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.
"Aluminum powder has long been known to be a very energetic material," says Bergthorson. But other metals such as magnesium—remember the magnesium flash light—zinc, and even lithium and silicon could do as well. "If we look at the elements available in the Earth's crust, iron is the third most abundant element, and the idea of the iron economy is that once you got this thing going, you will be recycling that iron. Initially you will need a production from ore, but at some point you would not be mining iron anymore, you would simply be recycling it back and forth, adding the iron in at the point of use," says Bergthorson.
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."
How metal powder would power, for example, a locomotive is the big question. The idea of using powder in an internal combustion engine was quickly discarded, and the researchers turned to a modernized version of the good old steam engine, equipped with an external combustion engine, such as a Rankine-cycle steam engine or Stirling engines, which are now as efficient as internal combustion engines.
Experiments are underway, says Bergthorson:
We demonstrated the burners in our lab, and we have shown that we can stabilize flames, and we can generate heat at significant power levels. But we have not yet coupled it to a heat engine, although we know that there are cyclone technologies that exist and that can separate the produced oxide powder out quite effectively. But we have demonstrated the scientific understanding.
However there may be a shortcut. "There are parallel systems that exist: the coal power plant: you take in coal and then crush it to powder and deliver it to large boiler systems. If you would want to back up power for solar and wind energy, you could stockpile metal fuels and burn them in a retrofitted coal-fired power plant that has the appropriate collection systems for the combustion exhaust on it. The coal power plant infrastructure is already there," says Bergthorson.