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

Abstract

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/Li 2 NH, MgH 2 /MgNH, CaH 2 /CaNH, SrH 2 /SrNH, and BaH 2 /BaNH. Only the LiH/Li 2 NH pair has satisfied the given criteria, and initial estimates suggest that a 62% yield is obtainable.

Towards green and efficient chemical looping ammonia synthesis: design principles and advanced redox catalysts

“…59 As shown in Fig. 4d, the thermodynamic feasibility of the A-H/ A-N-H pair mediated in H 2 -CLAS is in the sequence LiH/Li 66 More recently, Chen and co-workers measured the equilibrium concentration of LiNH 2 , KNH 2 , and Ba(NH 2 ) 2 amides in a high-pressure hydrogen atmosphere at different temperatures based on a highpressure gas-solid system equipped with a conductivity meter, providing essential information on the mechanistic understanding of metal amides hydrogenation for ammonia synthesis in H 2 -CLAS. 67 Metal nitrides are of interest as media for two-step H 2 -CLAS via nitrogen chemical looping.…”

“…Recently, Metcalfe and co-workers estimated and compared the formation enthalpies of different pairs, including LiH/Li 2 NH, MgH 2 /MgNH, CaH 2 /CaNH, SrH 2 /SrNH, and BaH 2 /BaNH, and found that only the LiH/Li 2 NH pair has satisfied the criteria for H 2 -CLAS. 66 More recently, Chen and co-workers measured the equilibrium concentration of LiNH 2 , KNH 2 , and Ba(NH 2 ) 2 amides in a high-pressure hydrogen atmosphere at different temperatures based on a high-pressure gas-solid system equipped with a conductivity meter, providing essential information on the mechanistic understanding of metal amides hydrogenation for ammonia synthesis in H 2 -CLAS. 67…”

Mixed oxides as multi-functional reaction media for chemical looping catalysis

“…95,123,130,197,198,[209][210][211] For instance, a recent study adopted the chemical looping strategy to selectively oxidize ammonia with excellent NO selectivity for nitric acid production. 192 A few other studies explored the feasibility of using nitrides/imides/hydrides [131][132][133][134][189][190][191][212][213][214] as intermediates for ammonia synthesis and using alkali molten salt as the medium for combined CO 2 capture and utilization. 215 Engineering the chemical potential of redox oxides has also shown to be able to circumvent reaction equilibrium limitations in conventional processes while maintaining high selectivity, making it possible to achieve previously unattainable yields for water gas shift reactions.…”

2022 Pioneers in Energy Research: Anders Lyngfelt

“…Ugwu and Amini 40 43 present the design and evaluation of a hydrogen energy storage system using metal oxides via a chemical looping scheme. Pereira and Metcalfe 44 report the impact of gas−solid reaction thermodynamics on the performance of a chemical looping ammonia synthesis process. Purnomo 45 studies the effect of the mass conversion degree of an oxygen carrier on char conversion and its implication for chemical looping gasification.…”