Design of an Ammonia Synthesis System for Producing Supercritical Steam in the Context of Thermochemical Energy Storage

Concentrating solar power plants typically incorporate thermal energy storage, e.g. molten salt tanks. The broad category of thermochemical energy storage, in which energy is stored in chemical bonds, has the advantage of higher energy density as compared to sensible energy storage. In the ammonia-based thermal energy storage system, ammonia is dissociated endothermically as it absorbs solar energy during the daytime. The stored energy can be released on demand (for electricity generation) when the supercritical hydrogen and nitrogen react exothermically to synthesize ammonia. Using ammonia as a thermochemical storage system was validated at Australian National University (ANU), but ammonia synthesis has not yet been shown to reach temperatures consistent with the highest performance modern power blocks such as a supercritical steam Rankine cycle requiring steam to be heated to ∼650°C. This paper explores the preliminary design of an ammonia synthesis system that is intended to heat steam from 350°C to 650°C under pressure of 26 MPa. A two-dimensional pseudo-homogeneous model for packed bed reactors previously used at ANU is adopted to simulate the ammonia synthesis reactor. The reaction kinetics are modeled using the Temkin-Pyzhev reaction rate equation. The model is extended by accounting for convection in the steam to predict the behavior of the proposed synthesis reactor. A parametric investigation is performed and the results show that heat transfer plays the predominant role in improving reactor performance.