Could we burn iron for energy instead of fossil fuels?

According to Wikipedia, iron ore is basically various oxides of iron, and humans use energy to de-oxidize it.

So if you burn iron you're basically un-doing that process.

Iron might be feasible as an energy storage system. Article says it has low specific energy so I'd question if it could ever really be feasible for transporting energy or useful as a vehicle fuel (for ground vehicles let alone aerospace).

If new geosurvey technology revealed a bunch of giant veins of pure un-oxidized iron, or if we start replacing all our steel buildings with plastic or carbon nanotubes or something and producing so much metal scrap and recycled steel that it starts ending up in landfills? Then it might be worth extracting the energy from iron. But neither of those scenarios seems particularly likely.

Also I'd be worried about temperature issues. If your iron burning engine is going to be, well, running hot enough to burn iron, you probably shouldn't make it out of steel. You'd need a material that's more expensive and harder to work. It's doable but seems like it would cause annoying fabrication issues and increase costs.

Question to those who are better educated: I seem to remember from school several decades ago that one of the problems of iron production is reducing the carbon content of iron, which takes a tremendous amount of energy. Would that not negatively impact the CO2 balance of iron as a fuel?

Energy from burning iron is about tenth of energy from burning gasoline. Lithium ion batteries are also a tenth of gasoline, and they are reusable. It is also a lot easier to distribute energy as electricity than iron dust. It takes energy to refine iron or to turn iron oxide back into iron, which would be less efficient than using electricity directly.

Finally, manufactured fuels like hydrogen or ammonia should do the job for places like ships that can’t use electricity.

Can you produce electricity from the process directly? If you cannot use a fuel cell, the efficiency and usefulness drop quickly. There are better metals we could use e.g. sodium. Sodium (Na) can under circumstances outlined in the expired US patent US3730776A react with water (H2O) where there is no flame but instead flow of electrical current. You get a sodium hydroxide (NaOH) solution + a lot of energy + oxygen if you do it well, if you do it less well, you also get hydrogen and less energy.

The well known Castner Process can be used to split molten salt NaOH and get sodium, hydrogen and oxygen in the process. That does work under lower temperatures than the electrolysis of NaCl which is more efficient if you only look at this part of the cycle to get sodium. If you use sodium as fuel, the cycle as a whole is important for recycling. There is plenty of sodium in salt as all the oceans are salty and there are salt mines in many places. Also, it melts at about 100°C so you can pump it if you can keep it at about the temperature of boiling water. Yes, NaOH is caustic but neutralizes quickly in the nature which cannot be said for oil spills. You can keep NaOH in normal steel containers. For instance, it is common to use NaOH when cleaning clogged toilets. However you wouldn't pour gasoline down the drain. Obviously, iron is very dense, however in a useful form for burning, you need iron powder or thin threads/ foil. We would need to electrify iron foundries, which is hard to do completely as e.g. the arc furnace uses graphite electrodes. (I have worked at a steel foundry.)

You can read more about the sodium fuel cell and the context here: https://orgpad.com/s/5BfLP-cxj-7

Sorry if dumb question but how would that compare to using the iron for energy storage in an Edison battery?

The idea is obviously to use iron for energy storage. You grind the iron until it is a fine dust, and store the dust somewhere. When the time comes, you burn the iron in a power plant. Modern coal power plants burn the coal in a fluid bed, so probably some retrofitting can be done, to switch from particles of coal to particles of iron. The idea is that while burning coal produces CO2, burning iron produces iron oxide Fe3O4, which is a form of rust. You then store that rust, and when you have spare electricity (for example during the summer, when solar panels generate more electricity than there is demand), you remove the oxygen and return to iron powder. The idea would be that this type of energy storage is cheaper than other round-trip chemical reactions, the most obvious being burning hydrogen and then electrolyzing water.

The advantage of the hydrogen-based energy storage is that you don't need to store the output of the oxydation reaction, because water is all around us. The iron-based solution requires you to store lots of this rust, in large storage places. In many places storage is reasonably cheap, so this would not be such a big cost (but it would be an additional cost nonetheless).

With hydrogen, people have run the numbers. It's not easy. Not easy by a mile. It's very difficult to produce hydrogen out of water at a cost below the cost of the natural gas that would produce the same electricity. Even if you add some subsidies to account for the zero-emissions, it's still very difficult to make this work. Why? Mainly because of capital costs. To make a plant that produces enough hydrogen to feed a typical 1GW power plant you need billions of dollars. And you need that for every 1GW natural-gas power plant that you want to convert to hydrogen. It's doable in principle, but it was never done in practice. Nothing remotely approaching that scale was ever done, and it's not clear anything like that can be done this decade.

With iron, you need to replace the electrolysis with something close to an iron smelting furnace. We have lots of those furnaces around already, busy separating iron from ore. It's one of the toughest problems of the road to net zero: how do you make iron green? People say you can do it via green hydrogen, but then if you make the green hydrogen why would you not just use that for energy storage, rather than add one more step in your energy storage economy?

So, unfortunately, although I like a lot the idea of finding a cheap chemical solution for the electricity storage problem, I'm not that optimistic that iron can provide it, at least not in this form.

Zinc-air is mature technology with high energy density, you can get electricity directly without any fire risk and zinc is abundant enough. If we despite all that are unable to get it used beyond hearing aids, then what are chances for iron?

See also https://en.wikipedia.org/wiki/Chemical_looping_combustion

There's also a lot of waste aluminium, which might be usable in furnaces.