Metal Powder: the New Zero-Carbon Fuel?
IEEE Spectrum, December 16, 2015. Image credit: rgouveia
The two solid fuel boosters that burned for two minutes helping the U.S.’s old space shuttle fleet to reach its orbit each contained 80 tons of aluminum powder, which corresponds to 16 percent of the total weight of the solid fuel. “This idea of burning metals as a fuel sounds pretty far out there, but this is something that has been done in rockets forever,” says Jeffrey Bergthorson, an aeronautics engineer at McGill University in Montreal, Canada. He and colleagues at McGill and at the European Space Agency published this week in Applied Energy a study outlining how metal powder could serve as a zero-carbon fuel to power transportation and the grid.
Bergthorson and his colleagues’ idea is not to use metal powders as a primary energy source, but as a way to store, transport and trade it as a zero-carbon fuel. If this sounds similar to the idea of a hydrogen economy, it is. In the hydrogen economy the gas is manufactured by solar or other renewable forms of energy and then distributed as a fuel that can drive cars and other transport systems. Bergthorson proposes that instead iron powder would be distributed as a means to drive power plants, ships, locomotives and even cars.
“Storing energy will be an important part of the green-energy equation,” says Bergthorson. And for this purpose, metal powders have an advantage over hydrogen and batteries. Metals have a much higher energy density, specifically the energy density by volume, than other materials proposed in low-carbon schemes, including hydrogen. And compared to hydrogen, where storage and transport are still a major problem, metal powder is easy as pie. “If you think about shipping energy by ships, as we do today, we have a much higher energy density as biomass. We ship wood chips all over the globe as one of the ways to trade clean energy, but we can do this with metal fuels on a much larger scale,” says Bergthorson.
After combustion, of course, you’re left with a pile of rust—iron oxide. The usual way of recycling it into iron is to reduce it with coal in a blast furnace. But that, of course, results in carbon emission. But Bergthorson is hopeful. “There are novel techniques to reduce iron oxide using pure hydrogen, or the use of biomass in chemical looping combustion, using gasified biomass or gasified coal, or by electrolysis, which is not yet commercially developed.”