(495b) Performance Targets for Renewable Electricity-Based Electrochemical Ammonia Production

The adoption of low-carbon resources such as wind and solar in the power sector are increasing interest in production of dense, carbon-free fuels such as ammonia. For this reason, it may be attractive to develop fully electrochemical ammonia production (EAP) processes that can be operated flexibly with intermittent power. In the present work, we holistically evaluated the cost-optimal design and operation of an EAP process that capitalized on the unique flexibility of electrochemical methods.

Our approach involves formulating the least-cost design and operation of the EAP process as a mixed-integer linear programming model. The model includes hourly operation and sizing of major process components to minimize levelized ammonia production costs while meeting plant-level production requirements. The analysis captures partial loading relevant for EAP by

1. approximating nonlinearities in reactor power consumption through piecewise approximations
2. limiting the range of operating current densities
3. adding inter-temporal constraints governing material and energy storage operation and sizing.

We used the model to explore system dynamics and to determine performance requirements necessary to produce ammonia at competitive costs. The flexible, fully electrochemical approach allowed for reduction in high-cost electricity and hydrogen storage in favor of inexpensive ammonia storage. We found that the energy consumption of ammonia synthesis from H₂ and N₂ should be <20 MWh/ton for competitively-priced ammonia. Capital costs associated with ammonia synthesis and separation are significant, making up 15-50% of total ammonia costs. Reactor cost contributions were found to decrease with increased maximum current densities for ammonia production, and remained approximately constant if the current densities exceeded 750 mA cm^-2. A relatively low pressure (~20 bar) of the ammonia synthesis loop was found to be desirable. The obtained specifications for the energy efficiency and rates necessary for ammonia synthesis can act as minimum viable targets for researchers working on electrochemical nitrogen reduction.