Steel’s quiet efficiency play: preheat the scrap, tune the oxygen, catch the heat

Steelmakers are finding pragmatic gains by reusing heat they already paid for. Preheating incoming scrap to a few hundred degrees Celsius trims the power bill in EAFs. In BOFs, “post‑combustion” (burning carbon monoxide back to carbon dioxide inside the vessel) transfers more heat into the bath. And across both routes, hot gases and molten slag carry recoverable energy that can raise steam or preheat charge materials.

Across sites and technologies, the pattern is consistent: kilowatt‑hours per tonne (kWh/t; electricity per tonne of steel), gigajoules per tonne (GJ/t; fuel/heat per tonne), and even tap‑to‑tap time can all fall, often with paybacks measured in months to a few years. The data below come from industry reports, peer‑reviewed analyses, and trials (citations inline).

Scrap preheating in EAFs (electric arc furnaces)

Preheating scrap before melting is a mature energy measure. Conventional bucket preheating—using hot off‑gas to warm scrap to about 315–450 °C—typically cuts electricity by 40–60 kWh per tonne of steel (1library.net).

Modern systems yield larger savings. Single‑shaft furnaces (Fuchs‑type) preheat roughly one‑third of the charge and can lower EAF power needs by up to ~18% (about 77–110 kWh/t) (1library.net). Continuous feeding systems like Tenova’s Consteel heat scrap counter‑currently with off‑gas on a conveyor, achieving “~~360 kWh/t~~” of total energy use (compared to ~420–450 kWh/t in older EAFs) and about 60 kWh/t savings (www.iipinetwork.org).

In practice, scrap preheating often cuts specific electricity by 10–25% (for example, 110 kWh/t is ~25% of a 440 kWh/t baseline) and shortens tap‑to‑tap times by 5–8 minutes (1library.net) (1library.net). Ancillary benefits include roughly 0.3 kg reduction in electrode use and ~1 kg reduction in refractory per tonne (1library.net).

Paybacks are rapid. Retrofitting Consteel on a 500 ktpy EAF is cited at about $7.8 per tonne of steel with savings near $3/t (≈1.3‑year payback), and shaft preheaters can pay back in about one year (www.iipinetwork.org) (www.iipinetwork.org).

In Indonesia—where EAFs already use roughly 75% scrap feed—such upgrades yield immediate cuts (pmc.ncbi.nlm.nih.gov). Adding heated hot‑metal to the charge (a proxy for scrap heat) cut arc power from 87.4 MWh to 72.9 MWh in an Indonesian 130 t EAF (≈16% cut), and implementing CO post‑combustion could further trim consumption by ~30% (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Optimizing oxygen blowing in BOFs (basic oxygen furnaces)

In a conventional BOF, only about 8–12% of the chemical energy of carbon is captured in the melt; most exits as CO‑rich gas (www.mdpi.com). Post‑combustion—burning CO back to CO₂ above the bath—captures more heat and shortens blow times.

Dual‑flow post‑combustion lances introduce oxygen through two concentric nozzles: the inner jet decarburizes, the outer burns CO above the bath. CFD‑optimized designs doubled the CO→CO₂ conversion from ≈10% to ≈17% of the CO, while transferring most heat into the steel; this raised BOF thermal efficiency and allowed higher scrap addition (www.mdpi.com). At ArcelorMittal Dofasco’s combined‑blowing BOF (with bottom tuyeres injecting the off‑gas into the shaft), both scrap charging and post‑combustion increased, enabling a 4% higher scrap rate and a commensurate CO₂ drop (www.mdpi.com) (www.mdpi.com).

U.S. studies estimate that advanced post‑combustion control could reduce BOF total energy requirements by about 30% (www.iipinetwork.org). Bottom‑gas stirring (submerged tuyeres) lowers flux/oxygen needs and improves yields; in one 80 t BOF trial, optimizing top and bottom lance heights reduced end‑point C and O by ~20% (from 0.0032 to 0.0026) and cut Fe loss to slag by 2.2%—implying less oxidation and heat loss (onlinelibrary.wiley.com).

