The Overlooked Water Cycle in Green-Hydrogen DRI

The Overlooked Water Cycle in Green-Hydrogen DRI Why the water used to make green hydrogen does not simply disappear The transition from natural-gas-based direct reduced iron (DRI) to green-hydrogen DRI is usually discussed in terms of carbon emissions and renewable electricity. But there is another material balance worth examining: water. Hydrogen does not simply disappear when it reduces iron ore. It becomes water. The fundamental chemistry Fe₂O₃ + 3H₂ → 2Fe + 3H₂O For every 3 kmol of hydrogen consumed, 3 kmol of water are produced. On a mass basis, producing 111.69 kg of iron consumes approximately 6.05 kg of H₂ and produces approximately 54.05 kg of H₂O. Therefore, per tonne of iron, the theoretical reduction reaction consumes approximately 54 kg H₂/t iron and produces approximately 484 kg H₂O/t iron. Where did the green hydrogen come from? Green hydrogen is produced by electrolysis. In simplified form: 2H₂O → 2H₂ + O₂ The theoretical water requirement is approximately 9 kg of water for every kilogram of hydrogen produced. Producing the approximately 54 kg of hydrogen theoretically required to reduce one tonne of iron from hematite therefore requires approximately 486 kg of water. The subsequent iron-ore reduction reaction produces approximately 484 kg of water again. The small difference is essentially rounding. So where did the water go? Water → Hydrogen → Iron-ore reduction → Water In the electrolyser, water is split into hydrogen and oxygen. The hydrogen is then used in the DRI shaft furnace to remove oxygen from iron oxide. The reduction reaction recreates water. Inside a hot DRI shaft furnace, that reaction product initially leaves principally as water vapour in the top gas, together with unreacted reducing gases and other components depending on the process configuration. This raises an important engineering question: why should all electrolyser water be regarded as permanently consumed? If the water vapour in the DRI off-gas is cooled and condensed, a substantial portion of the chemically generated water can potentially be recovered. It would require treatment to the quality needed for reuse, and a commercial green-iron plant will still have real make-up-water demand arising from cooling, purification, blowdown and other losses. The point is therefore not that hydrogen-based DRI has zero water demand. Rather, the stoichiometry shows that much of the water directly associated with producing and consuming green hydrogen is not destroyed. It changes chemical form and can potentially participate in a designed water-recovery loop. What does this mean at 0.2 MTPA? For a theoretical production rate of 200,000 tonnes of iron per year, the simplified hematite reduction chemistry corresponds to roughly: • 10,800 tonnes/year of H₂ consumed • 96,800 tonnes/year of H₂O formed by the reduction reaction • Approximately 97 million litres/year of reaction water These are stoichiometric values, not a complete commercial plant water balance. Actual values will depend on ore chemistry, metallisation, hydrogen utilisation, recycle-gas design, operating conditions, cooling systems and water-recovery efficiency. A different way to frame green iron We frequently hear the statement: “Green hydrogen requires enormous quantities of water.” That is directionally correct when describing electrolyser feedwater requirements, but for hydrogen-based iron reduction it is incomplete unless we also ask what happens to the water after the hydrogen has done its job. A large part of it has become water again. Perhaps future green-iron plants should therefore be designed not simply around a hydrogen balance and an energy balance, but around an integrated hydrogen-oxygen-water balance, with water recovery engineered into the process from the beginning.