Greening Ammonia toward the Solar Ammonia Refinery

In light of the targets set out by the Paris Climate Agreement and the global energy sector's ongoing transition from fossil fuels to renewables, the chemical industry is searching for innovative ways of reducing greenhouse gas emissions associated with the production of ammonia. To address this need, research and development is under way around the world to replace the century-old Haber-Bosch process for manufacturing ammonia from N₂ and H₂, powered by renewable electricity. This involves replacing H₂ obtained from steam-reformed CH₄ to H₂ that is instead obtained from electrolyzed H₂O. This transition will enable the changeover from the Haber-Bosch production of NH₃ to electrochemical, plasma chemical, thermochemical, and photochemical generation of NH₃. If ammonia can eventually be produced directly from N₂ and H₂O powered by just sunlight, at a technologically significant scale, efficiency, and cost, in a “solar ammonia refinery,” green ammonia can change the world! It is well known that the century-old Haber-Bosch process, N₂ + 3H₂ → 2NH₃, is thermally powered by fossil energy, resulting in a greenhouse gas intensive process, which needs to be replaced by one driven instead by renewable energy. To address this issue, we need to replace the energy-intensive synthesis process of ammonia from N₂ and H₂ by one powered instead by renewable electricity. This involves replacing H₂ obtained from steam-reformed CH₄ to H₂ that is instead obtained from electrolyzed H₂O. Consequently, the required energy is dramatically decreased, and the conversion no longer requires CH₄ derived from natural gas because the H₂ comes directly from H₂O. Herein, we present a critical overview of past and current research on ammonia synthesis that is envisioned to evolve to the “solar ammonia refinery” of the future. A high-level analysis of the energy requirements of the proposed greening ammonia strategies further supports the “solar ammonia refinery.” An overview of current research theme of catalytic ammonia synthesis and potential candidates for “solar ammonia refinery.” A high-level analysis of the energy requirements for different ammonia synthesis strategies is shown.