Life cycle assessment of renewable hydrogen transport by ammonia

Ammonia is a promising hydrogen carrier for enabling the efficient transport of hydrogen, as observed by the many hydrogen transport projects using ammonia. For the clean energy future, understanding environmental impacts of the transport system is important. This study conducts life cycle assessment (LCA) for the marine transport of renewable hydrogen using ammonia as the hydrogen carrier. The LCA considered renewable hydrogen produced from four systems; wind-powered electrolysis, gasification of forest residue, anaerobic digestion of food waste, and landfill gas reforming; followed by Haber-Bosch ammonia synthesis using the renewable hydrogen and nitrogen produced from air separation. The ammonia was then transported 11,000 km by sea to a destination facility where it was decomposed using either Ru or Ni catalysts to obtain hydrogen. Among the four hydrogen transport systems operated with renewable energy, electrolysis-hydrogen system presented the highest global warming impact of 3.31 kg CO₂ eq/kg H₂ due to electricity use for the electrolysis, whereas simpler processes based on a landfill gas system led to the lowest impact of 2.27 kg CO₂ eq/kg H₂. Process energy consumption was the major contributor to global warming impact with 27%–49.2% of contribution. The consumption of metals and energy during wind turbine construction resulted in the most significant impact in six out of 12 midpoint impact categories for the electrolysis-hydrogen system, which also led to the highest endpoint impacts. The endpoint impacts of the four systems were in the order of electrolysis > food waste > forest residue > landfill gas (from high to low) for both endpoint human health and ecosystems impacts. Ammonia decomposition using Ru catalysts exhibited slightly lower global warming impact than Ni catalysts, while final purification of hydrogen by vanadium membrane presented 4.8% lower impacts than the purification by pressure swing adsorption. Large-scale hydrogen supply chains can be achieved by technological improvement and support of policies and financial schemes.