We burn coal in boilers to produce heat which is used to make steam and electricity. But, burning coal produces copious amounts of carbon dioxide. Is there any other material that can replace coal in boilers?

Yes, say scientists. Iron.

The use of metals as energy carriers is being investigated seriously by the scientific community. Among them, iron seems to have become the darling because of its properties — abundant availability, high energy density in terms of volume. Its energy density is slightly higher than coal. A cubic metre of iron (7.8 tonnes) packs 16.1 KWhr of energy; comparatively, a cubic metre of coal (1.6 tonnes) can hold 15.8 KWhr of energy. This means that if you burn a certain quantity of iron, it will produce more heat than if you burn the same quantity of coal.

Scientists are investigating other metals also. While iron seems to be hot, other metals that could be good carriers of heat are boron, magnesium, aluminium silicon and titanium.

All these metals can be used as energy carriers and turned back into metals as many times as required.

Making energy carriers

Now, the question is, how do they become energy carriers. The answer is simple. To take iron as an example — we know that the metal exists in two oxide forms — magnetite (Fe3O4) and hematite (Fe2O3). To get pure iron from these ores, oxygen needs to be removed with a (reduction) process that takes in energy. Once you have energy that needs to be stored, this can be used to extract iron from the ore. This iron is now an energy carrier which if you put it in a furnace and burn it like coal, it produces heat.

The next question is, even if you have energy—such as surplus wind and solar—how do you reduce the ore, that is, extract iron from iron oxide? Conventionally carbon (coke) is used, but a greener alternative would be to use hydrogen as a reducing agent. The problem with green hydrogen is that, as of now, it is pretty expensive.

Researchers Paulo Debiagi et al of the University of Darmstadt, Germany, note in their paper published in Applications in Energy and Combustion Science, that innovative technologies for reducing iron are emerging. One such technology involves the removal of oxygen in the iron ore by passing an electric current through it, either in an aqueous solution or through high temperature (over 2,0000 C) molten oxide electrolytes.

In due time

The main benefit of using metals as energy carriers is that they can be long duration energy storage (LDES) systems — lots of energy can be stored for months or even years. Secondly, if green hydrogen is in abundance you don’t have to convert all of it into green ammonia and end up in a glut — you can use the green hydrogen to make pure iron.

However, there are questions such as economic feasibility around this idea. Nobody is saying that this is a mature technology—only, it is a technology whose time is just around the corner.