Cellulose Dreams

The search for new means and materials for making ethanol

Alternative energy is hitting the headlines. Last year, former Vice President Al Gore scored a surprise hit with his climate-change documentary An Inconvenient Truth. Currently, drivers are steeling themselves against gasoline prices that could shoot well past $3 per gallon. The war in Iraq continues to draw attention to the United States’ dependence on imported oil and has prompted calls for a shift toward domestic sources of fuel.

WASTE NOT. Cornstalks, gathered and baled here in a Nebraska field after harvest, can serve as a high-cellulose feedstock for making ethanol. W. Wilhelm/USDA

FROM GRASS TO GAS. Switchgrass, a wide-ranging perennial, is harvested from a University of Tennessee test plot for studies of its value for biofuel production. Oak Ridge National Laboratory/DOE

More and more, policy makers are touting a homegrown solution—literally—to the nation’s energy and global warming problems: ethanol made from plants. Mixing ethanol into gasoline reduces overall greenhouse-gas emissions from vehicles because plants recycle carbon: The fuel that they yield produces carbon dioxide, just as fossil fuels do, but the plants consume carbon as they grow. And because ethanol molecules contain oxygen, their presence makes gasoline burn more completely, reducing carbon monoxide and other harmful tailpipe emissions.

Ethanol figures significantly in the Bush Administration’s promotion of biofuel. Currently, the United States produces about 6 billion gallons of ethanol annually, mostly from corn. The President’s Twenty in Ten initiative sets a goal to reduce gasoline usage by 20 percent in 10 years—in part by increasing the production of renewable fuels to 35 billion gallons per year by 2017.

But to reach that goal, corn alone won’t do. Researchers are looking to trees, grasses, and waste organic matter as possible raw materials for ethanol production. The basic idea is to extract the cellulose locked up in plants’ cell walls, break it down into its component sugars, and ferment those sugars into ethanol.

In February, the Department of Energy (DOE) announced that it would spend up to $385 million over the next 4 years to work with commercial partners on six ethanol pilot plants. Then in June, DOE granted $375 million to fund three new Bioenergy Research Centers to develop technology for cellulosic ethanol and other biofuels.

“I equate what we’re doing to society saying, ‘We’re going to the moon,’ or ‘We’re going to sequence the human genome,'” says Tim Donohue, a bacteriologist at the University of Wisconsin–Madison. “To me, this is a critically grand scientific mission that we’re just setting off on today.”

Billion-ton vision

People figured out long ago how to make alcohol from grains, and now a similar process is used to turn corn into ethanol for fuel. First, corn kernels are ground into a coarse flour and combined with water and the enzymes alpha-amylase and glucoamylase, which convert starch into sugar.

After this mash is cooked and sterilized—to destroy the two amylase enzymes—yeast is added to ferment the sugars into ethanol. The final step, distillation, separates the ethanol from water, solids, and other chemical products of fermentation.

Corn sugar is almost entirely glucose, which yeast readily ferments into ethanol, Donohue says. But because corn is a foodstuff for people and animals, diverting large amounts of it into ethanol production could push up prices and even cause shortages. That’s why plants that aren’t currently used in other ways are attractive alternatives to corn.

In 2005, the Oak Ridge National Laboratory in Tennessee issued a report for the Departments of Energy and Agriculture estimating that the United States could produce 1.3 billion tons of plant matter that, if turned into ethanol, could fill more than 30 percent of the nation’s petroleum needs. Agricultural waste forms a large part of that estimate.

But in order to reach that ambitious billion-ton goal without impinging on food supplies, high-cellulose crops, such as poplar, switchgrass, and wheatgrass, must also be grown specifically for ethanol production. All three plants are relatively undomesticated, so there’s plenty of opportunity for breeding them to improve their value as cellulose sources, says Brian Davison of the Oak Ridge lab.

The nagging problem with these plants and others is that the cellulose in their cell walls is hard to get out, a problem that researchers call “recalcitrance of biomass.”

“Nature developed plants so they’re not easily degraded,” says Martin Keller, also at Oak Ridge. The rigid cell wall has a complex structure built from three polymers: cellulose, hemicellulose, and lignin. Cellulose consists of long chains of glucose molecules (simple sugars with six carbon atoms) organized into tiny fibers. These fibers form a scaffold that supports hemicellulose, a polymer composed mostly of xylose (simple sugars with five carbon atoms). Lignin is a compound of various polymers that gives the plant strength and rigidity, but how it links with cellulose and hemicellulose is not well understood.

Current ethanol-making strategies require a number of difficult steps to dismantle a cell wall. Treatment with heat, pressure, or acids first removes hemicellulose and lignin from the long cellulose fibers. The cellulose and hemicellulose are then separately processed into ethanol, though doing so is challenging. The cellulose fibers don’t dissolve well in water, making it difficult for the amylase enzymes to access the cellulose and break it down into glucose. Microbes are used to break hemicellulose into its component sugars and ferment them, but they produce a lot of by-products and not much ethanol.

Finding a way to process cellulose and hemicellulose together would make the process simpler and cheaper. Researchers are also looking to genetically modify plants to produce softer types of lignin or cellulose, making their cell walls easier to break down. The poplar tree’s genome was sequenced last year, for example, so researchers could begin identifying genes important for cell wall synthesis.

