The idea of burning metal as a fuel may seem like an odd goal. Metal seems like one of the least combustible materials possible. However, all of this changes when metals are ground down to a fine powder. Take thermite, for instance, which is fuelled by metal powder and gets hot enough to weld underwater. Metal fuels are also widely used in sparklers and fireworks, and also in solid fuel booster rockets for space shuttles.
Fine powders behave much differently than big blocks of material because tiny particles have much more surface area per unit volume. To be useful as a fuel, metals like iron, a front runner for this application, would need to be milled down to a powder as fine as flour or icing sugar. As it turns out, almost anything so fine is combustible, even certain rocks.
Prof. Jeffrey Bergthorson at McGill University believes that metal powders could be the next clean alternative fuel, strong enough to power cars or trains. His work confirming that metal fuels could produce stable flames similar to those produced by fossil fuels was published this month in Applied Energy.
With so many alternative energy sources in development, why are fuels still needed?
Many sustainable energy sources, such as wind and solar, aren’t powerful enough to directly drive vehicles like cars, and batteries are still not efficient enough for even larger vehicles like trains. Meanwhile, the more well-known alternative fuel is hydrogen, but its production still relies on natural gas reformation, which is not carbon neutral.
Metal powders have high energy density, boast low emissions, and are recyclable
Carbon-based fuels like gasoline release carbon dioxide into the atmosphere, and these emissions are part of what drives climate change. Carbon capture technology is improving, but absorption by our oceans means that capturing carbon after its release remains problematic.
On the other hand, metal powders form stable metal oxides when burned that can easily be collected at the source, and later recycled. Like fossil fuels, metal fuels have high energy density, which makes them a strong candidate for engine fuels.
A stable flame has been achieved – the next step is to build a working prototype
Prof. Bergthorson has reported that his team successfully stabilised a flame in a flow of tiny metal particles suspended in air in the lab, and that the properties of the flames were very similar to those produced by burning fossil fuels.
The next step is to build a working prototype burner, and to couple it to a heat engine. Cars that run on gasoline use internal combustion engines, but metal fuels could see a return to external combustion engines, which use heat from an outside source, much like the coal-fired steam locomotives of the industrial era.
To produce a steady flame, Prof. Bergthorson’s proposed design will blow air through a stream of metal powder, and these will be injected together into a combustion chamber. A cyclonic chamber will then separate the metal oxide ash, and the heat produced will run the engine. The exhaust will simply be clean nitrogen gas.
Recycling oxides from the spent metal ash back into purified metal powder
Another obstacle to achieving low-carbon status will be finding a low-carbon method to refine the spent metal ash to recycle it back into a useable metal fuel. In addition to Prof. Bergthorson’s work on an engine prototype, his team is developing carbon neutral recycling processes. This work focuses on iron powders, as these are already commercially manufactured for the metallurgy, chemical, and electronic industries. Iron is also an abundant and low cost metal.
Critics, however, are quick to point out that mining additional iron for this application will require more infrastructure that would not be carbon-neutral to construct, and that milling iron down to fine powders will be more energy intensive than regular smelting. Also, while metal fuels compare well with conventional fuels in terms of energy by volume, they are also dense and fare less well by weight, which could make them less practical.
If these obstacles can be overcome, this could be an interesting alternative to fossil fuels for a low-carbon society.
