According to World Steel Association, the global crude steel production was 1.9billion metric tonnes in 2023 with BF-BOF process accounting for ~70% and the balance being EAF. The overall carbon intensity in steel making was an estimated 1.9tCO2  per tonne of steel resulting from an energy consumption of 21GJ/t steel. The energy consumed and the resulting carbon intensity varies across production routes – BF-BOF and EAF. The energy consumption in a BF-BOF is in the range of 17-23 GJ/t steel and carbon intensity being ~2.3t CO2/ t steel. Whereas, EAF has a far lower carbon footprint at 0.7-1.3tCO2/ t steel and the energy intensity being 2-3GJ/t steel based on scope 1 emissions alone.

Understandably, steel industry decarbonisation efforts are focused on addressing the emissions via BF-BOF through process improvements and/or radical shift in the process route by replacing BF-BOF with EAF-DRI system. The energy consumed in steel production may be broadly segmented as process heat and electricity. The share of each of these varies based on the steel production route  – BF-BOF or EAF. In case of BF-BOF, process heat demand accounts for majority of the energy consumption, while in EAF based process, electricity accounts for most of the energy consumption.

Figure 2: World Crude Steel Production Route (2023) (Source: Vijay Thangavelu)

Steel production involves heating the raw material charge to very high temperatures >1000 deg C with the heating agent being metallurgical coal in BF-BOF and electricity in case of EAF.  Majority of the energy is consumed in the blast furnace where coal in the form of coke reacts with the incoming iron oxide to produce molten iron. The external energy inputs at the steel refining stage in the BF-BOF process is quite limited and any energy consumed at this stage is mainly attributed to chemical energy in the form of oxygen reacting with carbon present in the hot metal.

The carbon emissions associated with the BF-BOF process may be mitigated to a certain extent via efficient energy management techniques. This involves capture and re-use of high heat exhaust gases in the form of blast furnace and BOF gas. Modern blast furnaces are equipped with top recovery turbines where in energy locked in the BF gas is used to produce electricity and any unspent gas is further channelled to downstream reheating processes where slab / billet is reheated before rolling them into finished steel products such as hot rolled coil, rebar, wire rod and others. Within EU, most of the BF-BOF systems are equipped with some form of energy recovery and re-use infrastructure to enable very low carbon footprint within the remit of the available technologies for this process. However, all of these process improvements only make a marginal contribution towards carbon mitigation and hence the need for a shift from BF-BOF to EAF based process.

The EAF in contrast consumes 2-2.5 GJ/t steel which is significantly lower relative to BF-BOF process. The EAF is predominantly a scrap melting furnace, wherein scrap pre-heated/ or not, is melted to produce molten crude steel for further refinement and finally casting into semifinished products such as slab/ billet. The energy consumed in the EAF is electrical energy supplemented by some chemical energy in the form of natural gas and oxidation. Electrical energy accounts for a larger proportion of the energy input. Chemical energy takes the form of carbon injection, natural gas and oxygen as the balance of the energy inputs. Natural gas is used for a variety of applications such as ladle heating and others.

Figure 3: EAF Energy Input (Source: Vijay Thangavelu)

Out of the total energy consumed in the EAF, ~50% is transferred to the metal, 25% into exhaust gas and the balance being in slag, radiation, cooling etc. Although some of the exhaust gas may be used in reheating the scrap input, the energy potential of the gas is not as well harnessed as in the case of BF-BOF system. The EAF off-gas is hardly used to generate any electricity or reheating of slab/ billet in steel reheating furnaces There are several challenges associated with the effective use of EAF waste gas. These include inconsistent composition of the off-gas output, variability in flow rates and high dust content. The dust content of EAF off gas is quite high , >12g/cu.m, leading to complications in handling and the need for an extensive dedusting system.

Figure 4: EAF Energy Output (Source: Vijay Thangavelu)

Unlike the BF-BOF system, EAF based steel mills have limited recourse to inhouse exhaust gases with adequate heating value. Thus these mills rely quite significantly on external sources especially natural gas for performing all the auxiliary functions such as ladle heating and others.

Steel mills especially in the EU are pursuing transition from BF-BOF to EAF based system as a part of the steel decarbonisation strategy. The EAF system will be further augmented by DRI to manage shortfall of any high quality scrap, which is critical for producing flat products used in the automotive, energy and other such highly demanding sectors. Unlike scrap, DRI is a high grade metallics with low levels of residues such as copper and others, and is used as a scrap substitute in EAF. Although the energy consumption in an EAF-DRI system may be higher relative to 100% scrap melting, the energy consumption is still lower than a typical BF-BOF system and the associated carbon emissions. The carbon emissions associated with EAF-DRI (natural gas) is estimated to be in the range of 1.3-1.4tCO2/ t steel. The emissions are expected to be even lower in case of renewable based EAF-Hydrogen DRI system.

With the new EAF-DRI installations in the EU anticipated to be hydrogen based and with hydrogen being an expensive resource, it is important focus on mitigating energy losses and work towards using the exhaust gas more productively within the EAF steel mill production system. The waste gas characterisation is critical and in parallel the existing systems will have to be modified or fully altered to utilise the full potential of the exhaust gas.

The harnessing of EAF exhaust gas heat is crucial to improve the energy efficiency of the EAF based steel production route and contributes significantly towards optimum use of hydrogen and ultimately renewable energy. Some of the areas under consideration include pre-heating of incoming scrap with clean and dedusted off-gas, usage of oxy-fuel burners for ladle heating exploring the potential of generating electricity from the high temperature off gas. Economising the use of hydrogen input and electricity is crucial to ensure the sustainability of EAF-hydrogen DRI steel production route.

While several cross sectoral studies and interdisciplinary studies are being undertaken to address this challenge, it is also important to assess this from a market perspective. In the near to medium term, this will help the steel mills better utilise their allowances and in an evolving carbon price regime in the long term, this would result in an even greater savings.

References:

[1] World Steel Association
[2] Estep
[3] J. Dock and T. Kienberger; Techno-economic case study on Oxyfuel technology implementation in EAFsteel mills – Concepts for waste heat recovery and carbon dioxide utilization; Cleaner Engineering and Technology 9 (2022) 100525
[4] Teske et. al; 1.5 °C pathways for the Global Industry Classification (GICS) sectors chemicals, aluminium, and steel; SN Applied Sciences (2022) 4:125
[5] Various journal and news articles

Author

Vijay Thangavelu
VT Advisory
London
+44 7957749983
vthangavelu@vtadvisory.co.uk