Metallic fuels studied as
future energy option
An ORNL researcher predicts that a car with a modified engine powered by metallic nanoparticles could drive three times as far as today's gasoline-powered internal combustion engine. Metal fuels also offer great potential for unmanned vehicles and battlefield power sources for military uses.
David Beach, leader of the Materials Chemistry Group at DOE's Oak Ridge National Laboratory, explains that, like hydrogen, a metal fuel is an energy carrier and burns cleanly. But unlike hydrogen, metal fuels—such as iron, aluminum, and boron—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. 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.
Metal nanoparticles, however, burn faster and more completely at lower temperatures with no gas phase combustion.
“These particles oxidize fast enough that they never reach the peak combustion temperature,” Beach says.
At the American Chemical Society meeting in March 2005, Beach's group displayed transmission electron micrographs of iron nanoparticles before and after burning in oxygen. Their poster included a sharp image of iron oxide particles, showing complete combustion.
“We displayed the results of a radiometry experiment in which we measured the iron nanoparticles' peak combustion temperature, which is 1100 Kelvin," Beach says. “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 says that 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 says. “We recover most of the heat using a recuperator and get much closer to the highest efficiency theoretically achievable in an engine.
“An even better energy carrier would be boron if boron nanoparticles could be made at a reasonable cost. Boron is three times better than gasoline in terms of heat per unit weight and heat per unit volume.”
Submitted by DOE's
Oak Ridge National Laboratory