Heat recovery from an autothermal ammonia synthesis reactor for solar thermochemical energy storage

Abstract The previously proposed ammonia synthesis system for ammonia based solar thermochemical energy storage is complex with two catalyst-filled reactors, i.e., a heat recovery reator to heat the working fluid and an autothermal synthesis reactor to preheat the feed gas. In this paper, an autothermal heat recovery reactor (AHRR) is proposed, which cannot only heat the working fluid but preheat the feed gas simultaneously with ammonia synthesis. A model is proposed to simulate heating of the working fluid, e.g., sCO 2 , and the feed gas in an AHRR. Based on the model results, the AHRR not only requires less catalyst volume but has the potential for lower heat losses to the environment than the previous system. A sensitivity analysis is performed to analyze the potential model error. The analysis shows the model is most sensitive to the activation energy of the catalyst E a . A parametric study has been performed to investigate the effects of diameters, mass flow rates, and inlet gas temperature for reaction on the reactor wall volume of the AHRR. The results show that smaller diameters of center tube and outer annulus are preferred for reducing the reactor size due to enhanced heat transfer for the secondary flows. But smaller catalyst bed thickness is not preferred because of larger pressure drop. Increasing sCO 2 mass flow rate also reduces the reactor size by enhancing heat transfer. Although increasing inlet gas temperature for reaction decreases the reactor size as well as the required pumping power, there is a trade-off between reducing the reactor size and increasing the size of the heat exchanger for preheating the feed gas from the storage. The study provides a baseline for a further optimization including economic analysis.