Blast Furnace Ironmaking

Blast furnace top gas — the off-gas exiting at the furnace top at 100–250 °C — contains approximately 23–27% CO, 20–22% CO₂, 3–5% H₂, and 50–55% N₂. This gas, with a calorific value of 3.5–4.0 MJ/Nm³, is cleaned in bag filters and used as fuel throughout the integrated steelplant. Its primary uses are firing the Cowper stoves (which preheat the hot blast), the sinter plant ignition hoods, coke oven underfiring, and the slab reheating furnaces upstream of the hot strip mill.

The Cowper stove is the enabling technology for hot blast. A large refractory-filled vertical vessel is fired on BF top gas until its checker brick reaches ~1,400 °C ("on-gas" phase), then cold blast is passed through it to absorb heat and emerge as hot blast at 1,000–1,250 °C ("on-blast" phase). Three stoves per furnace run in rotation so that one is always delivering hot blast. Each 100 °C increase in blast temperature reduces coke rate by approximately 8–10 kg/tHM.

Modern blast furnace plants recover pressure energy from the top gas using a Top Pressure Recovery Turbine (TRT), exploiting the differential between furnace top pressure (1.5–3.0 bar for modern high-top-pressure furnaces) and the gas cleaning system pressure. A large TRT generates 30–50 MW of electricity from what would otherwise be throttling losses — a significant contribution to plant energy efficiency.

Blast furnace productivity is expressed as the productivity index — tonnes of hot metal produced per day per cubic metre of inner volume (t/day/m³). World-class furnaces achieve 2.5–3.0 t/day/m³ through a combination of high-top-pressure operation, oxygen enrichment, high-quality burden (high CSR coke, high reducibility sinter and pellets), efficient PCI, and optimised burden distribution using rotating chute charging systems (Paul Wurth bell-less top).

Campaign life — the period of continuous operation between major relines — is the defining productivity metric for the blast furnace owner. A campaign ends when the hearth or stack lining wears through to the cooling plate, as evidenced by rising shell temperatures or thermocouple readings in the hearth wall. Modern campaigns of 15–25 years are achieved through: titanium ore "bear" additions that precipitate TiC and TiN on the hearth floor (protecting the carbon brick from erosion by the circulating liquid iron), copper stave cooling systems, and online monitoring of hearth liquid level through electromagnetic sensors.

A major reline is a capital-intensive exercise — $100–300M for a large furnace — requiring 6–18 months of planning and 3–6 months of outage. The decision to reline versus operate to the safe end-of-campaign boundary is one of the most consequential operational decisions an integrated steelplant makes.

The BF-BOF route accounts for approximately 70% of global steel production and emits roughly 2.0–2.2 billion tonnes of CO₂ per year — about 7% of global anthropogenic emissions. Four main decarbonisation pathways are being pursued in parallel:

Hydrogen injection at tuyeres: Replacing pulverised coal with H₂ at the tuyere zone eliminates CO₂ from that fraction of reductant. Each 1% substitution of PCI by H₂ reduces CO₂ by approximately 10–15 kg/tHM. The challenge is thermal: H₂ combustion and reduction reactions are endothermic at BF conditions, so replacing coal with H₂ cools the raceway and reduces RAFT. Operators must compensate by increasing blast temperature or oxygen enrichment. thyssenkrupp's H2Hamburg project demonstrated 2.5% H₂ injection at commercial scale in 2022; Salzgitter and TATA Steel IJmuiden have further trials underway. The theoretical limit before thermal balance fails is approximately 30% H₂ substitution of PCI without major plant modifications.

Top gas recycling (TGR-BF): BF top gas is stripped of CO₂ by pressure swing adsorption (PSA) or membrane technology, and the CO-rich stream is reinjected at tuyere level. This improves CO utilisation, reduces coke rate by 15–20%, and cuts CO₂ by 20–25%. The ULCOS pilot at SSAB Luleå (Sweden) demonstrated this at industrial scale. TGR-BF still produces CO₂ — it requires CCS (carbon capture and storage) for the stripped CO₂ stream to achieve deep decarbonisation.

BF-BOF to DRI-EAF transition: The most radical option. Replacing the BF-BOF route with a hydrogen-based DRI shaft furnace and EAF eliminates process CO₂ entirely when green H₂ is used. SSAB's HYBRIT project (Sweden) produced the world's first H₂-reduced steel in 2021 and targets commercial scale by 2026. ArcelorMittal, thyssenkrupp, and Salzgitter all have 2030–2035 transition targets. Capital cost of full conversion is £400–800M per million tonnes of annual capacity. The pace of transition in Asia — where most BF capacity is concentrated — depends on green hydrogen scale-up and electricity costs, and full BF phase-out by 2050 is unlikely given the capital stock and infrastructure dependencies involved.