Steel

Steel is an indispensable construction material today and a critical component of building and transport infrastructures. However, even with the emerging shift among some steel producers towards 100 per cent renewable energy in the production phase, and although steel is very well suited to recycling (potentially reducing up to 75 per cent of embodied carbon), the highest goal is to avoid the use of steel in buildings where possible and to shift to proven low-carbon alternatives, since steel is a non-renewable material and demand is increasingly outpacing the supply of recycled steel sources.

Global Steel Use in Buildings and Infrastructure Is Rising

More than half the world’s steel is used in the construction of buildings and infrastructure.

Steel is the second most abundant material used in buildings, at 360 million tons in 2008 (Cullen, Allwood and Bambach 2012). It is perhaps the building material that is most associated with modernisation and is a cultural indicator of economic progress, given its role in developing infrastructure. Annual steel production in 2021 was 1,950 million metric tons, with current growth rates of around 3 per cent (World Steel Association 2022). Production is anticipated to increase substantially by 2030 (IEA 2020).
‍
Of the total iron and steel produced worldwide, 55 per cent goes into the built environment sector, split across buildings (33 per cent) and infrastructure (22 per cent) (Cullen, Allwood and Bambach 2012). In commercial and tall buildings, steel is used for the primary structure as well as to reinforce concrete. It is also used widely as the primary material for fitting out mechanical systems for heating, ventilation and cooling (HVAC).
‍
As a primary and preferred structural building material, steel combines tensile strength with low cost, but it can also come with a high human cost. There are many points of potential forced labour along the steel supply chain due to the hazardous conditions and lack of transparency, ranging from extraction and smelting to production, rolling and erecting (Grace Farms Foundation 2022).

Steel Is Emission Intensive, Driven by Blast Furnace Technology

Unless policies incentivise greater material efficiency, cost structures will favour using more material.

The iron and steel sector is energy and emissions intensive, accounting for 8 per cent of global final energy use and 7 per cent of direct energy-related CO2 emissions (IEA 2020). In steel production, most emissions are generated during three processes: when steel is produced from primary raw materials (using a blast furnace or basic oxygen furnace), when carbon is needed as a reducing agent (provided as coke derived from coal, releasing CO2) and from the energy used to heat the melt.
‍
A number of technical options exist for increasing the material efficiency of steel (thereby reducing use and overall emissions). These include: adopting lightweight design, reducing yield losses, diverting manufacturing scrap, re-using components, creating longer-life products and intensifying steel use (Allwood et al. 2013; Raabe, Tasan and Olivetti 2019). An example for extending the building lifetime is using galvanised steel for rebar in concrete, since galvanising protects the steel from corrosion and therefore avoiding the risk of failure. However, unless policies incentivise greater material efficiency, existing cost structures will tend to favour more material over less labour (Allwood 2013).
‍
There are two main approaches to reduce carbon emissions from steel production: 1) to continue to use carbon-based methods but to couple this with carbon capture technologies, and 2) to replace the carbon (coke) used in reduction, the chemical conversion of iron ore into pig iron, with alternative reductants such as hydrogen or direct electrolysis (Rissman et al. 2020). Moving to renewable energy sources in production offers the greatest potential to reduce the embodied carbon of steel (Raabe, Tasan and Olivetti 2019).

Figure 5.8 The time delay in the recycling of metals

The long lifetime of steel products has limited the amount of scrap available.

Note: Most metals have an average lifetime of around 20 years before they become available for recycling, and even longer when used in buildings. These long lifetimes combined with high growth rates in the past explain why metals will continue to be made primarily from virgin materials rather than from scrap. Source: UNEP 2011.

CIRCULAR STEEL

Making Steel From Scrap Will Reduce Emissions, But Recycling Rates Are Already High

Making steel from scrap saves 60-80% energy, but the scrap supply is limited.

An alternative way to produce steel is by using scrap as a raw material (“secondary production”), which occurs in an electricity-powered electric arc furnace. Producing steel from scrap requires 60-80 per cent less energy than primary steel production (UNEP 2013; IEA 2020) and does not entail chemical reduction (hence no input of coke, or heated coal). These massive energy savings also result in cost savings for producers; thus, the use of scrap as an input material is already very high, leaving only modest room for improvement. In certain markets, steel is already being recycled at over 90 per cent (IEA 2020).
‍
The biggest challenge to wider use of secondary steel production is the limited amount of scrap. A large gap exists between the supply of recycled material and rising demand. As a result, 35 per cent of all steel is made from scrap (World Steel Association 2023). The reasons for the limited amount of scrap are the long lifetime of steel products (20 years or more in buildings), combined with the rapid growth in demand in recent decades (see Figure 5.8). As long as demand continues to rise, the gap between scrap supply and demand will further widen, preventing the major carbon benefits from using scrap rather than virgin metal. This concept also applies to long-lived materials such as concrete, plastics and glass.

KEY STEPS TOWARDS

Decarbonizing Iron and Steel

Improve the quality and collection methods of scrap steel

In a circular steel economy using only scrap, measures would be needed to minimise contamination.

Address concerns about the quality of recyclables, with copper contamination being of highest concern (Daehn, Serrenho and Allwood 2019; Cooper et al. 2020).

Measures to minimise contamination include design for recycling, better sorting and the deployment of scrap refining technologies (Cooper et al. 2020).

Improve production with direct reduced iron ore technology and renewable energy

Transitioning all steel production to best available technologies can save up to 26 per cent energy; better boilers can save up to 10 per cent, and using heat exchangers in refining can reduce power demand by 25 per cent (Napp et al. 2014; Gonzalez Hernandez, Paoli and Cullen 2018; Fennell et al. 2022).

Primary steel production through the direct reduced iron process followed by an electric arc furnace avoids the need for coke as a reducing agent, leading to a reduction of 61 per cent in carbon emissions if methane-derived gas and renewable energy are used, respectively (Fennell et al. 2022).

Using only hydrogen for the direct reduced iron process could reduce emissions by 97 per cent (Fennell et al. 2022), but this will depend on access to competitively priced green hydrogen, which is limited in supply and faces upscaling challenges (Castelvecchi 2022; Odenweller et al. 2022).

Avoid overuse of steel by selecting the appropriate product for the building’s lifetime

Materials need to be selected with the entire building lifetime in mind, not just the production stage.

Carbon steels are the default metal of choice for reinforced concrete and as structural materials.

Using corrosion-resistant stainless steels in marine environments makes it possible to design for longer building lifetimes, avoiding costly and carbon-intensive maintenance and repair. Correct material selection in marine environments is critical because most urban growth will occur along coastlines.

In the International Energy Agency’s most ambitious decarbonisation scenario, extending the lifetime of buildings would contribute to more than 90 per cent of the CO2 emission reductions for both steel and cement by 2060 (IEA 2019).

Avoiding over specification is a key opportunity to minimise material use, both in material selection and in the amount of material used. For example, there is no need to use corrosion-resistant stainless steel rebar for inland applications.

180 York St, New Haven, CT 06511, United States

cea@yale.edu | +1203.432.2288