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| Fig. 1: Carbon emissions emanated from tough-to-decarbonize sectors. [4,9] (Image Source: K. P. Kua) |
Steel is a vital material for society in the modern world. Every year, approximately 2 × 1012 kilograms of steel are produced worldwide. [1] Steel is ubiquitously utilized in construction, transportation and automotive, machinery and consumer goods manufacturing, electrical appliances, energy technologies, biomedical equipment, and art. [2,3]
Globally, the total amount of carbon dioxide released into the atmosphere by the iron and steel industry is reported to be 2.6 to 3.8 × 1012 kilograms per year, which accounts for 7 to 9% of anthropogenic greenhouse gas emissions across the world. [4-6] Iron and steel manufacturing is one of the hardest industries to abate because of its heavy reliance on fossil fuels and enormous capital requirements (Fig. 1). [4,7-9] It is also energy-intensive and consumes around 4.2 × 1019 joules of energy per year, amounting to 8% of global final energy demand. [10-11]
Generally, there are two routes for steel production: blast furnace-basic oxygen furnace (BF-BOF) and electric arc furnace (EAF). BF-BOF on average can produce 1000 kilograms of crude steel by using 1370 kilograms of iron ore, 780 kilograms of coal, 270 kilograms of limestone, and 125 kilograms of steel scrap. On the other hand, EAF can fabricate 1000 kilograms of crude steel with the use of 710 kilograms of steel scrap, 586 kilograms of iron ore, 150 kilograms of coal, 88 kilograms of limestone, and 2.3 × 109 joules of electricity. [12,13] 72% of the world's steel is manufactured via BF-BOF route that utilizes coal, natural gas, or petroleum as fuel source, whereas the remaining 28% is produced via EAF route that uses electricity from both renewable and non-renewable resources. [13,14] 1.1 × 1012 kilograms of coal are used internationally, rendering the iron and steel industry the second largest consumer of coal, behind only electricity generation. [12,15] The carbon footprint attributed to EAF is estimated to be 1648.28 kilograms of CO2 per 1000 kilograms of steel produced, which is lower than BF-BOF that emits 4449.76 kilograms of CO2 per 1000 kilograms of steel produced (Table 1). [16]
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| Table 1: Comparison of carbon emissions between steel production from BF-BOF and EAF. [16] |
Industrial ironmaking is a multistep process in which iron ores (Fe2O3) are oxidized first into magnetite (Fe3O4) and subsequently to an intermediate material known as wüstite (Fe1-xO), and eventually be refined into pure iron (Fe) appropriate for steelmaking. [17]
To date, there are two breakthrough zero-carbon technologies for steelmaking: molten oxide electrolysis and green hydrogen-based direct reduction of nanoparticles in iron ores. [17,18]
Molten oxide electrolysis is a metal extraction process for high-throughput steel production. [19] Without coal combustion or other fossil fuels, it can utilize 100% renewable electricity to convert iron ores of any grade to high-quality metal in the liquid state. Molten oxide electrolysis simplifies the steel production process and decreases energy use. [18,20] It is estimated that manufacture of 1.279 × 1012 kilograms of steel using molten oxide electrolysis requires 1.84 × 1019 joules of electric energy, corresponding to almost 20% of total global electricity consumption. [16]
Molten oxide electrolysis is entirely carbon-free and releases only pure oxygen gas as its byproduct. [21,22] It does not require process water, hazardous chemicals, and precious metal catalysts. Iron ores, typically hematite (Fe2O3) are fed into an electrolysis cell, comprising a cathode that serves as a negative terminal of the cell and an inert, long-lasting anode that is submerged in an electrolyte containing dissolved iron oxide (iron ores). [21,23] When electric current is imposed between the two electrodes and heats the cell to above the melting point of iron metal (1600 °C being the optimum operational temperature), electrons decompose the bond of iron oxide to generate oxygen gas at the anode and pure liquid metal sediments at the cathode that subsequently settle at the bottom to be tapped off. Without the need for reheating, metal in the liquid state can be transmitted directly to ladle metallurgy, casting, and rolling for producing finished steel products (Fig. 2). [19-21,24]
The following chemical equations depict electrochemical reactions that occur in molten oxide electrolysis for steelmaking: [21,23]
Technical challenges facing molten oxide electrolysis encompass the necessary high temperature of over 1538°C to accomplish the metallurgical reaction, corrosion of most metals under anodic polarization, spontaneous reduction of iron oxide on contact with most refractory metals, choice of material and design of current collector for cathode, choice of material for anode to facilitate oxygen evolution following reaction, and choice of electrolyte that is stable and can act as a solvent for the metal oxide feedstock on top of demonstrating ionic conduction and a high rate of mass transport. [18,25]
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| Fig. 2: Schematic diagram illustrating molten oxide electrolysis process for steel production. [20] (Image Source: K. P. Kua.) |
Green hydrogen-based direct reduction of nanoparticles in iron ores offers zero-carbon ironmaking that generates pure water as its sole byproduct. A hydrogen-based reduction of iron ores reactor operates at 800 to 1000°C. For a carbon-neutral operation, water electrolysis techniques driven by renewable electricity are instrumental to generate clean hydrogen as a reductant for steel production. [26,27] In a proof-of-concept experiment, iron ore fines (Fe3O4) undergo two stages of reduction (Fe3O4 → Fe1-xO, then Fe1-xO → Fe). The reactor contains adequate active surface oxygen atoms to react with hydrogen directly for the removal of water (O + H2 → H2O). Fe3O4 nanoparticles exhibit fast reaction rates, with the majority being reduced to Fe1-xO at 800 °C. Wüstite (Fe1-xO) is spontaneously formed at a lower temperature of 300 °C owing to the kinetically preferential stabilization of nanoparticles at this phase. At 350 °C, a conversion to pure iron (Fe) occurs. Future clean ironmaking process seeks to bypass the wüstite (Fe1-xO) phase and go from magnetite (Fe3O4) straight to pure iron (Fe) and hence will require less energy. [17]
As solar and wind are anticipated to deliver 70% of global electricity generation in 2050 and constitute the core renewable energy resources in steel manufacturing, the transition of both aforementioned sustainable steelmaking methods to be powered by 100% clean, non-combustion, zero-emission energy will make enormous strides toward environmental and energy sustainability, economic growth, and civil society development. [28] In order to address market barriers and compete in an extremely efficient steel market, clean steelmaking technology will need to be as nimble, cost-effective, and flexible as modern steel manufacturing techniques or require some form of monetary benefits or financial opportunities, such as tax measures, rebates, grants, performance-based incentives, loan programs, guarantees, and credit enhancements implemented by governments or private sector companies around the world to empower its progress and support its manufacturing scale-up.
© Kok Pim Kua. 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.
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