How green can steel be?
Took the train to work today? Turned on the radiator? Checked your phone? One particular material, above all others, plays an essential role in our daily lives: steel. There is no product, which isn’t itself made of steel, or which isn’t processed or moved by something made of steel.
Not all steel is the same though. There are a multitude of alloys (mixtures of the main element iron, carbon and other elements) and implementations. One aspect remains the same among all of them: steel production is very energy and CO2 intensive. The global steel production of approximately 1.9 billion tons is responsible for roughly 10 % of global CO2 emissions. In the ranking with countries that produce the most emissions, it would be in third place, just before India.
Seeing as we are completely reliant on steel (in most, especially mass intensive, applications it can’t be substituted) we are faced with a massive challenge considering the looming climate crisis. What makes producing steel in a ‘green’ way so difficult? And which side effects do we have to consider? Let’s get into it!
As mentioned above 1.9 billion tonnes of steel are produced globally. 25% of that are produced via the recycling route, meaning the remelting of steel scrap in electric arc furnaces. This route is comparatively low in emissions and could be, assuming the usage of green energy, climate neutral. But the other 75 % are produced in the primary way, which is the carbothermic reduction of iron ore. Carbothermic means that carbon reacts due to enormously high temperatures with iron oxide and converts into iron. This happens in blast furnaces around the globe. Iron ore and coke are delivered into the shaft-like oven. The carbon burns to carbon monoxide due to air blown into the furnace and undergoes the following reaction with the iron oxide:
Fe2O3 + 3CO (g) = 2Fe + 3CO2 (g)
That's how it's done nowadays. But how do we get to a sustainable way? One option would be to increase the share of recycled steel. Currently, recycled quantities aren’t nearly enough to cover the need. So let’s look at production out of iron ore again. One option (the only one currently discussed on an industrial scale) to decrease the CO2 output is the chemical reduction via hydrogen. In special furnaces' hydrogen (H2) is used instead of carbon monoxide (CO) as reduction gas, resulting in this reaction:
Fe2O3 + 3H2 (g) = 2Fe+3H2O (g)
Instead of climate damaging CO2, the byproduct is now gaseous water. The process itself is still being investigated in some industrial countries and will be available in the midterm. The biggest challenge is the enormous amount of energy required to produce hydrogen. It can be roughly calculated as follows. As shown in the chemical equation above, for every part Fe2O3 you need 3 parts H2 to achieve the desired reaction. As a result, we get two parts iron, therefore the ratio of produced iron to used hydrogen is 1.5.
Using the molar mass (as in the mass per amount of substance) we can calculate the required amount of hydrogen per ton of produced pure iron. Afterward, small amounts of carbon will be added to achieve the needed qualities.
A ton of pure iron therefore requires at the minimum 54 kg hydrogen. This is the thermodynamic minimum, meaning it is impossible to achieve this reaction under this value. Any real process will require more than that. If the hydrogen would be produced in a green way, the process would theoretically be climate neutral. The currently prevalent method for producing green hydrogen is electrolysis. Here, water is broken down into its building blocks of hydrogen and oxygen. The energy source is usually electricity, and if that was supplied CO2 neutrally, we speak of green hydrogen.
Let’s have a look at the situation for the Austrian industry. Production of raw steel in Austria weighs in at about 7.2 million tons annually. Due to process related as well as recycling management reasons, the usage of scrap contributes roughly 20% of the overall amount. Considering the scrap usage, we are left with an amount of about 5.8 million tons of raw iron produced annually, which will be further processed to steel.
Using the results of our previous calculation, we can deduce that we would need about 312 000 tons of hydrogen. Hydrogen has a massive energy of 33.6 kWh/kg. Using electrolysis to produce it, you have to account for a conversion loss. The current efficiency factor is approximately 70%. Multiplying these values gives us a minimum amount of energy in the form of electricity of 15 TWh. If you wanted to cover this demand using wind energy, and assuming full load values of 2300 hours per year, we would need 1627 wind turbines with 4 MW. This is as mentioned an absolute minimum, so realistically it would be more.
Moreover, we only looked at the reductions step, there are more processing steps in a steel mill. There is ore processing, raw iron and steel production as well as casting-, rolling- and refining processes. If you want your steel mill to be producing climate neutrally, all energy and heat input would need to be covered by green energy and gas. The Austrian steel producer voestalpine published an energy demand of 33 TWh for the long term conversion of its mills.
Is that a lot? Yes, it is an enormous amount. The planned expansion of renewable energy in Austria until 2030 is only 27 TWh. So what to do? Should we stop producing steel? That is absurd, since we cannot substitute steel products, especially in its infrastructure application. That leaves us with the option of producing it climate neutral in the long term.
We have to acknowledge the physical limitations and optimize our energy usage, especially in cross-industry interactions. The fundamental laws of the universe dictate limitations for our technological advancements. The goal of intelligent optimization is to come as close to these limitations as possible. Additionally, we need another massive expansion of renewable energy sources.
There is some consolation: the global community will come close to a point of saturation in regard to recyclable material. This includes steel. Therefore, we will need less “new” steel which has to be produced from ores, which entails a significantly lower energy demand. Until then the aforementioned hydrogen reduction remains, at least likely, our best bet for green steel.