Modeling of ammonia synthesis to produce supercritical steam for solar thermochemical energy storage

In ammonia-based solar thermochemical energy storage systems, solar energy is stored by production of hydrogen (H2) and nitrogen (N2) via ammonia dissociation and released when the hydrogen and nitrogen react exothermically to heat a working fluid for electricity generation. In our lab, a concentric-tube ammonia synthesis reactor has been built with steam in the tube and ammonia synthesis in the shell packed with a porous bed of iron catalyst. It achieves heating of supercritical steam at ∼26 MPa from ∼350 °C to ∼650 °C; to our knowledge this is the first time that ammonia synthesis has been used to heat steam to a sufficiently high temperature for a supercritical steam power block. In this paper, a model is proposed to simulate heating of supercritical steam in an ammonia synthesis reactor. The model is a two-dimensional, steady-state, pseudo-homogeneous, packed bed reactor computational model that solves the energy and mass species conservation equations, along with the Temkin-Pyzhev rate equation. The model results for temperature distributions match well with experimental results from our reactor. A sensitivity analysis is carried out for the model to study the effects of six input parameters on the gas and steam temperature profiles. The results show the process in our reactor is heat-transfer-limited and most sensitive to activation energy. The process is also very sensitive to inlet ammonia mass fraction. Improving heat transfer and decreasing inlet ammonia mass fraction are crucial to improve the capability of the reactor to heat a higher steam mass flow rate per unit of synthesis gas mass flow rate.