Kurt Kleiner, New Scientist 18 Aug 09;
Faced with global warming and potential oil shortages, the US navy is experimenting with making jet fuel from seawater.
Navy chemists have processed seawater into unsaturated short-chain hydrocarbons that with further refining could be made into kerosene-based jet fuel. But they will have to find a clean energy source to power the reactions if the end product is to be carbon neutral.
The process involves extracting carbon dioxide dissolved in the water and combining it with hydrogen – obtained by splitting water molecules using electricity – to make a hydrocarbon fuel.
Syngas process
It uses a variant of a chemical reaction called the Fischer-Tropsch process, which is used commercially to produce a gasoline-like hydrocarbon fuel from syngas, a mixture of carbon monoxide and hydrogen often derived from coal.
Robert Dorner, a Naval Research Laboratory chemist in Washington DC and first author of a new paper on the technique, says that CO2 is rarely used in the Fischer-Tropsch process because of its chemical stability.
But CO2's abundance, combined with concerns about global warming, make it an attractive potential feedstock, Dorner says. Although the gas forms only a small proportion of air – around 0.04 per cent – ocean water contains about 140 times that concentration, he says.
Iron catalyst
The navy team have been experimenting to find out how to steer the CO2-producing process away from producing unwanted methane to produce more of the hydrocarbons wanted.
In the conventional Fischer-Tropsch process, carbon monoxide and hydrogen are heated in the presence of a catalyst to initiate a complex chain of reactions that produce a mixture of methane, waxes and liquid fuel compounds.
Dorner and colleagues found that using the usual cobalt-based catalyst on seawater-derived CO2 produced almost entirely methane gas. Switching to an iron catalyst resulted in only 30 per cent methane being produced, with the remainder short-chain hydrocarbons that could be refined into jet fuel.
Heather Willauer, the navy chemist leading the project, says the efficiency needs to be much improved, perhaps by finding a different catalyst.
Appealing source
"The idea of using CO2 as a carbon source is appealing," says Philip Jessop, a chemist at Queen's University in Kingston, Ontario, Canada.
But to make a jet fuel that is properly "green", the energy-intensive electrolysis that produces the hydrogen will need to use a carbon-neutral energy source; and the complex multi-step process will always consume significantly more energy than the fuel it produces could yield. In addition, each step in the process is likely to add cost and problems.
"It's a lot more complicated than it at first looks," Jessop says.
A paper on the navy research was presented at the American Chemical Society annual meeting in Washington DC on Sunday.
