Sustainable metals, specifically green steel production

This lecture, by Dierk Raabe, is a part of a series organised by Prakash Srirangam of Warwick University.

Abstract: metallic materials encompass such diverse features as strength, hardness, workability, damage tolerance, ductility and toughness, often combined with corrosion resistance, thermal and electric conductivity and magnetism. Today we produce and consume 2 billion tons of metals every year. The accelerating demand for metals in key sectors such as green energy supply, infrastructures, construction, robotics, passenger safety and transportation is resulting in predicted production growth rates of up to 200% until 2050. Most metals require a lot of energy when extracted and produced and these processes emit large amounts of greenhouse gases (for steel it a factor of 2 in CO_2 emission per mass unit of metal produced while for nickel it is even a factor of 12-45). Therefore, metal production has become a huge environmental burden: production of metals leads to a consumption of about 10% of the global energy used and 40% of all industrial CO_2-equivalent emissions. The lecture presents several aspects related to this field, with a focus on methods for improving the sustainability of steels, in areas including reduced-CO_2 primary production, recycling and scrap-compatible alloy design.

Iron- and steelmaking alone stand for about 7-8% of all global greenhouse gas emissions, which qualifies this sector as the biggest single cause of global warming [1,2]. This originates from the use of fossil carbon carriers as precursors for the reduction of iron oxides. Carbon is turned in blast furnaces into CO and – through the redox processes reducing iron oxide – into CO_2, producing about 2 tons CO_2 for each ton of steel produced. Mitigation strategies pursue the replacement of fossil carbon carriers by sustainably produced hydrogen and / or electrons as alternative reductants, to massively cut these CO_2 emissions, thereby lying the foundations for transforming a 3000 years old industry within a few years [1,2]. As the sustainable production of hydrogen using renewable energy is a severe bottleneck in green steel making, at least during the next decade (transforming this industry would need about 250-300 million tons of green hydrogen each year, i.e. about 5 orders of magnitude more than produced around the globe today), the gigantic annual steel production of 1.85 billion tons requires strategies to use hydrogen and / or electrons very efficiently and to yield high metallisation at fast reduction kinetic.

This presentation presents progress in understanding the governing mechanisms of hydrogen-based direct reduction and plasma reduction of iron oxides [2-5]. The metallisation degree, reduction kinetics and their dependence on the underlying redox reactions in hydrogen-containing direct and plasma reduction strongly depend on mass transport kinetics, Kirkendall effects, nucleation phenomena during the multiple phase transformations, chemical and stress partitioning, the oxide's chemistry and microstructure, the acquired (from sintering) and evolving (from oxygen loss) porosity, crystal plasticity, damage and fracture effects associated with the phase transformation phenomena occurring during reduction [5-8]. Understanding these effects, together with external boundary conditions such as other reductant gas mixtures (including also ammonia [8]), oxide feedstock composition [9], pressure and temperature, is key to produce hydrogen-based green steel and design corresponding direct reduction shaft or fluidised bed reactors (with and without plasma support), enabling the required massive C0_2 reductions at affordable costs. Possible simulation approaches that are capable of capturing some of these phenomena and their interplay are also discussed.

Author: Dierk Raabe is the Director of the new Max Planck Institute for Sustainable Materials in Duesseldorf, Germany. He studied music, metallurgy and metal physics. After his doctorate 1992 and habilitation 1997 at RWTH Aachen he received a Heisenberg fellowship and worked at Carnegie Mellon University. He joined Max Planck Society as a director in 1999. His main interest today is to make industrial production of materials more sustainable, focusing on basic research where the leverage for CO2 elimination is particularly large. His specific interests lie in sustainable metals (specifically green steel and sustainable Aluminium alloys), physical metallurgy of metallic alloys, steels, hydrogen, aluminium alloys, atom probe tomography, machine learning and sustainable manufacturing. He received the Gottfried Wilhelm Leibniz Award (highest German Science Award), The Acta Materialia Gold Medal and two ERC Advanced Grants (highest European Research Grant). He is professor at RWTH Aachen in Germany and at KU Leuven in Belgium. He is a Doctor Honoris Causa at the Norwegian Technical University Trondheim.