Environment

Metal makes for a promising alternative to fossil fuels

Metal makes for a promising al...
Stabilized flames of different metal powders burning with air, compared to a methane-air flame
Stabilized flames of different metal powders burning with air, compared to a methane-air flame
View 1 Image
Stabilized flames of different metal powders burning with air, compared to a methane-air flame
1/1
Stabilized flames of different metal powders burning with air, compared to a methane-air flame

Clean fuels come in many forms, but burning iron or aluminum seems to be stretching the definition – unless you ask a team of scientists led by McGill University, who see a low-carbon future that runs on metal. The team is studying the combustion characteristics of metal powders to determine whether such powders could provide a cleaner, more viable alternative to fossil fuels than hydrogen, biofuels, or electric batteries.

Metals may seem about as unburnable as it's possible to be, but when ground into extremely fine powder like flour or icing sugar, it's a different story. The simile is an apt one because the metal powders are similar to flour or sugar in more than particle size. Almost anything ground so fine will burn or even explode under the right conditions.

Grinding a powder so fine vastly increases the ratio between the surface area and the volume of the grains, so they burn very readily. In fact, they burn so readily that it's the reason why flour mills are so well ventilated. The slightest spark in floury air and a mill can blow up like a munitions dump. The same goes for sugar, metals, or even some types of rock.

This fact is already employed in a number of areas. Iron or aluminum, for example, can be ground up and turned into colorants for fireworks, solid rocket fuel powerful enough to lift a payload into orbit, or thermite that can burn hot enough to cut steel rails. What the McGill team hopes to do is harness this principle and turn it into a practical power source for everyday use.

The McGill team explains that metal powders aren't a primary energy source, like petroleum, but a storage medium for energy sources, such as nuclear or hydroelectric, which would be used to refine the metals into a pure, flammable form. The metal powders would be used for external combustion or heat engines, such as steam engines.

Under laboratory conditions, the team found that the flames produced from metal powders were quite similar to those of hydrocarbon fuels and they calculated that the energy and power densities of a metal-burning engine would be comparable to those of a conventional internal combustion engine.

The trick is to get the powders to burn in an even, steady flame. The team have mapped out a possible burner design that works by blowing air through a stream of metal powder. These combine and are injected into a combustion chamber. A cyclonic chamber separates the resulting metal ash out afterwards and clean nitrogen gas forms the exhaust, while the heat is used to run the engine.

The McGill team sees a number of advantages to using metal powders as fuel. Such powders would be transportable without the need for special tanks or cryogenic cooling. They are also much less bulky than hydrogen, and have higher energy density than batteries. Since they run heat engines, the technology can be scaled to be small enough for use in vehicles or large enough to run power plants.

Another plus is that metal powders are recyclable. As they burn, metal powders create stable, non-toxic solid-oxides that can be collected, refined back to pure metals, and used again with a minimum of carbon dioxide or other emissions.

If a metal powder engine does become practical, the McGill team says that iron will be the most likely candidate. Not only is it relatively cheap, but iron powders are already manufactured in the millions of tons for the metallurgy, chemical, and electronic industries. The main difficulty would be to ensure that iron refining is as carbon-neutral as possible.

The team is currently working on building a prototype burner that can be hooked up to a heat engine, as well as developing carbon dioxide-free recycling processes.

"We are very interested in this technology because it opens the door to new propulsion systems that can be used in space and on earth," says team-member David Jarvis, who is head of strategic and emerging technologies at the European Space Agency. "The shift away from fossil fuels for vehicle propulsion is a clear trend for the future. While not perfected and commercialized today, the use of low-cost metallic fuels, like iron powder, is a worthy alternative to petrol and diesel fuels. If we can demonstrate, for the first time, an iron-fueled engine with almost zero CO2 emissions, we believe this would then trigger even more innovation and cost reduction in the near future."

The team's research was published this month in in Applied Energy.

Source: McGill University

19 comments
bjh616
I read in New Scientist magazine 15-20 years ago, about a car running on metal. The only drawback according to the designer was the weight of the metal in the return tank. The metal itself had a similar energy per kg as petrol. I don't how the cylinders avoided being coated with the fuel metal. He used different metals as fuel. He kept the metal oxides and they could be reprocessed into fuel using electricity so the system could be renewable.
Gavin Roe
carbon isn't all bad, it is very light by comparison to iron or aluminum and collected can also be transformed
PadlokYknow
'recycling' metal oxides would take as much energy as the oxidation originally gave off, just sayin...
EcoLogical
Burning metals in an ICE is a bad, bad idea! You still get NOx and other toxins including particulate matter that will no doubt go out the tail pipe. A much better way to convert the energy in a metal is in a metal-air fuel cell. www.phinergy.com and www.alcoa.com are doing it with about 2 or 3 times (200% ~ 300%) the efficiency of burning and zero emissions (the metal oxide sludge is collected and recycled). All batteries use metals as fuels but they also contain all the reactants within the cell. It's like having an oxygen tank for the ICE and then an exhaust collection tank within the vehicle chassis. The only difference for a fuel cell is the reactants are external to the reactor. In an ICE the reactants are also external but the toxic byproducts are emitted out the tail pipe. It's those emissions that are causing enormous problems. Please, let's stop this 'burning' mentality and stop treating the air we all have to breathe as a dump!
Peter Kelly
Two issues here: First, "Another plus is that metal powders are recyclable." Let's see how well that's accepted when having to take the sack of fine ash to the recycle centre, and heaven help the potential lawsuit because someone is diagnosed with a rare cancer owing to exposure to it! Second, "The main difficulty would be to ensure that iron refining is as carbon-neutral as possible". You don't say? Apart from the fact that iron isn't refined at all, it is extracted from iron oxide using vast amounts of energy, usually gas, which I suspect would be better spent being piped to homes to heat them!
natosoco
This is pretty fascinating. I'd love to see something like this become mainstream.
ozAndy
I read about a system that used aluminium as an energy storage medium. Electricity is used to create the metal from the oxide. The engine then used Al metal wire plus water and a small current to split water and create hydrogen gas and Al oxide. This is done just prior to the carby or fuel injection system. The hydrogen is burned using oxygen from the air creating water in the exhaust which is then (mostly) recovered. This is probably where the "car that runs on water" BS came from. I'm not sure how efficient it is (Oxide must travel back to be stripped of oxygen so 2 way transport) but apparently a 600Km range was possible with only a few Kg of Al wire spool. The devil is always in the details but it sounded pretty cool and more viable than hydrogen as a storage medium.
Nik
A large proportion of coal fired power stations output was absorbed grinding the coal to dust, how much more enrgy would be used in grinding iron, after and including the mining and refining process? My guess is that it wouldn't be cost effective, when coal can be reduced to gas, and coke, much more easily.
Catweazle
Where does the energy come from to refine the metals in the first place? Unicorn flatulence?
MBadgero
The obvious difference is that fossil fuels are an energy source (although non-renewable) and metals are an energy storage technology. Many other synthetic fuels are possible.