Impact of Gas-Solid Reaction Thermodynamics on the Performance of a Chemical Looping Ammonia Synthesis Process

Novel ammonia catalysts seek to achieve high reaction rates under milder conditions, which translate into lower costs and energy requirements. Alkali and alkaline earth metal hydrides have been shown to possess such favorable kinetics when employed in a chemical looping process. The materials act as nitrogen carriers and form ammonia by alternating between pure nitrogen and hydrogen feeds in a two-stage chemical looping reaction. However, the thermodynamics of the novel reaction route in question are only partially available. Here, a chemical looping process was designed and simulated to evaluate the sensitivity of the energy and economic performance of the processes toward the appropriate gas-solid reaction thermodynamics. Thermodynamic parameters, such as reaction pressure and especially equilibrium ammonia yields, influenced the performance of the system. In comparison to a commercial ammonia synthesis unit with a 28% yield at 150 bar, the chemical looping process requires a yield greater than 38% to achieve similar energy consumptions and a yield greater than 26% to achieve similar costs at a given temperature and 150 bar. Entropies and enthalpies of formation of the following pairs were estimated and compared: LiH/Li₂NH, MgH₂/MgNH, CaH₂/CaNH, SrH₂/SrNH, and BaH₂/BaNH. Only the LiH/Li₂NH pair has satisfied the given criteria, and initial estimates suggest that a 62% yield is obtainable.