IT MIGHT make airports smell like a Thai restaurant, but a group of American biochemists think soya oil is just the thing to give aviation fuel a greener future. They say that an aircraft fuel based on soya oil and traditional jet fuel will slash consumption of fossil fuel, and help slow the rise in greenhouse gas levels by using carbon from renewable sources. They will tell a meeting of the American Chemical Society in Anaheim, California, next week how it can be done. Commercial jets run on a petroleum fuel called Jet A. Like all fossil fuels, it releases carbon dioxide into the atmosphere when burnt. Biofuels like soya oil, on the other hand, are "carbon neutral" because the carbon they release came from the atmosphere only recently. Meanwhile air traffic is a growing contributor to global warming. In 2002 the UK's Royal Commission on Environmental Pollution predicted that air travel could account for nearly 75 per cent of the UK's greenhouse gas emissions by 2050. "If further research fails to dispel current concerns, then, at some stage, commercial aviation is going to need a completely different fuel," says David Wardle, a fuels expert at the University of Auckland in New Zealand. And, he says, biofuel blends could be one answer. So far, attempts to create a suitable fuel from blends of jet fuel and vegetable oils have been unsuccessful. One stumbling block is the requirement that aviation fuel stays liquid down to -40 degrees C. Vegetable oils generally freeze at around 0 degrees C. But now biochemist Bernard Tao of Purdue University in Lafayette, Indiana, and his graduate student Shailendra Bist have developed a blend that freezes at -40 degrees C. They convert fatty acids within the oil into volatile, combustible esters, some of which freeze at higher temperatures and some at lower temperatures. "Remove the components that freeze at higher temperatures, and you will be left with something that will freeze at low temperatures," says Bist. It's here that they have made the breakthrough. The standard method of removing unwanted esters involves chilling the biofuel and removing any crystallised solids. Repeat this at ever lower temperatures and you create a fuel with a very low freezing point. But the process can take days, and is wasteful because desirable esters "co-crystallise" out with the unwanted ones. Yields can be as low as 25 per cent. Tao's team has developed a similar fractionation technology that takes less than an hour and has yields as high as 80 per cent. They are unwilling to discuss details pending a patent filing, but say they can now make a 40 per cent blend of biofuel and 60 per cent Jet A with the right freezing properties. It is the highest percentage ever reported, says Wardle. The fuel is now being tested on a turbo-prop engine to assess its emissions.
New Scientist issue: 27th March 2004
Written by ANIL ANANTHASWAMY
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