The steel industry is one of the largest contributors to global carbon emissions, accounting for approximately 7-9% of total CO2 output. Traditional blast furnaces rely heavily on coke, a carbon-rich fuel derived from coal, to reduce iron ore into molten iron. The chemical reduction process in blast furnaces involves carbon monoxide (CO) reacting with iron oxides to produce iron and CO2. This reaction is inherently carbon-intensive, making steel production a prime target for decarbonization efforts. One promising pathway is the partial substitution of coke with hydrogen (H2) in blast furnaces, which can reduce CO2 emissions while maintaining steel output.
The chemical mechanism of hydrogen-based iron ore reduction differs from traditional coke-based methods. In a coke-driven blast furnace, the primary reducing agent is carbon monoxide, formed by the reaction of coke with oxygen:
C + O2 → CO2
CO2 + C → 2CO
The CO then reduces iron oxide (Fe2O3) in a stepwise process:
3Fe2O3 + CO → 2Fe3O4 + CO2
Fe3O4 + CO → 3FeO + CO2
FeO + CO → Fe + CO2
When hydrogen is introduced, it competes with CO as a reducing agent. The reactions proceed as follows:
3Fe2O3 + H2 → 2Fe3O4 + H2O
Fe3O4 + H2 → 3FeO + H2O
FeO + H2 → Fe + H2O
The key advantage of hydrogen-based reduction is that water vapor (H2O) is the primary byproduct instead of CO2. However, the thermodynamics of hydrogen reduction differ significantly. Hydrogen reduction is endothermic, meaning it absorbs heat, whereas CO reduction is exothermic and contributes to maintaining the high temperatures required in the blast furnace (typically above 1200°C). This creates a critical challenge: hydrogen injection can lower the furnace temperature, potentially disrupting the smelting process.
To compensate for the cooling effect, additional energy input is necessary, often in the form of increased natural gas or oxygen injection. This introduces trade-offs in carbon savings. While hydrogen reduces direct CO2 emissions from the reduction process, the need for supplemental heating may offset some of the benefits. Current estimates suggest that hydrogen injection can achieve incremental carbon savings of up to 20-30% in traditional blast furnaces, depending on the injection rate and furnace configuration. Full decarbonization, however, requires a complete shift to direct reduced iron (DRI) processes using 100% hydrogen, which is not yet feasible at industrial scales for blast furnaces.
Gas permeability is another limitation. Hydrogen has a lower density and higher diffusivity than CO, which can lead to uneven gas distribution in the blast furnace. This affects the efficiency of iron ore reduction and may cause operational instability. Optimizing hydrogen injection points and flow rates is essential to ensure uniform reduction and maintain furnace productivity.
Major steelmakers are actively exploring hydrogen injection in blast furnaces through large-scale trials. ThyssenKrupp, for instance, initiated a pilot project in 2019, injecting hydrogen into one of its blast furnaces in Duisburg, Germany. The project demonstrated that hydrogen could partially replace pulverized coal injection (PCI) without compromising steel quality. However, the trials also highlighted the need for process adjustments to manage temperature fluctuations.
ArcelorMittal has pursued a similar approach in its Hamburg plant, testing hydrogen injection alongside carbon capture and storage (CCS) technologies. The company aims to integrate hydrogen more extensively but acknowledges that scaling up requires significant infrastructure investments, particularly in green hydrogen supply. Currently, most industrial hydrogen is produced via steam methane reforming (SMR), which emits CO2. For hydrogen injection to deliver meaningful carbon reductions, it must rely on green hydrogen produced via electrolysis using renewable energy.
The barriers to full hydrogen adoption in blast furnaces are multifaceted. First, the cost of green hydrogen remains high compared to fossil-based alternatives. Electrolysis requires substantial renewable electricity, and current production capacities are insufficient to meet the steel industry’s demand. Second, retrofitting existing blast furnaces for hydrogen compatibility involves capital-intensive modifications, including new injection systems and heat management solutions. Third, the global steel industry operates on thin profit margins, making large-scale transitions economically challenging without policy support or carbon pricing mechanisms.
Despite these challenges, hydrogen injection represents a pragmatic intermediate step toward decarbonizing steel production. It allows steelmakers to reduce emissions incrementally while maintaining existing infrastructure. In the long term, however, a transition to hydrogen-based DRI or electrolytic iron production will be necessary to achieve near-zero emissions.
Ongoing research focuses on optimizing hydrogen utilization in blast furnaces, including advanced process control systems and hybrid injection strategies combining hydrogen with other reductants. Collaborative efforts between industry, governments, and research institutions are critical to overcoming technical and economic hurdles. The success of current trials by ThyssenKrupp and ArcelorMittal will provide valuable insights into the scalability of hydrogen injection and its role in the steel industry’s decarbonization roadmap.
In summary, partial substitution of coke with hydrogen in blast furnaces offers a viable pathway to reduce carbon emissions in the near term. While challenges related to temperature control, gas permeability, and hydrogen supply persist, incremental carbon savings of 20-30% are achievable with current technologies. The steel industry’s transition to hydrogen will depend on continued innovation, cost reductions in green hydrogen production, and supportive policy frameworks to accelerate adoption.