POSTED 8 FEBRUARY 2007
As we've seen, corn ethanol is not going to cure the twin crises of global warming and fuel shortage. But beyond the corn-fuel cornucopia looms a much larger source of ethanol: cellulose. Cellulose is the general term for a combination of ingredients found in the plant cell wall, including hemicellulose, lignin, and the cellulose molecule itself; which is a stable, long-chain structure made of sugar sub-units.
If you can get at those sugars, you can ferment them, but they are difficult to isolate. Cellulose can come from wood wastes, crop wastes, and energy crops like switchgrass and fast-growing trees. Most plans for "cellulosic ethanol" would not compete with food crops; a major advantage over corn ethanol. Overall, the U.S. Department of Energy wants to find a source for 1 billion tons of biomass per year. This mountain of cellulose would mainly be converted into fuel, but also make raw materials for the chemical industry.
Switching to switchgrass?
Although crops like switchgrass are getting considerable attention, essentially no cellulosic ethanol is on the market, due to the high cost of processing. For one thing, cellulose biomass has a low energy density, so it is expensive to transport. Removing cornstalks or other crop wastes for biomass can also hasten erosion of soil, and reduce its humus content, and guidelines for safe biomass removal remain to be written.
Before cellulose can be fed to fermenting microbes, it must be broken apart, which takes a lot of energy because cellulose is so strong (to see the problem from a different point of view, cellulose is simply good at its job -- supporting plants). "Over millions of years of evolution, plants have done everything they could to make sure cellulose is not broken down," says Bob Wallace, biomass project manager at NREL.
Wallace says enzymes (organic molecules that work as catalysts) are a promising way to break cellulose down to glucose. Over the past few years, he says, NREL has achieved a 20-fold reduction in the cost of the fungal enzymes used for degrading cellulose. (The helpful fungi were discovered devouring soldiers' cellulose belts in the South Pacific during World War II.)
"We want to get another two- or three-fold reduction in the enzyme cost," Wallace says, adding that the cost of cellulosic ethanol has dropped fast, from about $5 to 6 per gallon in 2000 to about $2.25 now, mainly due to those cut-rate enzymes. For comparison, corn ethanol now costs $1.10 to $1.20 per gallon at the distillery. "The goal is by 2012 to be competitive with corn ethanol, and by 2020, to be competitive with gasoline," says Wallace.
As the computer proves, technologies tend to get better and cheaper with time. In biofuels, a key area for improvement is the yeast that convert sugars to alcohol. These little fungi put themselves in an awkward spot because their metabolic byproduct, ethyl alcohol, is toxic to them. Your (skinned) knee probably knows that alcohol is a great microbicide, and wine stops fermenting at about 12 percent alcohol as the miraculous microbes start to shut down, doubtless woozy from their own ethyl alcohol.
As yeast ferment, they foul their own nest.
Many scientists have tried to breed yeast that tolerate a higher concentration of ethanol and produce stronger alcohol faster. A group at MIT has just reported a new way to create a yeast that tolerates a higher ethanol concentration and makes the moonshine 69 percent faster. The researchers used a new tactic: instead of moving genes, they altered genes that control other genes through what they call transcription factors.
Graphic: American Solar Energy Society
The new yeast seemed at ease in the fermenter, says MIT chemical engineer Gregory Stephanopoulos. "The cells grow better and faster. They produce more ethanol, and produce it faster. On all counts, they are more robust" (see #6 in the bibliography). "We have a different method for engineering tolerance to ethanol," Stephanopoulos added, "but it could also apply to other toxic compounds which are present" in other types of biomass.
Microbial future shock
Currently, given the questions surrounding corn ethanol, many people believe cellulosic is the preferred source of moonshine-motorfuel. "Cellulosic ethanol is a far better feedstock for the energy balance," says Wallace. "As the price and conversion costs come down, it will really make the questions about corn a moot point."
Further down the line, experimental technologies may be used to extract the solar energy stored in many organic wastes. Tim Donohue, a professor of bacteriology at the University of Wisconsin-Madison, who has just submitted a large grant to the Department of Energy for a regional biomass research center, says the challenge reaches beyond cellulose. He envisions "microbial biorefineries" that generate electricity, ethanol, hydrogen, and other useful chemicals from plant wastes. Some of these biorefineries would contain photosynthetic bacteria that get some power directly from sunlight.
In the short run, Donohue admits, ethanol is the name of the game. "There is a lot of push to make ethanol faster, cheaper; it's being done because we know how to do it. In 10 or 15 years, we hope to have a whole suite of facilities that can convert biomass into different forms of energy. Some will work on animal and plant waste at home, or at farms or farm coops. They will be at water treatment plants, or in large factories with organic material that can be converted into energy, heat and useful products. It's a whole new landscape in the energy field, and in 10 to 15 years, it will look very different from what we have today."
Got the energy to dig into our bibliography?
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive