A Dual Technical and Environmental Assessment of the Transition from Grey to Green Hydrogen in the Haber–Bosch Process: A Modelling and Simulation Approach

The transition from fossil-derived (grey) hydrogen to electrolytic (green) hydrogen in Haber-Bosch ammonia synthesis introduces substantial technical, energetic, and environmental challenges at the process level. This study provides an integrated technical and environmental assessment per 1 ton of NH₃, combining detailed process simulation with system-level mass, energy, and emission accounting. Methodologically, the novelty lies in the coupled modelling framework that links electrolyser dynamics (including purity variations and intermittency) with the closed-loop behaviour of the ammonia synthesis section, allowing the quantification of stability limits and integration requirements under variable renewable operation. The approach further introduces a unified per-ton NH₃ indicator set that simultaneously evaluates electrical demand, compression duty, CO₂ intensity, and water consumption.

Stoichiometrically, the process requires 0.177 ton H₂/ton NH₃. Using proton-exchange-membrane electrolysis at 50 MWh/ton H₂ results in an electrolytic demand of 8.85 MWh/ton NH₃, and including a representative compression penalty of 1.5 MWh/ton H₂ yields a total electrical requirement of about 9.12 MWh/ton NH₃. From an environmental perspective, conventional SMR hydrogen (8.5 to 9.5 ton CO₂/ton H₂) corresponds to 1.50 to 1.68 ton CO₂/ton NH₃, whereas renewable energy-based electrolysis cuts direct process CO₂ emissions by >95%, contingent on the grid carbon factor for upstream electricity. Water use, quantified at around 9 to 10 kg H₂O per kg H₂, implies 1.6 to 1.8 m³ of deionized water per ton NH₃.

Simulation results show that hydrogen purity fluctuations and pulsating electrolyser output significantly affect loop conversion, recycle duty, and process stability, and that heat-integration and storage strategies are required to maintain steady operation under renewable intermittency. The unified performance indicators and dynamic-integration results generated in this work provide new design benchmarks for retrofitting existing plants and deploying next-generation low-carbon ammonia production systems.