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Sustainable Energy

Biodiesel: A New Way of Turning Plants into Fuel

A breakthrough process for converting biomass into biodiesel fuel promises a cheaper way to go green.

Eco-dreamers have long hoped for a way to drive around without contributing to global warming, but the slow pace of progress in alternative fuel technologies has kept that vision from materializing. Now, a promising new process, designed by researchers at the University of Wisconsin and outlined in a paper that appeared in the journal Science on June 2, could be a significant step toward turning that dream into a reality.

The paper details a new way to produce biodiesel fuel, which is made out of plant matter. Traditional biodiesel refining uses only the fatty acids of a plant, which typically make up less than 10 percent of the mass of dried plants. Rather than converting only the fat, this new method promises to turn all of the dried plant material, including roots, stems, leaves, and fruit, into biodiesel or heat energy.

Ethanol, the most popular and commercial biofuel, has long been refined out of plant matter, but it requires the costly, energy-intensive step of distilling every molecule of water out of the solution. In contrast, the new biodiesel process is based on aqueous phase reactions, which don’t need to go through the expensive distillation phase.

“The biggest advance we have to offer is the lack of that distillation process,” says George Huber, one of the paper’s authors and a graduate student at the University of Wisconsin who will soon be teaching at the University of Massachusetts at Amherst. “That means that our process is exothermic.” In other words, it doesn’t need a lot of extra energy. And that’s important, because the largest cost in the current biofuel refining process is energy.

The new method is divided into four parts. First, a stream of processed biomass consisting of water and sugars is fed over a nickel-tin catalyst to strip off some of its hydrogen atoms. Then the stream is treated with acids that take out most of the water. The resulting “goo” is then transported over a solid base catalyst, which forms it into long carbon chains, called alkanes. Finally, those alkanes are run through a platinum-silica-alumina catalyst at high temperatures, while the hydrogen from the first step is fed into the reactor. The resulting liquid has almost the exact same chemical structure as traditionally refined biodiesel and burns the same way in diesel engines. And the only byproducts are water and heat.

If the process can be scaled up to industrial levels, it could be a major step toward the creation of a transportation fuel that is relatively clean burning, doesn’t contribute to global warming, and provides U.S. farmers with billions of dollars of new income.

According to Bill Jones, Chairman of the Board of Pacific Ethanol, a leading biofuel company, the oil industry currently views the emerging bio-fuels industry with fear, rather than acceptance.

“But eventually they’ll come around,” he says. “They’ll understand that this isn’t just competition, it’s a whole new market for them to get into.”

He points out that the Brazilian petroleum industry also resisted government attempts to promote biofuels, but it is now a big supporter – more than half of Brazil’s oil imports have been replaced with biofuels (see the Technology Review April cover story on world-changing ideas).

Others don’t need to be convinced, though. Charles Wyman, a distinguished professor at Dartmouth College in Hanover NH, whose specialty is the biological conversion of cellulosic biomass to ethanol and other products, says this new methodology could give biodiesel a fighting chance to succeed in the commercial marketplace by allowing manufacturers to make either ethanol or biodiesel fuel.

“Once you break down all the sugars in the plant material, the only option we had before was to make ethanol,” Wyman says. “This presents more options.”

In the future, a single manufacturing center, after refining the biomass into sugars, could make biodiesel or ethanol, depending on market demand. However, Wyman also points out that the economic battle hasn’t necessarily been won.

“In the end it’s the price at the gas station where these technologies win or lose, not in the laboratory,” he says.

To insure that both biodiesel and ethanol become more competitive in the marketplace, Wyman says that a key breakthrough is needed to make diesel fuel or other products such as ethanol competitively from sugars. According to him, advances in this area could beat wholesale gasoline prices.

And some believe that breakthrough is on the horizon. Advancements in the last two years in enzyme technology by the National Renewable Energy Laboratories and private companies such as Iogen and Novozymes have substantially reduced the costs of cellulose transformation, which is tantalizingly close to making the whole system economically competitive with cheap gas.

The new process being developed by James Dumesic, professor of chemical and biological engineering at the University of Wisconsin, and Huber will help to reduce those costs by limiting the amount of waste, since any type of plant matter can be fed into their system. Unlike current ethanol refineries, which can work only with high-glucose content materials such as corn, the biodiesel fuel generated by this process uses the cellulose, roots, and stems of any plant.

That means the waste biomass of America’s vast agriculture industry – everything from corn stover (the stems and leaves of the plant) to peanut shells and fallen leaves – can be used. A recent U.S. Department of Agriculture study (see Notebook) estimated that more than 1.3 billion tons of such waste is produced every year. If all of it were turned into biodiesel, it would provide enough fuel to replace one-third of the petroleum consumed in the United States. Furthermore, turning currently unused farmland into grassland to be harvested for biodiesel production would easily account for the other two-thirds of petroleum needs.

That, of course, means another beneficiary of such a transformation would be family farmers, according to Pacific Ethanol’s Jones. Ethanol refineries owned by cooperatives of farmers already supply the bulk of U.S. ethanol production, and biodiesel refineries could be modeled on the same program.

Honing this new process, though, is only the first step in the very long process of transforming the country to a biodiesel nation. For that to happen, the entire U.S. commercial car fleet would have to switch from internal combustion engines to diesel ones, of course; but the move might be attractive, since the new engines would cause less pollution (biodiesel vehicles would produce far fewer pollutants like sulfur and nitrogen oxides.)

Such a sea-change in the U.S. transportation infrastructure won’t happen quickly. More likely, biodiesel production will start slowly, then ramp up to an industrial scale, if it’s competitive with diesel and gasoline.

Still, Huber thinks that his team has taken a major step toward harnessing one of the world’s most-prevalent, yet least-utilized energy resources.

“If this is a success,” he says, “I can say that I helped to convert our biomass resources to fuel our transportation system.”

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