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Blooming Prairies
Whether ethanol made from cellulosic biomass is good or bad for the environment, however, depends on what kind of biomass it is and how it is grown.

In his office in St. Paul, David Tilman, a professor of ecology at the University of Minnesota, pulls out a large aerial photo of a field sectioned into a neat grid. Even from the camera’s vantage point far above the ground, the land looks poor. In one plot are thin rows of grasses, the sandy soil visible beneath. Tilman says the land was so infertile that agricultural use of it had been abandoned. Then he and his colleagues scraped off any remaining topsoil. “No farmer has land this bad,” he says.

In a series of tests, Tilman grew a mixture of native prairie grasses (including switchgrass) in some of the field’s plots and single species in others. The results show that a diverse mix of grasses, even grown in extremely infertile soil, “could be a valuable source of biofuels,” he says. “You could make more ethanol from an acre [of the mixed grasses] than you could from an acre of corn.” Better yet, in a paper published in Science, Tilman showed that the prairie grasses could be used to make ethanol that is “carbon negative”: the grasses might consume and store more carbon dioxide than is released by producing and burning the fuel made from them.

The findings are striking because they suggest an environmentally beneficial way to produce massive amounts of biofuels without competing with food crops. By 2050, according to Tilman, the world will need a billion hectares more land for food. “That’s the land mass of the entire United States just to feed the world,” he says. “If you did a lot of biofuels on [arable] land–it is very easy to envision a billion hectares for biofuels–you will have no nature left and no reserve of land after 50 years.” Instead, ­Tilman argues, it makes sense to grow biomass for fuels on relatively infertile land no longer used for agriculture.

But down the hill from Tilman’s office, his colleagues in the applied-economics department worry about the practical issues involved in using large amounts of biomass to make fuel. For one thing, they point out, the technology and infrastructure that could efficiently handle and transport the bulky biomass still need to be developed. And since the plant material will be expensive to move around, biofuel production facilities will have to be built close to the sources of feedstock–probably within 50 miles.

The amount of biomass needed to feed even one medium-size ethanol facility is daunting. Eidman calculates that a facility producing 50 million gallons per year would require a truck loaded with biomass to arrive every six minutes around the clock. What’s more, he says, the feedstock is “not free”: it will cost around $60 to $70 a ton, or about 75 cents per gallon of ethanol. “That’s where a lot of people get fooled,” he adds.

Since no commercial cellulosic facility has been built, says ­Eidman, it is difficult to analyze the specific costs of various technologies. Overall, he suggests, the economics look “interesting”–but cellulosic ethanol will have to compete with corn-derived biofuels and get down to something like $1.50 a gallon. Eidman believes it will be at least 2015 before biofuels made from cellulose “are much of a factor” in the market.

Exiled
While chemical engineers, microbiologists, agronomists, and others struggle to find ways of making cellulosic ethanol commercially competitive, a few synthetic biologists and metabolic engineers are focusing on an entirely different strategy. More than fifteen hundred miles away from the Midwest’s corn belt, several California-based, venture-backed startups founded by pioneers in the fledging field of synthetic biology are creating new microörganisms designed to make biofuels other than ethanol.

Ethanol, after all, is hardly an ideal fuel. A two-carbon molecule, it has only two-thirds the energy content of gasoline, which is a mix of long-chain hydrocarbons. Put another way, it would take about a gallon and a half of ethanol to yield the same mileage as a gallon of gasoline. And because ethanol mixes with water, a costly distillation step is required at the end of the fermentation process. What’s more, because ethanol is more easily contaminated with water than hydrocarbons are, it can’t be shipped in the petroleum pipelines used to cheaply distribute gasoline throughout the United States. Ethanol must be shipped in specialized rail cars (trucks, with their relatively small payloads, are usually far too expensive), adding to the cost of the fuel.

So instead of ethanol, the California startups are planning to produce novel hydrocarbons. Like ethanol, the new compounds are fermented from sugars, but they are designed to more closely resemble gasoline, diesel, and even jet fuel. “We took a look at ethanol,” says Neil Renninger, senior vice president of development and cofounder of Amyris Biotechnologies in Emeryville, CA, “and realized the limitations and the desire to make something that looked more like conventional fuels. Essentially, we wanted to make hydrocarbons. Hydrocarbons are what are currently in fuels, and hydrocarbons make the best fuels because we have designed our engines to work with them.” If the researchers can genetically engineer microbes that produce such compounds, it will completely change the economics of biofuels.

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Credit: Steve Hebert/Polaris

Tagged: Energy

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