Monday, April 03, 2006

Alternative Aircraft Fuel

There are many possible ways to power cars that don't require petroleum products: ethanol, biodiesel, hydrogen, and electricity seem to be the leading contenders today. But there aren't many viable alternatives for fueling jet aircraft. Kerosene-based jet fuel provides a great balance of high energy density, low freeze point, and ease of handling. An article in Technology Review, however, suggests a nifty way to use a byproduct of processing coal into coke for industry that could stretch petroleum-based fuels and possibly even allow for optimized turbine design. If practical, it might even be implemented quickly enough to provide struggling airlines with a bit of relief on fuel prices.

The method described is much simpler than the standard way to turn coal into oil substitutes, as was done in Nazi Germany and still practiced in South Africa today. That approach involves gasifying the coal and creating the desired hydrocarbons from scratch, using Fischer-Tropsch synthesis. The approach suggested in the article goes back to an earlier coal technology that simply cooks liquids out of the coal. Some of the first cars were powered by coal-derived fuels a century ago, particularly in Europe, where petroleum was less plentiful than in the US.

Although this doesn't sound nearly as sexy as some of the alternative fuel technologies people are pursuing for cars, it addresses a segment of oil demand for which other substitutes are lacking. Although planes can be designed to run on liquid hydrogen, as an early design of the SR-71 Blackbird reconnaissance jet was in the late 1950s, its lower energy density by weight and handling difficulties make is much less practical than petroleum distillates. Jet fuel accounts for roughly 8% of US oil consumption, so this is a large and growing demand segment.

There are two problems with the technology suggested in the article, and the author identifies one: very limited supply, relative to the volumes of oil-based jet fuel required. The other is potentially even trickier, namely metals contamination. The refinery "cycle oils" with which the coal oil is to be combined are produced in the fluid catalytic cracking units that are the "upgrading" heart of a modern refinery, at least in markets with high gasoline demand such as the US. But cycle oils carry off small quantities of the catalyst from the cracking units. Although this is a much bigger problem in the "heavy cycle gas oil", even the light cycle oil will contain a bit of catalyst, and that quantity may be too much for a high-performance turbine engine. The coal oil may have similar contamination problems, depending on the quality of the source coal. This issue would need to be thoroughly assessed, before jet engine manufacturers would certify such a fuel for use in their turbines.

So what we have here is a clever, relatively low-tech solution to one of the trickier challenges in any beyond-oil scenario. It needs more evaluation, and it may not be the total solution, but it certainly seems worth pursuing.

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