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Observing the renewable energy transition from a European perspective

Iron Powder as a Fuel

Project SOLID of the University of Eindhoven/the Netherlands. Burning iron from [0:44]

The world of science and technology is wrestling with the question how to power the engines of the future, post fossil fuel. Batteries, hydrogen fuel cells, biomass, exotic fuels like ammonia, methanol and several others. There is one overlooked possibility though: iron. Few people realize that iron can burn, a process also known as oxidation or “rusting”. If you have fine iron powder at your disposal, burning can go really fast:

Researchers at four universities around the world, Eindhoven (NL), Bochum (D), Orleans (F) and McGill (CA), are working on the possibility of metal powder-as-a-fuel, notably iron. The idea is to burn iron powder in an external combustible space and use the generated heat to drive an engine, for instance a Stirling engine or Rankine cycle-based generator, see video at the top of this post:

[wikipedia.org] – Stirling engine

Stirling engines have a high efficiency compared to internal combustion engines, being able to reach 50% efficiency. They are also capable of quiet operation and can use almost any heat source.

[wikipedia.org] – Rankine cycle

McGill University in Montreal is also busy researching the possibilities of metal powder as fuel:

Fuel Specific Energy MJ/kg Specific Energie kWh/L
Petrol 46.4 12.9
Iron 5.2 11.3
Zinc 5.3 10.6

[wikipedia.org] – Energy density

[teamsolid.org] – Technical University Eindhoven SOLID project site
[tue.nl] – Iron powder: a clean, alternative fuel for industry that has to quit natural gas

[deingenieur.nl] – First System to Use Iron Powder as Fuel Has Been Built

Why is iron so suitable for this process? ‘Firstly, iron has a high energy density, and burns at a high temperature of up to 1,800 °C… Some industrial processes need temperatures of up to 800 or 900 °C, which is way beyond the scope of heating air with electricity via heat pumps’… For example, iron powder can be made with different shapes of grain, but it has not yet been determined which shape is most suitable… Another challenge the team has to deal with when scaling up is handling the emissions generated by the process. NOx is released at such high temperatures, and possibly also particulates, and both will have to be filtered… The most important obstacle is perhaps the unfamiliarity of iron as fuel. Although some four universities around the world are carrying out research into metal fuels, it’s really unknown territory for the students.

[sciencedirect.com] – Direct combustion of recyclable metal fuels for zero-carbon heat and power

Metals are promising high-energy density, low-emission, recyclable energy carriers…. Metal fuels, produced using low-carbon recycling systems powered by clean primary energy, such as solar and wind, promise energy densities that are competitive to fossil fuels with low, or even negative, net carbon dioxide emissions… This paper proposes a novel concept for power generation in which metal fuels are burned with air in a combustor to provide clean, high-grade heat… The metal-fuel combustion heat can be used directly for industrial or residential heating and can also power external-combustion engines, operating on the Rankine or Stirling cycles, or thermo-electric generators over a wide range of power levels… The energy and power densities of the proposed metal-fuelled zero-carbon heat engines are predicted to be close to current fossil-fuelled internal-combustion engines, making them an attractive technology for a future low-carbon society.

[tue.nl] – Iron powder clean alternative

On an industrial scale, fuel cost will be double that of fossil fuel. But if the cost of CO2-emissions are factored in, this increased cost could be bearable… The (TUE) students developed a 20 kW installation that burns iron and produces hot water and electricity via a Stirling engine. The next step will be 100 kW installation.

[ieee.org]

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.”… 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.

[metalot.nl] – Iron powder as fuel

In the future these so-called metal fuels will provide our coal-fired power stations and cars with the energy they need… The volumetric energy density of iron powder is at least three times higher than that of hydrogen’…‘And you do not have to transport this powder under high pressure or extremely low temperatures.’… by burning it to rust powder in an external combustion engine. You can also use it to store solar energy, according to postdoc Yuriy Shoshin. ‘We can already convert solar energy into hydrogen. Then we use the hydrogen to reduce rust powder to iron powder.’… iron is cheap, easily manageable and reusable. Shoshin: ‘We still have to adjust the reduction techniques to the process, but the reactions are known.’… ‘We expect to be able to reuse the iron for about a hundred times.’… But how can you derive energy from iron powder? ‘You burn it’, says Shoshin. First you distribute the iron powder in the air by means of an electrical field. Then a small spark activates the reaction of oxygen and iron in the air. The iron oxidizes into iron oxide. That reaction warms up the environment, which causes other iron particles to oxidize. ‘This reaction is similar to what happens in coal-fired power stations’, says Shoshin…. The researcher are still looking for a way to collect the rust particles after use, otherwise it will be difficult to reuse them. The Goey is now considering filtering, because with sizes of 1 µm the particles are quite easy to catch… In order to make the combustion easier Shoshin wants to use iron particles in the shape of a sponge in the future. ‘This morphology is generated during the reduction of iron and creates a larger surface. This makes the iron more reactive… Pouring this fuel into a normal combustion engine does not seem to be an option. The powder would get caught between the cylinder and the piston and this friction would cause the engine to break… At this time we are considering an external combustion engine or some kind of steam system similar to those used in the coal-fired power stations.’… Even though the technique still needs to overcome some obstacles, metal fuels are already drawing the attention of companies. De Goey is in contact with a coal-fired power station willing to test whether iron can replace coal. The people from Eindhoven think metal fuels will become indispensable in a few years time. ‘We really have to get rid of the coal, and metals are a good alternative’

[mcgill.ca] – Metal particles as the clean fuel of the future?
[cbc.ca] – Metal as fuel? Canadian scientists busy to make it happen
[vattenfall.com] – HYBRIT: Pilot Plant for creating Fossil-free steel
[newscientist.com] – Electrolysis may one day provide ‘green iron’ (2006)
[newscientist.com] – Powdered metal: The fuel of the future (2005)
[tue.nl] – IJzerpoeder: schone brandstof voor industrie die van het gas af moet
[metalot.nl] – Iron powder as fuel
[springer.com] – Electrolysis of iron in a molten oxide electrolyte
[wikipedia.org] – Donald Sadoway

Site comment: the advantages are obvious: iron powder is very easy to store, handle, trade and transport. One can achieve high temperatures during burning and heavy batteries are not necessary (but iron powder as fuel in a vehicle is rather heavy as well). However, the links above provide only material about the burning of iron oxide. What they don’t do is give information about the required reduction of iron-oxide to iron to make the complete cycle work. The efficiency of that process is crucial to the success of an iron-based fuel cycle. Don’t open that champagne bottle yet though:

[twitter.com] – Donald R. Sadoway
Sadoway’s molten oxide electrolysis makes liquid iron at 2.5 to 3.5 kWh/kg. Plus tonnage oxygen by-product!

Burning iron powder yields 5.2 MJ/kg or 1.44 kwh/kg, see table above. In other words, electrolysis round-trip efficiency is not that great: 41-57%. Note that this applies to efficiency of transforming molten oxide in molten iron. Additionally you must heat your oxide powder and next somehow convert molten iron into iron powder, which inevitably will come at additional energy cost.

[source] Applications of metal burning

[source] Energy content of diverse metals

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