While setting fire to an iron ingot is probably more trouble than it’s worth, fine iron powder mixed with air is highly combustible. When you burn this mixture, you’re oxidizing the iron. Whereas a carbon fuel oxidizes into CO2, an iron fuel oxidizes into Fe2O3, which is just rust. The nice thing about rust is that it’s a solid which can be captured post-combustion. And that’s the only byproduct of the entire business—in goes the iron powder, and out comes energy in the form of heat and rust powder. Iron has an energy density of about 11.3 kWh/L, which is better than gasoline. Although its specific energy is a relatively poor 1.4 kWh/kg, meaning that for a given amount of energy, iron powder will take up a little bit less space than gasoline but it’ll be almost ten times heavier.
It might not be suitable for powering your car, in other words. It probably won’t heat your house either. But it could be ideal for industry, which is where it’s being tested right now.
So what happens to all that rust? This is where things get clever, because the iron isn’t just a fuel that’s consumed— it’s energy storage that can be recharged. And to recharge it, you take all that Fe2O3, strip out the oxygen, and turn it back into Fe, ready to be burned again. It’s not easy to do this, but much of the energy and work that it takes to pry those Os away from the Fes get returned to you when you burn the Fe the next time. The idea is that you can use the same iron over and over again, discharging it and recharging it just like you would a battery.
Photo: Bart van OverbeekeThe combustion of the iron powder is visible through the glass in the combustion tube.
To maintain the zero-carbon nature of the iron fuel, the recharging process has to be zero-carbon as well. There are a variety of different ways of using electricity to turn rust back into iron, and the TU/e researchers are exploring three different technologies based on hot hydrogen reduction (which turns iron oxide and hydrogen into iron and water), as they described to us in an email:
Mesh Belt Furnace: In the mesh belt furnace the iron oxide is transported by a conveyor belt through a furnace in which hydrogen is added at 800-1000°C. The iron oxide is reduced to iron, which sticks together because of the heat, resulting in a layer of iron. This can then be ground up to obtain iron powder. Fluidized Bed Reactor: This is a conventional reactor type, but its use in hydrogen reduction of iron oxide is new. In the fluidized bed reactor the reaction is carried out at lower temperatures around 600°C, avoiding sticking, but taking longer. Entrained Flow Reactor: The entrained flow reactor is an attempt to implement flash ironmaking technology. This method performs the reaction at high temperatures, 1100-1400°C, by blowing the iron oxide through a reaction chamber together with the hydrogen flow to avoid sticking. This might be a good solution, but it is a new technology and has yet to be proven.
Both production of the hydrogen and the heat necessary to run the furnace or the reactors require energy, of course, but it’s grid energy that can come from renewable sources.
If renewing the iron fuel requires hydrogen, an obvious question is why not just use hydrogen as a zero-carbon fuel in the first place? The problem with hydrogen is that as an energy storage medium, it’s super annoying to deal with, since storing useful amounts of it generally involves high pressure and extreme cold. In a localized industrial setting (like you’d have in your rust reduction plant) this isn’t as big of a deal, but once you start trying to distribute it, it becomes a real headache. Iron powder, on the other hand, is safe to handle, stores indefinitely, and can be easily moved with existing bulk carriers like rail.
