Look around, and you’ll see it everywhere. From towering skyscrapers to cars, almost every man-made structure is made at least partially from steel. Whilst it is certainly a great material for construction, it is also extremely unsustainable - in fact steel production is one of the world’s leading sources of greenhouse gas emissions, and as you might realise, this has to change to avoid a climate disaster.
All is not lost however, as there are already solutions springing up around the globe, with the most promising being direct reduced iron using hydrogen used along with electric arc furnaces.
How steel is currently produced
Direct reduced iron using hydrogen
The electric arc furnace
Is it really green?
How steel is currently produced
Before we delve into how hydrogen direct reduced iron works, let’s see how steel is normally produced.
Liquid iron being removed from a blast furnace
Iron ore contains a common type of iron oxide (haematite - Fe2O3), as well as impurities such as silica (a major component of sand - SiO2). This ore is extracted and put into a blast furnace, along with limestone (CaCO3), and carbon-containing coke (no - not the fizzy drink, a purified form of coal). Hot air is blasted into the furnace, supplying energetic oxygen which reacts with the carbon (coke) to produce carbon dioxide. This reaction generates more heat, giving energy to all of the materials in the furnace. Next, more carbon reacts with the carbon dioxide to produce carbon monoxide gas (CO), which is a good reducing agent (ie, it is good at taking oxygen from other substances). This carbon monoxide then reacts with haematite (which is liquid at these high temperatures) to produce carbon dioxide, and liquid iron (Fe). This is called a redox reaction, as the CO is oxidised to CO2 (it gains oxygen, and this gained oxygen loses electrons), whilst the haematite is reduced to iron (it loses oxygen / gains electrons). Alternatively, at such high temperatures the carbon may react directly with the haematite, producing carbon dioxide and liquid iron. The limestone does not actually affect the iron, instead it thermally decomposes to form calcium oxide (CaO) and carbon dioxide. The Calcium oxide then reacts with the sand to remove it in a neutralisation reaction, forming liquid calcium silicate, aka slag.
The slag floats on top of the iron, and is removed and used for building roads. The iron itself still contains relatively high amounts of carbon, and so it is called pig iron - it is refined later to steel in the following way: It trickles to the bottom of the furnace and is removed (tapped). Oxygen gas is then bubbled through the mixture of iron and carbon, reacting with the carbon to form carbon dioxide, which then floats away. The iron is not re-oxidised because carbon is more reactive than it, and so the carbon reacts with the oxygen rather than it. After this has happened, we are left with liquid iron with a small percentage of carbon. Any other desired elements (such as chromium) are added, giving us the desired type of steel.
As you can see, there are large amounts of carbon dioxide emissions from this process. To resolve this issue, and prevent a climate catastrophe, the steel making system must become more sustainable.
Direct reduced iron using hydrogen
With this in mind, let’s discuss hydrogen direct reduced iron. Rather than using a blast furnace, this method uses a structure called a shaft. Iron ore pellets, along with heated hydrogen, are pumped into the shaft. At these temperatures, the iron ore pellets are still solid, but there is enough energy in the system to facilitate the reduction of the haematite by the hydrogen, though this occurs in several stages. Firstly, the haematite reacts with the hydrogen to form magnetite (another type of iron oxide - Fe3O4), and water. The magnetite then reacts with more hydrogen, forming ferrous oxide (yet another type of iron oxide - FeO), and water. Finally, the ferrous oxide reacts with yet more hydrogen to form iron and, you guessed it, water. However, you may have noticed that this so-called ‘sponge iron’ is not pure, it still contains impurities like silica from the iron ore. To remove these impurities, the sponge iron is moved to an electric arc furnace.
The electric arc furnace
An electric arc between two electrodes
Electric arc furnaces are already commonly used, however they cannot replace blast furnaces entirely by themselves as they effectively recycle used steel, meaning that the steel output is of an inferior quality. The addition of sponge iron resolves this issue, increasing the steel’s quality. Large electrodes in the electric arc furnace ionise the steel and iron, creating an arc of electricity which passes from one electrode, through the scrap, to another electrode. The heat created by this process melts the sponge iron, along with the pre-used steel that has been added to be recycled, and just like in the blast furnace, limestone is added to remove the silica impurities, which are then removed as slag. Unfortunately, the molten iron and pre-used steel mixture is still not high quality steel, as there is too little carbon. High quality steel contains a small percentage of carbon, as well as lots of iron, and in some cases other metals such as chromium. Therefore, a suitable amount of carbon must be added to the molten mixture, and once this has been completed, the high quality liquid steel can be tapped (removed).
Is it really green?
At this point, you may be questioning how green this whole process really is. After all, carbon has to be added to the electric arc furnace, the removal of silica produces carbon dioxide, there’s an awful lot of energy that’s required to heat the system, and the hydrogen could be produced unsustainably from natural gas.
It is true that carbon must be added to the electric arc furnace to produce steel, but at least half of this does not turn into polluting carbon dioxide - it is simply integrated into the steel. The carbon dioxide that is released by this process and the removal of silica is polluting, but there is significantly less of it released than with a blast furnace where carbon is involved at every stage. Furthermore, a large portion of this could be removed by carbon capture processes, and so this is a generally sustainable low (if not 0) carbon process. As for the hydrogen production, rather than using natural gas, electrolysis of plentiful salt water using renewable energy sources would provide a source of green hydrogen. For heating, renewable energy sources could also be used, along with efficient heat recovery stations that transfer heat from the water vapour leaving the iron reduction shaft back to the shaft. Additionally, the process also recycles used steel in the electric arc furnace, further benefiting this system’s environmental credentials.
As global concerns over sustainability grow, the steelmaking industry will be forced to adapt, and with several countries like Sweden already adopting the process, hydrogen direct reduced iron along with electric arc furnaces just might be the future of iron and steel production.
To learn more about sustainable technologies, whether they are on a domestic scale, industry wide, or even global and beyond, you can check out other articles on this blog.
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