Recovering waste heat from hot off‑gases

In EAFs, the hot exhaust (~1,100–1,200 °C) carries roughly 15–30% of the process heat. Mass–energy analyses show a 50–50 mix of chemical (from CO) and sensible heat in the gas. For 50% hot‑metal charging, Yang et al. estimate ~274 kWh/t of heat in the gas and another ~43 kWh/t in slag (www.degruyter.com).

Recovering even a portion yields big gains. Waste‑heat boilers or steam cycles on a 150 t EAF can recoup ≈130 kWh/t (at ~30% efficiency) (www.iipinetwork.org). Process simulations suggest an optimal scheme (combining off‑gas utilities and scrap preheat) could harvest ~170 kWh/t, cutting ~58 kg CO₂ per tonne of steel (www.degruyter.com). In practice, EAF off‑gas heat is used for preheating scrap (as above), raising saturated steam, or driving oxy‑fuel burners. Recovering ~2.8–15.1 MWh/year of electricity from a 150 t EAF is technically feasible (CO₂ savings up to ~12–73 kt/yr), though this depends on uptime and off‑gas handling (www.iipinetwork.org).

Where waste‑heat boilers are deployed, steam‑cycle reliability is tied to make‑up water quality; plants commonly deploy dedicated make‑up treatment, for example a demineralizer, and may dose the steam circuit with oxygen scavengers to protect metallurgy. (Acronyms: CO = carbon monoxide; CO₂ = carbon dioxide.)

BOF gas recovery and mill integration

BOFs also produce large off‑gas flows (~3,000–4,000 Nm³/t; normal cubic meters per tonne) at ~900–1,000 °C, rich in CO. Many plants clean and combust this gas. NEDO (2008) data show 0.125–0.92 GJ/t of BOF gas heat can be recovered via steam or export gas (www.iipinetwork.org).

At ArcelorMittal Ghent, recovered BOF gas replaced about 0.7 GJ/t (≈700 kWh/t) of external fuel, cutting total mill energy use by ~3% (www.iipinetwork.org). Reuse of BOF gas yields roughly 50 kg CO₂ avoided per tonne of steel, and global reuse could abate ~25 MtCO₂ per year (IEA estimate) (www.iipinetwork.org). Implementation involves scrubbers, cleaning, and onsite power/steam systems; capital costs can be about $20 per tonne of capacity with paybacks on the order of a decade (www.iipinetwork.org).

Water‑based gas cleaning trains typically incorporate solids separation; many flowsheets pair scrubbers with a clarifier as part of the recycle loop, alongside other water‑treatment ancillaries selected to match local conditions.

Slag heat recovery concepts

Ferrous slags at tapping (~1,200 °C) have ~1–2 GJ/t energy content (www.mdpi.com). Novel heat‑exchangers (RecHeat designs) can cool slag from about 1,200 °C to ~300–400 °C, heating air or process fluids from roughly 125 °C to ~340 °C (www.mdpi.com). In theory, oxidizing residual FeO in slag (for example, chemical looping) can also release heat or generate fuel gas.

Though not yet widespread, slag heat recovery could provide tens of kWh per tonne of steel if captured (for example, each 10% of a 1.5 GJ slag roast is ~150 kWh). In sum, integrating gas heat recovery (in boilers or generators) and slag heat exchangers offers one of the largest untapped efficiency gains in steelmaking, potentially shaving energy use by several GJ/tonne and cutting CO₂ in tandem (www.degruyter.com) (www.mdpi.com).

Sources and technical notes

For scrap preheating and EAF data, see industrial surveys and technical reports (1library.net) (www.iipinetwork.org) (www.degruyter.com). For BOF improvements, see Primetals (“Green LD”) and iipinetwork summaries (www.mdpi.com) (www.iipinetwork.org) (onlinelibrary.wiley.com). Waste‑heat figures are drawn from heat‑balance analyses and case studies (www.degruyter.com) (www.iipinetwork.org).