Wow, what's up with iron combustion?
November 14, 2020 7:39 PM   Subscribe

I recently saw an article about burning and then "un-burning" iron powder, allowing the same material to be burned over and over again as part of a renewable energy system. Dang, I have so many questions!

This sounds like a really exciting avenue of research, and I had never heard of anything quite like it. I don't really understand the science behind this, so I'm hoping some folks here might be able to put it into context. I'm going to roll off a bunch of questions that I'm specifically curious about, but really any existing resources that help explain this would be welcome.

I would guess there are some published articles from the university featured in the articles, but I haven't tried to look them up. They are probably well beyond my layperson's chemistry knowledge.

For background, here's the article where I learned about this: TU/e demonstrates iron fuel at brewery Bavaria, and here's another one from earlier this year: Iron powder as the battery of the future.

I'm just going to copy professor De Goey's quote from the first article, because I think it gets at the heart of what I find so interesting about this: "No CO2 is produced during combustion and only rust remains. It’s a circular process: you capture this rust powder and sustainably convert it back into iron powder."

I was really surprised that the process could be that reversible. Like, I'm used to thinking of combustion as a one-way chemical reaction. Are there other examples of this kind of "fully reversible combustion" that are more familiar? Science experiments or mundane commercial products that I'm not thinking of? Could we "recombine" gasoline from its combustion byproducts if we wanted to spend the energy on it, or is there something special about iron combustion? What makes this reversible, when reactions like battery charging and discharging tend to degrade the battery over time?

And, heck, how does iron combust anyway? Why doesn't it just melt? What even is combustion? Are all metals combustible under the right circumstances?

What temperature is iron powder combustion likely to run at? How does that compare with "consumer grade" combustibles, like gasoline or natural gas? It sounds like their focus is on industrial power generation (which makes practical sense), but what are the chances of personal-scale iron powder combustion? Like home heating or vehicle power?

What's the energy density of iron powder? How much energy per gallon (or whatever) would it contain? How does it compare with gasoline or natural gas?

Is there any prior work in this area that is well known? The articles make it sound like it's a brand new idea (outside of, like, fireworks) that we just never bothered to investigate because we didn't need to. Is that accurate?

Okay, there's probably more that I could ask, but I think that's enough. It's a big topic!
posted by Lirp to Science & Nature (17 answers total) 6 users marked this as a favorite
Metallic iron reacts with oxygen from the air to turn into rust on its own, at room temperature, albeit slowly. Burning is just doing it faster by giving it a boost (eg starting it at much higher temperature, say).

And then they are proposing to recycle (Reduce) it back in to the metallic fuel using hydrogen. The catch is that this isn’t free- running that reduction consumes energy, and extracting hydrogen from water consume energy. They are implying that the costs are worth it because they can all be done without carbon emissions.

(Sorry not going to go look up thermodynamic constants)
posted by janell at 7:53 PM on November 14, 2020 [5 favorites]

Are there other examples of this kind of "fully reversible combustion" that are more familiar?

Pass some current through water and you can split it into hydrogen and oxygen (aka electrolysis). Burn that hydrogen and you get water.
posted by pompomtom at 8:18 PM on November 14, 2020 [8 favorites]

> This iron fuel is a promising energy carrier

They point out it's an energy carrier, meaning an energy transport mechanism. A bit like a battery, or hydrogen as mentioned. As opposed to a fuel you extract from the ground and then burn, where you didn't make the fuel or capture the combustion byproducts.

(Ignoring losses.) So, you have 1 joule of energy from a windmill/solar panel, which you use to turn a tub of rust into a tub of metallic iron powder. Then keep it on site or ship it. Then months later, you combust the iron powder in a furnace, generating 1 joule of energy, which turns a steam turbine or something.

> Is there any prior work in this area that is well known? The articles make it sound like it's a brand new idea (outside of, like, fireworks) that we just never bothered to investigate because we didn't need to. Is that accurate?

No idea. It probably didn't make sense until we realized climate change was a problem. Try here:

> but what are the chances of personal-scale iron powder combustion? Like home heating or vehicle power?

Spitballing, I'd power your house or vehicle using electrical power, but then burn iron powder to generate electrical power at a municipal power plant. Or turn rust back into iron powder when there's spare energy from the grid.

On the individual level, manipulating powder is a pain in the ass, compared to liquids, or gasses. It jams, clumps, abrades, and so on.

Also, a home dweller or driver isn't going to want to transport the burnt iron oxide back to the shop.

> I was really surprised that the process could be that reversible. Like, I'm used to thinking of combustion as a one-way chemical reaction. Are there other examples of this kind of "fully reversible combustion" that are more familiar?

