Conception and optimization of an ammonia synthesis superstructure for energy storage

Abstract

Ammonia has a potential as carbon-free and high-energy density compound for chemical storage of renewable energies and its synthesis from green H2 requires to be as energetically efficient as possible. In this work, a superstructure optimization methodology for process synthesis is proposed and applied to an ammonia production process. The approach covers three different scales: process, equipment, and molecules. Process scale refers to finding the optimal process structure, while the equipment scale is related to the best set of operating conditions, and the molecular scale studies two catalysts (Fe and Ru) with their respective kinetic rates. The optimization intends to minimize the Levelized Cost of Ammonia (LCOA) and maximize the energy efficiency. The best trade-off is found with the Ru catalyst, with an LCOA of 766€/tNH3 and 57.2% of energy efficiency. In comparison with reference cases, the LCOA decreases 0.6% and the energy efficiency increases 1.5%. The main improvement is found in the pressure, 75bar, reduced in 25%. The production of 11.6tNH3/day equals to 2.5MW of stored power from 3MW supplied as H2 to the process, with a total energy consumption of 10.67kWh/kgNH3, including prior H2 and N2 production processes.