulose splitting enzyme, or cellulase. Those enzymes were later put to work making cellulosic ethanol during the energy crises of the 1970s. But with the dawn of the era of cheap oil they eventually proved too costly. Since then clothing manufacturers have been using them to give jeans that stonewashed look.

In the past decade, NREL researchers have been looking for better ways to dismantle cellulose, including improved cellulase enzymes. That research is tested at NREL’s ethanol pilot facility just down the road from the massive Coors brewery. NREL makes its brew from corn stover, an inedible plant fiber consisting of the stalks, leaves, husks and cobs left over after harvesting.

The two-story facility is a maze of insulated pipes and silvery tanks. Pounded stover is first hit with sulfuric acid and steam. Invented in Germany during World War II, the technology is old school—clunky and expensive. But it works. The result is a brown, tarry goop that smells and looks like molasses. Next, cellulase enzymes are employed to coax the cellulose molecules into giving up their sugars. Then it’s on to a series of fermentation tanks. This is also an expensive process, because five different kinds of sugars must be fermented, each one by a separate organism. One goal of biofuels research at NREL and elsewhere is to engineer a single organism that will produce the cellulose-splitting enzymes and then ferment all five sugars, all in one step.

The result of the fermentation process is a kind of beer with an ethanol content of 5 percent. While not the final step in producing pure ethanol fuel, turning fiber into sugar and then ethanol is the most critical part of the process. And finding ways to coax even more ethanol out of the fibers—to make a stronger beer—would drive down the cost of cellulosic fuel.
“We’d really like to make something that’s more like wine—a solution with 15 percent alcohol,” says George Douglas, NREL’s media relations manager. Making that "wine," combined with other research and development breakthroughs and a ramp up to a commercial scale of production, could bring the cost of cellulosic ethanol down to about $1.30 per gallon, he says. That would make it commercially viable. The hope is that this will be achieved by the year 2012.

NREL scientists are going to great lengths to accomplish this goal. They’ve even gone bio-prospecting in Yellowstone National Park looking for microorganisms that use cellulase enzymes to live off biomass falling into the park’s steaming hot springs. The idea is to use these enzymes to run the cellulose-splitting process at a higher, more efficient temperature. Such research has already paid off, helping to cut the cost of cellulosic ethanol in half during the past 10 years.

Cars obviously don’t enjoy running on beer very much. So in the last part of the production process, the 5 percent beer is centrifuged and distilled to separate the ethanol from impurities. When the resulting pure ethanol is fed into a flex-fuel vehicle adapted to run on an 85 percent ethanol/15 percent gasoline mix called E85, it will work almost as well as straight gasoline. Almost, but not quite, because ethanol’s lower energy content reduces fuel efficiency.

But since cellulosic ethanol can significantly reduce emissions of greenhouse gases, maybe that doesn’t matter so much. In fact, when all of the fossil energy used in making it is added up, and the net energy produced is calculated, driving a mile on cellulosic ethanol could be even better than a guilt-free experience. That’s due in part to a substance called lignin, one of the impurities separated during the purification process.

Lignin, which gives plants their structural integrity, is an excellent fuel for producing electricity. So commercial cellulosic ethanol plants will probably burn it to make their own power. NREL research shows that enough lignin is produced to power the entire process—and then some. The excess electricity can be pumped into the grid, ultimately displacing fossil fuels with renewable solar energy. (Lignin’s energy ultimately comes from the sun.) Taking this benefit into account, NREL researchers calculate that driving a mile on cellulosic ethanol could reduce greenhouse-gas emissions by more than 100 percent compared with gasoline.

Only time—and the major federal research and development effort now under way—will ultimately tell whether these estimates are borne out in the marketplace.

 

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