Steelmaking: Part 1 #42
How the backbone of our society is made
September 1, 2022
It’s about time for the next deep dive series! I learned about steelmaking this time and am excited to share the learnings with you.
Steelmaking is a big thing. Our humankind produces 1.95 billion tons of it annually. Steelmaking alone accounts for 8% (!) of the global final energy demand and emits 2.6Gt of CO2 annually.
In this Part 1, I’ll cover the basics of steel, the climate impact of steelmaking, and the conventional pathways of steelmaking.
In Part 2 of this deep dive series, we’ll explore the technologies that can help decarbonize these existing pathways and create entirely new pathways of steelmaking!
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🏗️The backbone of our society
Steel is all around us. It is used in buildings, infrastructure, cars, trains, cutlery, you name it.
Our humankind loves steel so much that we produced 1.95 billion tons just last year! And the steel demand is expected to increase by more than a third by 2050.
To put things into perspective, check this out: We mined 3.2 billion tons of metals in 2019. 3 billion tons (94%) of those mined metals were iron ore, which we use to make steel. Try to find that tiny pile of lithium on the chart.
🪨What is steel?
Steel is an alloy, a combination of iron and carbon.
A small amount of carbon improves the strength and fracture resistance. Typically, steel contains less than 0.25% carbon of the total weight.
If you ask why carbon and iron together form a strong material, check out this video by Billy Wu from Imperial College London.
🌏The climate impact of steelmaking
Steelmaking is an enormous burden on the climate. It is extremely energy intensive and produces CO2 emissions.
Steelmaking accounted for 8% of the global final energy demand. It produced 2.6 Gt of direct CO2 emissions in 2019, around 8% of the global CO2 emissions.
Every time we produce one ton of steel, we emit, on average, 1.85 tons of CO2 into the atmosphere.
To understand why steelmaking produces so much emissions and how we could decarbonize it, we must first understand the steelmaking process.
➡️Metallic inputs in steel production
1. Iron ore
Iron is the primary “ingredient” in steelmaking. Iron occurs naturally in iron ores on Earth’s crust. Typical iron ores are hematite and magnetite. Iron is chemically bonded with oxygen and sometimes with other elements in iron ore.
Metallic iron can be extracted from iron ore for steel production. If iron ore is the main input, we refer to these operations as primary production.
2. Steel scrap
Steel is recyclable, and we are already re-using it. On average, 85% of end-of-life-steel is collected for recycling globally. The recycling rates are higher for vehicles and industrial equipment, and lower for packaging and reinforcing steel (rebar).
When steel scrap is used as the metal feedstock in steel making, we refer to this as secondary production.
⚒️Commercial routes of steelmaking
There are currently three main ways of making steel:
- Blast Furnace - Basic Oxygen Furnace (BF-BOF)
- Scrap - Electric Arc Furnace (Scrap-EAF)
- Direct Reduced Iron - Electric Arc Furnace (DRI - EAF)
I initially struggled to grasp the different steelmaking routes. Thus, I created this illustration that you can refer back to when you continue reading.⬇️
1. Blast Furnace - Basic Oxygen Furnace (BF-BOF)
70% of the steel is produced via the BF-BOF route, making it the most common process for producing steel today.
First, 1) iron ore, 2) limestone, and 3) coal are pre-treated and fed into a furnace with as high temperature as 2000°C / 3600°F.
The pre-treated coal, coke, helps to extract metallic iron from iron ore. The coke is ignited, and it turns into CO2(😖) and carbon monoxide (CO). The carbon monoxide then reacts with iron ore and forms metallic iron and carbon dioxide. Carbon monoxide is also referred to as the reducing agent, because it reduces the iron ore (=steals oxygen from the iron ore).
Limestone is used to get rid of silicate materials found in iron ore. When limestone is heated, it turns into lime (calcium oxide) and CO2(😖). The lime reacts then with silicate materials and forms molten slag on top of the molten iron.
Next, the molten iron is put into another hot furnace called the Basic Oxygen Furnace. Oxygen is blown into molten iron, where oxygen reacts with impurities such as carbon, silicon, phosphorus, and manganese.
2. Scrap - Electric Arc Furnace (Scrap - EAF)
25% of steel is produced via the scrap - EAF route.
First, steel scrap is fed into an EAF. In an EAF, a powerful electric current is passed through the furnace via electrodes, generating an electric arc. The resulting heat melts the steel scrap.
Then, lime, carbon, and oxygen are added. They combine with the impurities of the steel scrap and form slag. The resulting slag is then easy to separate from steel.
If you are interested in learning more about the BF-BOF and Scrap - EAF pathways, I recommend checking out the video below.
3. Direct Reduced Iron - Electric Arc Furnace (DRI - EAF)
5% of the global crude steel is produced via the DRI-EAF route.
DRI is sponge iron that is produced from iron ore via a direct reduction process. The iron ore is not melted, so the direct reduction requires a lower temperature (1000°C / 1800°F) than a blast furnace (BF) (2000°C / 3600°F).
In direction reduction, the iron ore is exposed to a gas mixture of carbon monoxide and hydrogen, which turns iron ore into sponge iron. The gas mixture acts as the reducing agent, as coke does in a blast furnace. The gas mixture is usually derived from fossil fuels, like natural gas or coal.
The resulting sponge iron is then further processed into steel in an EAF.
In Part 2 of this deep dive series, we’ll dive into the decarbonization strategies for steelmaking. And yes, you guessed it right. These decarbonization strategies involve some awesome technologies!
Huge thanks to Andreas Weber, Founder at Ferrum Decarb, and Maija Luukka, Founder Associate at Foresign Data Machines, for helping me understand steelmaking!
As always, I am more than happy to hear feedback and connect with fellow climate people at firstname.lastname@example.org.
If you liked this article, please share it with your climate friends!🌍
- Chemistry LibreTexts (2020). Metallurgy of Iron and Steel. Link.
- Hay, T., Visuri, V.-V., Aula, M., Echterhod, T. (2021). A Review of Mathematical Process Models for the Electric Arc Furnace Process. Steel Research International, 92(3). Link.
- IEA (2020), Iron and Steel Technology Roadmap, IEA, Paris. Link.
- McKinsey (2020). Decarbonization challenge for steel. Link.
- World Steel Association (2022). About steel. Link.
- Fan Z. & Friedmann, S. J. (2021). Low-carbon production of iron and steel: Technology options, economic assessment, and policy. Joule, 5(4), pp. 829-862. Link.