Design and optimization of an ammonia synthesis system for ammonia-based solar thermochemical energy storage

In ammonia-based solar thermochemical energy storage systems, stored energy is released when the ammonia synthesis reaction is utilized to heat the working fluid for a power block. It has been shown experimentally that supercritical steam can be heated in an ammonia synthesis reactor to a high temperature that is consistent with modern power blocks (similar to 650 degrees C). In this paper, a design is proposed for the first time of an entire ammonia synthesis system consisting of a heat recovery reactor to heat supercritical steam and a preconditioning system to preheat the feed gas to sufficiently high temperature. The structural (wall) material cost of the system may be relatively large due to the use of high temperature creep-resistant material. Thus, the focus of this study is on minimizing the wall material volume. A parametric study has been performed to investigate the effects of diameter, mass flow rate, and inlet temperature on the reactor wall volume for each component of the system. The results show that smaller tube diameter is preferred because it enhances heat transfer and thereby reduces the reactor size. The results also show the necessity of optimizing the entire system simultaneously because of interactions between the different components. An optimization algorithm is used to design the entire synthesis system with minimum wall material volume per power delivered to the steam. The results show that the preconditioning system plays an important role in the required wall volume. A modular system design is also proposed, which subdivides the heat recovery reactor into different sections in order to tailor the design to local conditions. The modular design is shown to reduce the wall material volume.