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| Fig. 1: Diagram of the steel production lifecycle, comparing extractive steel production (left) with recycled steel production (right). Recycled steel production uses end-of-life materials to produce low-emission steel in a closed-loop system. (Image Source: J. Navarro) |
In 2022, global steel production reached approximately 1.885 billion metric tons making it one of the largest produced commodities. [1] Modern society's reliance on steel is unparalleled as it is crucial for supporting infrastructure, defense, manufacturing, and construction. However, the industry's environmental impact is substantial, contributing roughly 7% of the world's total CO2 emissions. [2] As steel demand grows, especially in rapidly industrializing nations like India and China, these emissions are going to increase, presenting significant challenge to global climate goals. Recycling steel significantly decreases the energy consumption and emissions compared to producing new steel from raw materials. Steel recycling involves collecting and reprocessing metal from end-of-life products like vehicles, appliances, and construction materials, and transforming them into reusable materials. This reprocessing helps to manage the carbon intensity of a growing global industry by using less energy and emitting fewer greenhouse gases than traditional steelmaking method, thereby supporting efforts to meet climate goals even as demand continues to rise.
Steel is produced through two primary processes: (1) extracted steel from raw material feedstocks and (2) recycled steel from end-of-life products shown schematically in Fig. 1.
Extractive Steel Production: This process begins by adding iron ore, coke, and limestone to a blast furnace (BF). Preheated air is blown into the BF, where it combusts the coke, generating the necessary heat. The resulting gases reduce the iron ore to form carbon-rich molten iron, along with a byproduct called slag. The molten iron is then transferred to a Basic Oxygen Furnace (BOF), where oxygen is injected to reduce the carbon content via combustion, producing steel from raw minerals. Due to its reliance on carbon-based fuels, extractive steel production has a high carbon footprint, emitting 1.4 tons of CO2 per ton of steel. [2]
Recycled Steel Production: This method involves melting scrap steel in an electric arc furnace (EAF) using an electric arc generated between graphite electrodes. Unlike extractive steel production, this process primarily relies on electricity, resulting in significantly lower CO2 emissions, averaging about 0.4 tons per ton of steel. [2] The primary source of emissions for EAFs comes from the electricity used, so their carbon footprint has the potential to be significantly reduced when powered by renewable energy sources.
The theoretical energy required to produce steel from scrap is based on the heat needed to raise the temperature from ambient conditions (25°C) to its melting temperature (1600°C). This calculation involves two main components: the energy needed to heat the material to its melting point and the energy required to change its phase from solid to liquid (heat of fusion) once it reaches that point. Where the heat capacity (Cp) is taken as 0.49 kJ/kg K and the heat of fusion is 270 kJ/kg. Together, these factors determine the minimum energy needed for scrap steel production.
The theoretical energy required to melt scrap steel is about 1.04 million kJ per ton, equivalent to 289 kWh per ton. To put this into perspective, melting one ton of steel requires about the same energy as a 400 HP Ford F-150 truck (~300 kW) driving for one hour. However, in practice, Electric Arc Furnaces (EAFs) consume more energy, typically ranging from 350 to 600 kWh per ton of steel produced. [3] This difference arises mainly from heat loss and electrical generation losses. The efficiency of EAFs varies between 48% and 83%, depending largely on the quality and composition of the scrap steel. As a result, producing one ton of steel using an EAF is roughly equivalent to a Ford F-150 driving for about 1.2 to 2 hours. It is a modern marvel that steel production can achieve such levels of energy efficiency, given the high temperatures and phase changes involved. In contrast, extractive steel production is significantly more energy-intensive, requiring approximately 5,556 kWh per ton of steel. [4] This makes extractive steel production up to 8.6 times more energy-consuming than using EAFs, highlighting the substantial energy savings and environmental benefits that come from recycling steel in EAFs. A ton of steel extractive steel has the approximate energy equivalence of driving a Ford F-150 from San Francisco to San Diego California.
Transitioning to recycling-based steel production requires the widespread availability of renewable electricity in developing regions and a sufficient volume of high-quality scrap. Although scrap steel alone cannot meet the growing demand for steel, it provides a practical path to reducing carbon emissions. By using recycled materials, EAFs create a closed-loop system that is more energy-efficient and aligns with circular economy principles, emphasizing resource reuse and waste reduction.
© Joshua Martinez-Navarro. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "World Steel in Figures 2023," World Steel Association, May 2023.
[2] "Iron and Steel Technology Roadmap," International Energy Agency, October 2020.
[3] J. Cappel, "EAF Efficiency," Cappel Stahl Consulting GmbH, November 2021.
[4] "Steel's Contribution To a Low Carbon Future and Climate Resilient Societies," World Steel Association, 2017.