I'm shaky on this, but:
The word reduction originally referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was "reduced" to metal. Antoine Lavoisier demonstrated that this loss of weight was due to the loss of oxygen as a gas. Later, scientists realized that the metal atom gains electrons in this process. The meaning of reduction then became generalized to include all processes involving a gain of electrons.
Or maybe:
posted by sebastienbailard at 8:41 PM on November 14, 2020 [1 favorite]

This even happens in nature, with the iron battery allowing microbes to extract energy from wildly different metabolic talents. Dropping you in the deep end:

The Microbial Ferrous Wheel in a Neutral pH Groundwater Seep . Yes, biologists call it the ferrous wheel, there are T-shirts.
posted by clew at 8:42 PM on November 14, 2020 [10 favorites]

I believe the proposed applications are for industries that need a lot of heat, where existing components can be refitted to burn the iron rather than a polluting fuel. I don't believe the proposed plan is to use it to produce electricity. Too much energy is lost in the round trip. It's also not an efficient substitute for regular-old electric heaters. But that does still leave some possible applications for it.
posted by Winnie the Proust at 8:46 PM on November 14, 2020

At the molecular level, any chemical reaction is reversible. Yes, you could take carbon dioxide and water - the results of gasoline combustion - and, with the suitable application of energy, synthesize gasoline.

Here's a simple example you can do at home: Get a 1.5V battery and a couple of insulated wires. Stick one wire on each end of the battery and put the other ends in a glass of salt water, a few inches apart. You'll see bubbles coming up from each wire - from one wire, you'll see hydrogen and the other oxygen. You can tell which is which by the fact that water consists of twice as much hydrogen as oxygen, so the hydrogen side will put out twice as many bubbles as the oxygen side.

Burning hydrogen - i.e. combining it with oxygen - creates water and heat. Theoretically, you could do this back and forth forever - split the water into H and O, and then use those to run a generator that generates the electricity you're using to split the H and O.

But lest you get any ideas about perpetual motion, this process will always have waste heat - the amount of energy you get from burning the H and O will always be less than the amount of energy you used to split the water in the first place.
posted by Hatashran at 8:46 PM on November 14, 2020 [2 favorites]

Popular Mechanics had a brief overview of the brewery news, including: "The researchers also explain why this hasn’t really been explored in the commercial sector before. It’s simple: Fossil fuels were the right combination of plentiful and relatively cheap to dominate the energy market for decades. Even today, Eindhoven University says, iron powder is funneled through a scant handful of suppliers around the world. Production would need to be scaled up for wider energy usage." [Examples: Höganäs; US Metal Powders reps in the Americas for Jiangxi Yuean Superfine Metal]

Burning issues: Philip de Goey gets € 2.5 million for research into metal fuels (April 2020 article) has a Q & A with Prof. de Goey.

In 1896, Rudolph Diesel ran one of his engines with a powdered coal fuel... "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. [...] 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." (Are Powdered Metal Fuels a Flash in the Pan, 2016)

Energy consumption in powder metallurgical manufacturing (2012, Powder Metallurgy journal; there's also the Advanced Powder Technology journal);
Metal Powder: the New Zero-Carbon Fuel? ["In the iron economy, you'd retrofit coal plants to burn iron powder, then recycle it using renewable energy"] (2015 article); Could metal particles be the clean fuel of the future? (article); these last two (similar articles from around this time) focus on this exciting 2015 paper co-authored by McGill researchers and D.J. Jarvis (a European Space Agency scientist): Direct combustion of recyclable metal fuels for zero-carbon heat and power.

An April 2020 follow-up to that paper, again via the McGill Reporter, Metal makes for promising, recyclable alternative to fossil fuels: “Our idea is to take that rocket science and make it into clean tech,” says Prof. Jeffrey Bergthorson, Associate Director of the Trottier Institute for Sustainability in Engineering and Design at McGill. [...] While laboratory work at McGill and elsewhere has shown that use of metal fuels with heat engines is technically feasible, no one has yet demonstrated the idea in practice. So the next step toward turning the lab findings into usable technology will be to build a prototype burner and couple it to a heat engine.

Bergthorson’s lab aims to begin testing such a metal-engine prototype later this year. “The goal is to demonstrate that we can generate electricity from metal fuels without producing any carbon dioxide emissions. Once proven in the laboratory, the technology will be ready to scale up to become a commercial technology.” In parallel, Bergthorson’s group is building a consortium to develop a prototype metal-water reactor technology.

posted by Iris Gambol at 9:52 PM on November 14, 2020 [2 favorites]

The main reason iron and its oxides get contemplated for this kind of cyclic process rather than carbon and its oxides is that iron's oxides are all solid at reasonable temperatures, while carbon's are gaseous. That makes iron's oxides easier to capture and handle.

