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Plant carbs harnessed to power cars

June 2, 2005 By Mark Peplow This article courtesy of Nature News.

Fuel born from carbohydrates could be clean and easy.

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The diesel fuel in your car could one day come from plants rather than oil wells, according to chemists who have converted plant chemicals into useful hydrocarbons.

Biomass fuels are often touted as a green alternative to oil. Although the carbon dioxide they produce when burnt is a greenhouse gas that contributes to global warming, in theory it should be sucked up by the following year's crop as it grows.

The easiest way to extract energy from plants is simply to burn them, and convert the heat to electricity. Although this is good for stationary power plants, it isn't ideal for cars. Electric cars have to be recharged frequently, which may make them unsuitable for long journeys.

A better idea is to convert plant material into fuel that vehicles can use directly. This has been done with the fatty acids in vegetable oils, which make up a small part of plant material. But now researchers have found a way to create fuel from the carbohydrates that make up about 75% of a plant's dried weight.

The result is a much more efficient use of plant material, report James Dumesic, a chemist from University of Wisconsin, Madison, and his colleagues in Science1.

The plant-derived hydrocarbons are just like conventional diesel, notes Jens Rostrup-Nielsen of Haldor Topsoe, a chemical technology company based in Lyngby, Denmark, so they can be distributed through existing infrastructure. This makes the fuel easier to use than hydrogen, for example, which requires a different kind of pumping station and storage system.

If all goes according to plan, Dumesic estimates one could grow enough plants in the United States to power a significant percentage of the country's vehicles.

Chain of fuels

Carbohydrates have proven an expensive source of fuel in the past. Glucose, for example, can be fermented into ethanol and then added to gasoline. But this process is very inefficient, largely because of the energy it takes to boil ethanol away from water at the end of the fermentation.

Dumesic's team reasoned that this energy-intensive process could be avoided if plant carbohydrates were converted directly to the long-chain hydrocarbons that make up diesel fuel. Because oil and water do not mix, these hydrocarbons float to the top of the reaction mixture, where they are easily siphoned off.

The chemists first used a platinum catalyst to make carbohydrates containing five or six carbon atoms react with hydrogen gas: plant material provides both the carbs and the gas2.

A magnesium-based catalyst then knits these molecules together to create the longer carbon chains required for diesel fuel. Adding more pressurized hydrogen, and removing any remaining oxygen atoms with a platinum catalyst, delivers the finished fuel.

If this can be streamlined into a simpler process, it would be able to compete commercially with ethanol production, says Dumesic.

The next challenge is to work out how to extract the all-important carbohydrates from plant matter. The chemists used a pure carbohydrate supply in their tests, and Dumesic says that plants may have to undergo extensive processing to remove unwanted chemicals.

"We don't know how dirty a biomass stream we can tolerate," he says. "That's where the uncertainty lies."

References

  1. Huber G., Chheda J. N., Barrett C. J. & Dumesic J. A. Science, 308. 1446 - 1450 (2005).
  2. Huber G., Salge J. R. & Dumesic J. A. Science, 300. 2075 (2003).

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