Green Hydrogen-Based Direct Reduction

Direct reduction removes oxygen from iron ore while the iron remains solid, producing direct reduced iron (DRI) or sponge iron. In hydrogen-based direct reduction, green hydrogen, produced using renewable energy, replaces carbon monoxide or coal as the principal reducing agent. Hydrogen combines with the ore's oxygen and leaves the reactor as water vapour.

Reduction mechanism

Iron ore, generally prepared as pellets, is contacted with hot hydrogen in a shaft furnace. Hydrogen removes oxygen from iron oxides through successive reduction reactions while the operating temperature remains below iron's melting point.

• The overall reaction for hematite is Fe2O3 + 3H2 → 2Fe + 3H2O.
• Hydrogen acts as the reducing agent because it is oxidised to water while iron oxide is reduced to metallic iron.
• Removal of the resulting water vapour and circulation of hydrogen help sustain the reduction process.

From ore to steel

The porous sponge iron leaving the reduction furnace is subsequently melted, commonly in an electric arc furnace, and refined into steel. Hydrogen can initially supplement and eventually replace fossil-derived reducing gases where the plant and ore quality permit.

• Green hydrogen is usually produced by water electrolysis powered by renewable electricity.
• Hydrogen-based shaft furnaces generally require suitably processed, high-quality iron ore pellets.
• Low-carbon electricity is also needed for hydrogen production, ore preparation and electric steelmaking to minimise overall emissions.

Environmental significance and limitations

Unlike carbon-based reduction, hydrogen reduction produces water vapour rather than process carbon dioxide at the reduction stage. It can therefore address a major source of emissions from primary steelmaking, but it does not automatically make the entire steel chain emission-free.

• Lifecycle emissions still arise from mining, pelletisation, transport, electricity use and plant construction.
• Large-scale deployment depends on affordable green hydrogen, renewable power, suitable ore, storage infrastructure and redesigned furnaces.
• Steel composition may still require controlled carbon addition, although carbon need not serve as the main ore-reducing agent.