Mission: biofuels

Efficient and environmentally sound production of cellulosic ethanol is foremost in the minds of researchers involved with DOE’s three newly funded Bioenergy Research Centers. “The centers provide a mechanism and a platform for coordinated activities between people in the plant sciences, the processing of plant materials, and the microbiology and chemistry of converting plant sugars into a variety of different types of fuels,” says Donohue, principal investigator for one of the three consortia, the Great Lakes Bioenergy Research Center, based in Wisconsin. “And they provide a way to think about how we develop these new practices in an economically viable and environmentally sustainable manner.”

Scientists at the second Bioenergy Research Center, headquartered at Oak Ridge National Laboratory, are focusing on poplar and switchgrass. Keller, director of the center, highlights the need to find new enzymes to break down the cell walls of these plants. The two amylase enzymes work well for extracting glucose from corn, but for other feedstocks, he says, “I predict we’ll need to develop a whole suite of enzymes.”

As to where such enzymes might be found, ethanol researchers like to point out that when a tree falls in the forest, it doesn’t stay around forever—it rots. The microbes growing on that tree must contain enzymes that can chew through the cellulose in wood. Identifying the microbes, however, might require development of new laboratory techniques. For example, if the microbes grow on wood particles but aren’t soluble in water, they can’t simply be grown in a water-based nutrient solution, as most cultured cells can, Keller says.

Researchers at the Great Lakes center are bioprospecting for useful enzymes in bacteria, fungi, and other microbes. In addition, they hope to engineer yeast and bacteria that can ferment xylose as well as glucose and that can tolerate high ethanol concentrations. Microbes could also be engineered to make other products such as hydrogen, biodiesel, and chemical precursors for various industrial processes.

Keller sees promise in getting a single organism to do all the degradation and fermentation without the need for extra enzymes—an approach called consolidated bioprocessing. Lee Lynd, a biochemical engineer at Dartmouth College in Hanover, N.H., won the 2007 Lemelson–Massachusetts Institute of Technology Award for Sustainability for his work on engineering Clostridium thermocellum bacteria to break down biomass and ferment the resulting sugars into ethanol. The bacterium naturally degrades cellulose and survives the high temperatures typical of industrial fermentation.

At the third center, the Joint Bioenergy Institute in Berkeley, Calif., chief executive officer Jay Keasling has a different point of view on alternatives to gasoline and diesel fuel. He says he’s not fond of ethanol because of its low energy density—the modest amount of energy that 1 kilogram of the fuel yields.

He says that his focus is on generating “biogasoline” from plant matter. His group is attempting to engineer microbes to turn cellulose into hydrocarbons that can go directly into a gas tank without the need for mixing in petroleum.

Ethanol, environment

The success of cellulosic ethanol as a fuel depends on whether experimental processing methods can be scaled up in an economically sound way. Because it’s risky to try these technologies on a large scale, DOE is sharing the cost with industry, according to Davison. The six biorefineries funded in February represent a “great survey of the technology available now,” he says, adding that he’s “confident that more than half of these biorefineries will work.” The pilot plants are being built by six companies across the United States. Some will focus on feedstocks grown specifically for energy, such as switchgrass, while others will use agricultural waste.

Still, many questions remain about the long-term practicality and environmental value of large-scale ethanol production. “Can it be produced efficiently and economically?” Davison wonders. “Can we produce enough plant matter to make a substantial impact? Then, can we do it sustainably?” He’s confident about answers to the first and second questions, he says, but not so confident about the third.

A sustainable crop, he says, can ideally be grown on the same land for 100 years with no inputs—that is, no fertilizer or irrigation. By contrast, even collecting existing waste biomass for ethanol may not be as environmentally benign as it seems. Farmers often plow cornstalks back into their fields to keep the soil fertile, so “if we immediately start taking cornstalks off the field, we need to worry about how the soil is going to be affected by the loss of carbon or other nutrients,” says Donohue.

Then there’s a much bigger question: Is ethanol worth pursuing at all? The answer depends on how it’s produced, says Daniel Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley. Large-scale farming of feedstocks would involve heavy machinery that burns energy and produces greenhouse gases. Transporting those feedstocks to a refinery and converting them into ethanol would also require energy.

Kammen and his colleagues have developed a model—published in the Jan. 27, 2006 Science and modified since—that takes these factors into account. They find that ethanol from corn requires 95 percent less petroleum to produce than gasoline does but cuts greenhouse-gas emissions by only about 18 percent.

Cellulosic ethanol, by contrast, cuts greenhouse-gas emissions by more than 90 percent. That’s mainly because producers of corn ethanol burn fossil fuels to heat fermentation tanks, while producers of cellulosic ethanol burn the lignin from their feedstocks.

Contrary to one charge made by some critics, however, the researchers concluded that it doesn’t take more energy to make ethanol than can then be obtained by burning it.

Despite the excitement currently devoted to ethanol, it’s likely to be only a short-term answer to the country’s growing fuel needs, Keller says. “Within the next 5 years, ethanol is where we can make a difference. But will it be the final answer? I don’t know.”

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