Butanol vs. ethanol

The science to produce butanol from plants has been around for decades. Although today butanol is mostly produced from fossil fuels to be used as a solvent, it was first made in the early part of the last century by fermenting feedstock such as molasses.

But the traditional fermentation process is too inefficient to make biobutanol in the large amount that would be needed for it to be used as a biofuel. That’s because, aside from butanol, the fermentation also yields tow other products: acetone and ethanol. In comparison, the yeast that is used to transform feedstock into ethanol doesn’t create any byproducts. That means it takes more feedstock to producte butanol than ethanol. Additionally, butanol in high concentration is toxic for the bacteria that makes it, so at a certain point, they shut down production. The yeast that produces ethanol is more resistant.

Because of these limitations, making butanol with existing techniques is about 50% more expensive than the $1.75 to $2 it costs to make a gallon of ethanol. The key to changing those economics is bioengineering a more tolerant bug thant transforms feedstock more purely into butanol. DuPont has been working on such a butanol bug since 2004 and is said to be making headway.

Meanwhile BP has been conducting trials showing that gasoline blended with butanol behaves more like regular gasoline than ethanol. That is because ethanol, unlike gasoline and other oil-based fuels, mixes easily with water. So if ethanol finds any water residues in transit, it can separate from the gasoline it is blended with. This is a big advantage for butanol over ethanol, because it means that ethanol needs to be hauled by truck or train, which creates more logistical headaches than a fuel that can be piped.

BP also says it has been able to add up to 16% of butanol to gasoline without the need to modify car engines. Researchers generally believe higher concentrations than that may be possible. In contrast, according to the Alliance of Automobile Manufacturers, a trade group in the U.S., ethanol concentration in gasoline higher than 10% can corrode engine parts, except in flex-fuel vehicles, which are designed to tolerate higher ethanol content. About 2% of the cars on the road in the U.S. are flex-fuel vehicles, according to the trade group.

One major challenge is figuring out how to make butanol out of non-food feedstocks. Butanol can be made form any form of sugar and sugar comes in many forms. For example, Green Biologics, a British company, is testing feedstocks such as paper pulp derivatives and food waste.

Source: WSJ, 30/07/08