Recent progress towards solar energy integration into low-pressure green ammonia production technologies

Large progress has been made in the last decades to reduce the carbon footprint of ammonia, which is an essential commodity of the food, chemical and energy industry. Apart from alternative routes for green feedstock production, such as hydrogen via electrolysis and nitrogen via solar thermochemical methods, alternatives are explored to replace the Haber-Bosch process. The present article reviews four promising mild condition ammonia production methods: solid state synthesis, molten salt synthesis, thermochemical looping and photocatalytic routes. Contrary to the Haber-Bosch method, which requires high pressures of 200-400 bar, they operate at low-pressures, furthermore such routes open the possibility for direct ammonia production from H2O and N-2 without the intermediate hydrogen production step. These advantages allow easier renewable energy integration; however, R&D activities are needed for scaling-up. An analysis is given on renewable energy integration with focus on solar resources both in the form of electricity and heat. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.