The tricky part, as is unfortunately pretty standard for this kind of breathless Breakthrough! reporting, is in persuading the oxide to let go of its captured oxygen so as to regenerate elemental iron for another trip through the fuel cycle. On a fundamental level, this is always going to require more energy supplied to the regeneration reaction than was released during the fuel's initial combustion. If you were to ignore all practical concerns it would require the same amount of energy released during combustion but the entire point of thermodynamics is that you never can ignore all practical concerns.

Turning iron oxide into iron has traditionally been done by smelting. As the Wikipedia article explains, this involves the use of a "reducing agent" which is traditionally carbon from coke or charcoal; the point of the reducing agent is that oxygen would rather be bound to that than to the iron, given the choice. Smelting therefore produces huge amounts of carbon dioxide.

Molten oxide electrolysis is an alternative that essentially replaces the chemical reducing agent with brute force electricity, making it feasible to run without involving carbon-based fuels. It's still a very high temperature process though, and I would expect the waste heat it makes to be considerable.

It will be interesting to keep an eye on the overall efficiency of energy storage technologies involving metal powders such as iron, boron and aluminium and see how these develop compared to those involving hydrogen.
posted by flabdablet at 10:25 PM on November 14, 2020 [2 favorites]

I'm used to thinking of combustion as a one-way chemical reaction.

Combustion is a one-way reaction in much the same way as rolling downhill is one-way movement.

A boulder sitting at the top of a hill has more gravitational potential energy than one at the bottom. Give the boulder at the top a bit of a push to add a little kinetic energy, and you easily create conditions where ongoing conversion of gravitational potential energy to kinetic energy becomes sufficient to keep the boulder pushing itself past all the bumps and friction it encounters as it rolls down the hill.

Similarly, a fuel at room temperature has more chemical potential energy than its oxides. Raise the temperature of the fuel a bit to add some thermal energy, and you easily create conditions where the ongoing conversion of chemical potential energy to thermal energy becomes sufficient to keep the fuel making its own temperature high enough to facilitate the reaction with atmospheric oxygen.

Just as the progress of a boulder down the side of a hill involves a certain amount of upward movement in the form of bouncing and vibration, the progress of a combustion reaction involves oxygen both combining with and splitting away from the fuel. Both reactions are ongoing. But the fact that the oxide has lower potential chemical energy than the separated fuel and oxygen do means that the products of the forward reaction end up dominating the end result.
posted by flabdablet at 10:40 PM on November 14, 2020 [1 favorite]

Another point to ponder on the "one-way combustion" thing: atmospheric oxygen can oxidize wood - a complex carbohydrate fuel - to produce (ideally) carbon dioxide gas and water (hydrogen oxide) vapor. This reaction releases energy in the form of heat.

Combustion also runs much faster at higher temperatures; a wood fire is essentially a physical arrangement of reagents that encourages some of the heat energy released by the reaction to raise the temperature of the remaining reagents, bringing on a positive feedback that keeps the reaction rate very high.

But where did the wood and the oxygen come from in the first place? From the reverse reaction - absorption of energy in the form of sunlight to allow water and atmospheric carbon dioxide gas to react and produce at first simple sugars and eventually complex carbohydrates, plus gaseous oxygen. You can see this happening all around you, inside every green plant, so it's definitely a thing - but it does need that solar energy input and it won't run unless that's supplied.

As long as it is supplied, the reaction products (wood and oxygen) remain relatively stable at the temperature at which this reaction is run; were this not the case, wood would be a much less useful building material, both for plants and for us.

Reactions such as combustion that release energy as they go are called exothermic (outward-heat) reactions. Those such as photosynthesis that absorb energy are called endothermic (inward-heat) reactions.

Reactions don't have to be exothermic to be self-sustaining: some will tend to run to completion while forcibly extracting heat energy from their surroundings, resulting in reaction products at lower temperatures than the reagents. One example of this that you might have encountered is those chemical "instant" cold packs for bruises and sprains, where you burst an inner bag of water held inside a sealed plastic bag full of urea crystals and the whole thing gets suddenly very cold.
posted by flabdablet at 3:12 AM on November 15, 2020

You know those little disposable hand warmer packets. They use iron powder to create their heat reaction. When you remove the outer wrapping, oxygen slowly diffuses from the outside air through the pouch material to generate the oxidation reaction that produces heat. There are other materials in the mix that slow down the reaction so they don't burst into flame. It's the same chemical reaction you are talking about but carefully throttled for speed.
posted by JackFlash at 9:41 AM on November 15, 2020 [2 favorites]

What's the energy density of iron powder? How much energy per gallon (or whatever) would it contain? How does it compare with gasoline or natural gas?

