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Study metallic fuels as future energy option

A car powered by metallic nanoparticles could drive three times as far as one powered by a gasoline-powered internal combustion engine, according to a scientist at the U.S. Department of Energy’s Oak Ridge National Laboratory.

David Beach, leader of the laboratory’s Materials Chemistry Group, said metal fuels also offer great potential for unmanned vehicles and military battlefield power sources. Like hydrogen, metal fuels like iron, aluminum and boron are energy carriers that burn cleanly, Beach said, but unlike hydrogen, metal fuels possess a higher energy content per unit volume, can be stored and transported at ambient temperatures and pressures, and can be combusted at high efficiency in a heat engine without the high costs of fuel cells.

“Large particles of metal do not burn until heated to the metal's boiling point. At this temperature, metal vapor combusts to form metal oxides,” Beach explained. “Unfortunately, this process leads to very high combustion temperatures, fouling of the internal surfaces of the combustion chamber, and the production of oxides of nitrogen.” Nanoparticles of metal, however, burn faster and more completely at lower temperatures with no gas phase combustion. “Nanoparticles oxidize fast enough that they never reach the peak combustion temperature.” Beach said.

At the American Chemical Society meeting earlier this year, Beach's group displayed transmission electron micrographs of iron nanoparticles before and after burning in oxygen, showing complete combustion.

“We measured the iron nanoparticles' peak combustion temperature, which is 1100 Kelvin," Beach said. “The temperature should be hot enough to achieve high energy efficiency but not so high that exotic materials, such as expensive ceramics, are required to contain the combustion. Cast iron can be used as the combustion chamber for nanostructured metal fuels.”

Beach said the exhaust gas of metal fuels in a heat engine, such as a gas turbine or Stirling engine, is very clean. “We take the oxygen out of the air and have nearly pure nitrogen left,” he said. “We recover most of the heat using a recuperator and get much closer to the highest efficiency theoretically achievable in an engine.

“Boron would be an even better energy carrier if boron nanoparticles could be made at a reasonable cost,” Beach continued. “Boron is three times better than gasoline in terms of heat per unit weight and heat per unit volume.”

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