Running on Iron
Metal nanoparticles show promise as future fuels.
In the laboratory and on Capitol Hill, hydrogen has been touted by many as the potential fuel of the future in an age of rising gasoline prices and restricted oil supplies. But David Beach, leader of the Materials Chemistry Group in ORNL's Chemical Sciences Division, believes metallic nanoparticles may be even more promising candidates as a long-term solution to the rising cost of transportation fuels.
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, reach combustion at high efficiency in a heat engine, and avoid 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, the process leads to very high combustion temperatures, fouling of the internal surfaces of the combustion chamber, and the production of nitrogen oxide pollutants. 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.
Beach is leading experiments to demonstrate that metal fuels could be developed for both civilian and military transportation. He works with ORNL's Solomon Labinov, who originated the idea; John Thomas, who is designing an engine to run on metal fuels; and Bobby Sumpter, who carries out first-principles modeling of the combustion using the extensive computing resources of ORNL's National Leadership Computing Facility.
They conducted initial experiments with iron nanoparticles for civilian applications. They demonstrated that combustion occurs entirely in the solid state, that nanostructuring is preserved through many cycles of oxidation and reduction, and that the particles are easily reduced with synthetic gas—the mixture of carbon monoxide and hydrogen that comes from heating coal and water in the absence of air.
"We have performed experiments with iron nanoparticles about 50 nanometers in diameter," Beach says. "These nanoparticles are partially oxidized to develop a 2-nanometerthick oxide coating that keeps the particles from spontaneously combusting. With the oxide coating, which we measured using X-ray diffraction, a temperature exceeding 150°C is required to make the particles ignite. We measured the peak combustion temperatures of these particles, the ignition temperature, and the extent of the reaction. Then we determined the products of the reaction.
"In our radiometry experiment we measured the iron nanoparticles' peak combustion temperature, which is 1100 Kelvin," Beach continues. "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, leaving nearly pure nitrogen," 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, but only 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."
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