There are two separate but related measures you need to consider. First is energy density, which is how much energy per volume. So a "gallon" of iron powder has slightly more energy than a gallon of gasoline but in the same ballpark.

But the other consideration is specific energy, which is how much energy per weight. Iron powder has only one-tenth as much energy per pound.

So you could fill your gasoline tank with about the same amount of iron power or gasoline, say 16 gallons, and drive the same number of miles -- except the tank of iron would weight 1000 pounds while the tank of gasoline would only weight 100 pounds.

Because of iron's poor specific energy, this proposed scheme only makes sense if you can complete the entire burn and recharge cycle in the same place. You wouldn't want to generate iron power in one location and then transport it to another location for burning. The transportation cost because of the weight would be prohibitive.
posted by JackFlash at 10:06 AM on November 15, 2020

And to make matters worse, the rust that is the output of the reaction weighs more (you've added oxygen) and takes up more space (is less dense) than the iron power fuel you started with.
posted by JackFlash at 10:16 AM on November 15, 2020

Thank you to everyone for the answers! I marked a few "best answers" that I found especially interesting and/or helpful.

Iris Gambol provided a lot of really good links that are worth highlighting, including the Q & A with Philip de Goey that has a little more depth and discussion of the open research questions than most of the popular science articles I had seen. Also the connection to similar research at McGill and this IEEE Spectrum article that referenced it. And the history of coal powder fuel in internal combustion engines was great context.

Thanks again, everyone!
posted by Lirp at 1:33 PM on November 15, 2020 [1 favorite]

Also possibly of interest: chemical looping combustion that uses iron and iron oxide as intermediaries in a multi-stage fuel burner. This allows carbon dioxide to be made at much higher purity than is achievable just by burning fuels in air.

Air consists of 4 parts mostly non-reactive nitrogen to each part oxygen, and in straightforward air combustion, most of the nitrogen passes straight through the fire essentially unchanged. The rest makes oxides of nitrogen that are generally undesirable pollutants. Separating the carbon dioxide produced by the combustion from all that nitrogen needs some fairly tricky chemistry that's quite expensive both in energy and money terms.

The chemical looper makes almost pure carbon dioxide whose capture needs only cooling and compression. This is achieved by burning partially oxidized iron in air; this removes oxygen from the air and binds it as more completely oxidized iron. The flue gas resulting from that step is almost pure nitrogen. The iron oxide powder is then reduced using the carbon-based fuel as a reducing agent in a separate reaction, regenerating the original partial oxide. It's pretty neat.
posted by flabdablet at 1:39 AM on November 16, 2020

Just to add in that this makes sense mainly as a storage mechanism, as in your original comment, "as part of a renewable energy system". Wind is the dominant renewable energy supply, and is obviously intermittent (i.e. variable output). As more and more wind is built, there's oversupply of renewable electricity at some times, undersupply at others, so recently there's been much more focus on ways of storing this.

Pumped storage is the traditional way, but expensive and limited places to store large volumes of water. Batteries are being built extensively, but similarly expensive, and limited in total amount of energy stored. So there's more effort recently to be able to store large volumes cheaply. Hydrogen is getting a lot of attention recently, as you can sotre large volumes in a pressurised tank, but being able to store energy in solid form would have lots of advantages.

Ireland already can meet 65% of demand from wind (system maximum due to stability, although they're working on increasing this), at which point wind gets switched off, so that wind energy is lost. And a lot more wind is being built, so this can only increase. Using that lost energy to deoxidise Iron and store it for later cheaply would make a lot of sense. Similar happens, but not quite as often in the UK, and the rest of Europe.

Additionally, prices in Europe go negative fairly often (negative price in Ireland, UK, Belgium and France this Monday morning), so you could get paid to use electricity and store it for later, when you could sell it and get paid again. Efficiency doesn't really matter so much when prices are zero or less, just the ability to store large volumes for realtively long times cheaply.
posted by Boobus Tuber at 4:17 AM on November 16, 2020 [1 favorite]

Combustion of iron itself isn't a new idea at all - look up "thermic lance" for an application that's been around for a long time.

Another possible extension to this idea is the thermite process - this burns aluminium powder with iron oxide [yes, the waste product from iron combustion] , yielding metallic iron [the metal you started with] and aluminium oxide, which can then be re-smelted using electricity into metallic aluminium.
posted by HiroProtagonist at 6:51 PM on November 17, 2020

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