Jun 15, 2023

Are Powdered Metal Fuels Just A Flash In The Pan?

It’s no secret that fossil fuels are quickly becoming extinct. As technology charges ever forward, they are disappearing faster and faster. Many of our current dependencies on fossil fuels are associated with high-energy applications like transportation. Since it’s unlikely that global transportation will ever be in decline for any reason other than fuel shortage itself, it’s imperative that we find something that can replicate the high energy density of fossil fuels. Either that, or go back to the drawing board and change the entire scope of global transportation.

Energy, especially solar and wind, cannot be created all over the world. Traditionally, energy is created in situ and shipped to other places that need it. The proposed solutions for zero-carbon energy carriers—batteries and hydrogen—all have their weaknesses. Batteries are a fairly safe option, but their energy density is pretty poor. Hydrogen’s energy density is higher, but its flammability makes it dangerously volatile to store and transport.

Recently, a group of researchers at McGill University in Canada released a paper exploring the use of metal powders as our zero-carbon fuel of the future. Although metal powders could potentially be used as primary energy sources, the transitory solution they propose is to use them as secondary sources powered by wind and solar primaries.

The idea of using powdered fuel as an energy source is not a new one. One of Rudolf Diesel’s late 1800s prototypes ran briefly on coal dust, a resource which was plentiful in the mines of nearby Ruhr valley. After running the engine for less than ten minutes, he found that sludge had already accumulated and figured it was from the ash produced during combustion. Coal dust was tested further in Germany and the results were largely the same—internal sludge buildup and a higher rate of wear. Coal fuel research picked up in the US after WWII and focused on using a coal slurry made with diesel. Still, the ash caused the piston rings to wear out faster.

As the decades went on, diesel firms experimented with using smaller and smaller introductory coal particles, and they also tried mixing them with water instead of diesel. In the 1980s, the United States Department of Energy launched a program to work with diesel companies on the water-coal slurry question. One of those companies, a division of General Electric, had made big strides by the early 1990s, but by then, oil prices were declining. The DoE program was axed.

Aluminium and other metals are an attractive alternative energy choice for a couple of reasons. Most importantly, they have a high energy density. That’s partially why aluminium powder is used in fireworks and rocket boosters. Metal can be used to make battery anodes, although metal-air batteries have to be much larger in order to compete with the power densities of traditional fuel cells. More resourcefully speaking, metal powders used as standalone fuels for direct combustion from the infrastructure level down to car engines.

There’s a problem with directly using metal powders as fuel for internal combustion engines, though. Much like coal dust and slurry, the combustion of aluminium and iron powder produces solid metal oxides. These oxides will coat the engine, wear it out faster, and eventually foul the pistons.

The irony is that those solid metal oxides are the key to renewability. They can be collected and recycled back into powdered fuel using existing infrastructure under wind and solar power. But the smaller they are, the harder they are to collect. And really, unless the recycling is efficiently carried out, metal powders aren’t really a good substitute for petroleum or diesel.

External combustion engines are a better application for metal powder fuel. The combustion system can do the dirty work at a safe distance. It can filter out the metal oxides with a cyclone and send only clean heat to the engine. Even more promising than aluminium is iron powder. It burns 1,000 Celsius degrees cooler than aluminium does, and it produces larger oxide particles that are easy to collect.

Although theoretically possible, a lot would have to change before we’re all driving (or being driven by) cars with, say, Stirling or steam engines that run on metal powder. Existing combustion schemes would have to be altered to support powdered metal fuel, and there would have to be a plan in place to collect everyone’s metal oxides for recycling. At the infrastructure level, nothing has been built that can convert metal powder at power densities comparable to fossil fuels.

It takes energy to make energy, and that includes making metal powder. Generally speaking, solid pieces of metal are either pulverized with machines, atomized with a stream of compressed air, or collected through electrolysis. Iron powder metallurgy in particular (Editor’s note: tee-hee) is typically performed using atomization or a reduction technique known as the sponge iron process. But these methods all depend on external energy. Atomization requires a gas, and the sponge iron process involves heating it up in a kiln to produce the intermediate sponge iron, followed by a lot of force to crush it into powder. At best, all of this processing could be done using renewable energy sources. At worst, it requires a fossil fuel.

Metal powder fuels alone won’t save us from our dependency on fossil fuels. No single type of alternative energy will. Most likely, it will take a combination of them working together to satisfy all the use cases before we can wean ourselves off of fossil fuels.

But if we make this shift, are we really just trading one non-renewable for another? Sure, iron and aluminium are fairly abundant, but we said the same thing about coal. Maybe once we’ve pulled all the metal out of the ground, we could last another couple of years or decades running on recycled oxides. Hopefully, we’ll have a new solution by then, like mining on the moon.