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Turning Whole Plants into Fuel in Four Simple Steps

A new process can turn plants into energy-dense fuel by combining the power of fermentation and chemical reactions
ethanol-refinery



COURTESY OF CARGILL

A recipe for fuel: take the carbohydrates like starch and cellulose that make up the majority of plants. Use enzymes to break them down into fructose, the sugar found in fruits and honey. Mix this fructose with salt water and hydrochloric acid. Add a solvent—in this case butanol also derived from plant matter—to protect the resulting hydroxymethylfurfural (HMF) from reacting with the water, then extract it. This versatile molecule can be used to create plastic polymers or other chemicals. And by the way, adding a copper-coated ruthenium catalyst can also convert the HMF to DMF (2,5-dimethylfuran), a fuel that provides more energy than ethanol.

"It should be a great fuel," says James Dumesic, a chemical engineer at the University of Wisconsin–Madison, who, along with his colleagues, discovered the new process, "DMF has the energy density of petroleum."

He notes that DMF could rapidly replace ethanol, because it not only provides more energy but also has a higher boiling point (allowing DMF to blend more easily with gasoline) and it does not react with water (ethanol absorbs atmospheric water vapor, which degrades its potency). Plus, this process, reported in Nature, works faster than the several days it takes Saccharomyces cerevisiae yeast to ferment plant sugars into ethanol, because it is chemically controlled and therefore can be completed in hours.

But DMF, despite its apparent benefits, has yet to be extensively tested as a stand-alone fuel in engines. "We make relatively small quantities," Dumesic says. "I don't know of studies at very high concentrations [of DMF showing] how good of a fuel it would be. But you can make a very good case for this as a blending agent," much as ethanol is currently used.

And DMF may yet fail another important test. Whereas the process may be environmentally benign—using plant-derived butanol and hydrogen as well as simple salt water—the resulting molecule may not be. "We can't find information pro or con about the toxicological impact of DMF. That has to be looked at carefully," Dumesic says. "Does this make sense from an environmental point of view? Or are we making another MTBE?" (MTBE, or methyl tertiary butyl ether, was added to gasoline beginning in 1979 to help it burn more fully, but the cancer-causing chemical was widely banned in the U.S. after it was discovered to be leaking into and contaminating ground water.)

If DMF does pass that test, however, it could be available shortly and cost no more (and potentially less, depending on the utility of side products like HMF) than ethanol. "We could make this happen within the next few years if we are told from an environmental safety point of view that this would be a good thing to do," Dumesic says. "The process we are talking about here is very much like a petroleum process and the knowledge of the petroleum industry in scaling things up could all apply here."

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