Design and Comparison of Ammonia Reactors for Power-to-Ammonia

Abstract

This work investigates the design and performance of three general types of ammonia reactors for Power-to-Ammonia (P2A). Specifically, the reactors are an adiabatic quench cooled reactor (AQCR), adiabatic indirect cooled reactor (AICR) and internal direct cooled reactor (IDCR).We present rigorous steady-state and dynamic models for the three reactor types. Steady-state optimisation at nominal load shows that the AICR and IDCR yield the highest reactant conversion at 30.0 \% and 29.4\% respectively, while the reactant conversion in the AQCR is significantly lower at 26.1 \%. We perform open-loop simulations around the optimal operating point. This illustrates the unstable oscillatory dynamics of the AQCR, while the AICR and IDCR show stable and fast decaying oscillations. However, a stability analysis revealed that the optimal operating points are situated on the verge of reactor extinction for all the reactors. Thus, even minor disturbances pose a risk of extinguishing the reactors when optimally operated.We optimise the reactors over an operating window from 30\% to 130\% of nominal load relevant for P2A operations. The AICR and IDCR exhibit similar reactant conversions over this operational range and showcase adaptability to varying loads as significantly higher conversions are achieved at lower loads. In contrast, the AQCR exhibits a less pronounced increase in reactant conversion at low loads.Considering the location of the optimal operating points close to reactor extinction, we suggest operating the reactors at a 15 K higher feed temperature than optimal (safety margin). We show that a 15 K safety margin only decreases ammonia conversion slightly over the operating window and ensures stable operation even for a 10\% decrease in reaction rate.

Keywords: Power-to-AmmoniaSteady-state optimisationReactor designStability analysisDynamic simulationsFlexible operation