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Analysis of the Theoretical and Practical Energy Requirements to Produce Iron and Steel, with Summary Equations that Can Be Applied in Developing Future Energy Scenarios

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A Correction to this article was published on 13 May 2022

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Abstract

This paper derives from first principles simple relationships that can be used to compute energy requirements for the production of hot metal (pig iron) in a blast furnace (BF) or direct reduced iron (DRI) in a direct reduction furnace (DRF), and the transformation of hot metal and DRI into crude steel in a basic oxygen furnace (BOF) or electric arc furnace (EAF) with the addition of scrap iron or scrap steel of varying purity. These relationships account for the impact of changing iron ore grade, in the amount and type of impurities in iron, and the impact of the addition of fluxes and the production of slag. Changing proportions of hot metal and scrap to the BOF, or of DRI and scrap to the EAF, are accounted for. The energy flow analysis presented here, combined with the mass flow analysis presented in a companion paper, provides a foundation for tracking the impact on energy use and iron losses of alternative pathways that might be used in the future as part of a broad-based effort to reduce energy use and associated greenhouse gas emissions.

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Notes

  1. Added limestone is converted to CaO in the blast furnace, a process called calcination.

  2. Energy use associated with production of prepared iron as lumps, pellets and sinter is omitted here but will be considered in a follow-up paper that traces energy use from mining through to manufactured steel products.

  3. Due to quantum–mechanical effects, specific heats increase rapidly from zero at 0 K, but still increase slowly with temperature as temperature increases beyond 300 K, asymptotically approaching the theoretical limit expected from the equipartition of vibrational energy. See Snow [7] for a more complete discussion.

  4. For Cases 5 and 6, offgas otherwise available is consumed, so the offgas term adds to the net energy requirement.

  5. Of which 2.11 GJ/t for the reaction enthalpy change, 1.30 GJ/t to heat the iron, and 5.28 GJ/t as feedstock energy.

  6. Oxygen can be enriched to a concentration of up to 29% [8]. This reduces the required N2 and its heating, but requires energy for cryogenic separation of O2 from air. These opposing impacts on energy use are neglected here.

  7. This is the fuel efficiency taking into account production of CO by reactions involving SiO2, MnO and CaCO3. Also shown in Table 7 are fuel efficiencies (which are higher) based solely on the iron-reduction reactions.

  8. Precipitation of Fe3C in DRI as DRI forms occurs when the reacting gas contains > 20% CH4 with H2/CO ~ 5 [14].

  9. Consumers of DRI want a minimum C content in DRI. Thus, Stena (https://www.stenametalinc.com/hot-briquetted-iron-hbi/hot-briquetted-iron-hbi) guarantees a minimum C content in their HBI product of 0.6%, with a typical C content of 0.7%. In considering production of DRI with H2 produced from C-free electricity sources in order to reduce CO2 emissions, Yilmaz and Turek [17] assumed continued use of a minimal amount of natural gas so as to provide 0.5% C in DRI. A greater amount of combustible C (see “Production of Steel in EAFs” section) results in a lower N content of the output steel by avoiding the need for anthracite or for injection of fine graphite (Kirschen et al. [5]).

  10. The process can use fuels already consisting of a mixture of CO and H2.

  11. Fruehan et al. [12] give the minimum theoretical energy to produce pure Fe DRI as 8.36 GJ/t, based on heating the ore to 900 °C and then reducing it. Here, the energy requirement computed this way is 8.37 GJ/t if the energy required to heat the CH4 fuel to the reaction temperature is ignored, or 8.72 GJ/t if it is included.

  12. With increasing C content, the multiplier for R3 also decreases (as there is less CO from methane reforming available for absorption, due to the decrease in the R22 and R23 multipliers), but the R8 multiplier increases (to absorb the H2 produced by carburization).

  13. For EAFs, Kirschen et al. [5] indicate efficiencies in transferring electrical and fuel energy inputs to the melt of 0.6–0.8 and 0.5–0.6, respectively.

  14. The melting requirement increases with the scrap fraction because the added scrap is cold, while the hot metal is assumed to arrive with a fraction fhm of the required sensible heat gain. The melting energy also increases with the C fraction because the specific heat of C is greater than that of iron on a unit mass basis, and because more O2 must be heated (which also causes as slight increase in fCO).

  15. As shown in the Electronic Supplementary Material (Sect. 4), the net effect of a cycle involving production of cementite when DRI is formed, and destruction of cementite when DRI is refined, is the reaction CH4 → C + 2H2 and a total reaction energy that is equal to that for this direct reaction. Electronic Supplementary Material Sect. 5 shows that the net reaction for reduction of SiO2 and MnO in the BF and re-oxidation in the BOF is the reaction nC + n/2O2 → nCO.

  16. In these calculations, it is assumed that all the heat released from chemical reactions contributes to the EAF energy requirements. In reality, some portion would be lost as sensible heat in the offgas, but this would not affect the theoretical minimum energy requirement, as this assumes that all the offgas sensible heat (and chemical energy) is used in some way (so that it can be credited against the EAF energy requirement).

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Acknowledgements

I am indebted to Lauri Holappa (Aalto University, Finland) and one anonymous reviewer for their meticulous and thorough reviews of initial drafts of this paper, leading to significant improvements.

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This research received no funding support in the public, commercial, or not-for-profit sectors.

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  1. Department of Geography, University of Toronto, Toronto, M5S 3G3, Canada

    L. D. Danny Harvey

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The contributing editor for this article was Sharif Jahanshahi.

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The original version of this article was revised: The graphical abstract and Fig. 2 in this article as originally published omitted non-energy-containing offgases CO2 and H2O from the Direct Reduction Furnace.

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Harvey, L.D.D. Analysis of the Theoretical and Practical Energy Requirements to Produce Iron and Steel, with Summary Equations that Can Be Applied in Developing Future Energy Scenarios. J. Sustain. Metall. 6, 307–332 (2020). https://doi.org/10.1007/s40831-020-00276-5

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