(508g) Feasibility of Chemical Looping Ammonia Synthesis from Metal Nitrides and Hydrides and Their Alloys.

Ammonia is a commercially important chemical to the food, drug and energy industry. In 2015, 160 million tons of synthetic ammonia was produced to meet global demand which is expected to rise 1.5% yearly [1]. The Haber Bosch process is a century old process, responsible for the majority of industrial ammonia synthesis through the direct combination of its elements: N₂ + 3H₂ ↔ 2NH₃ (T=450^oC, P=200atm). The Haber Bosch process is an energy intensive polluting process, accounting for 1% of the world’s annual energy demand while emitting 400 Mt/yr of CO₂ [2]. The significant negative environmental impact of the Haber Bosch process has led to a search for alternative means to synthesize ammonia [2]. Chemical looping ammonia synthesis (CLAS) has been proposed as a low energy alternative to the Haber Bosch process [3]. Through a multistep process, CLAS circumvents the equilibrium of the Haber Bosch process and its high pressure requirement [3]. CLAS processes mediate ammonia synthesis by fixing nitrogen to a compound, then reacting that compound with a hydrogen carrier to yield ammonia, e.g., 1) 3MH₂ + N₂↔ M₃N₂ + H₂, 2) M₃N₂ + 6H₂↔ 3MH₂ + 2NH₃ (T=100^oC, P=1atm), where M is the material (hydrogen or nitrogen) carrier. Identifying carriers M, capable of mediating ammonia synthesis in a chemical loop is difficult, due to the excessive number possible material-reaction combinations, the majority of which are infeasible [3]. Additionally, tradeoffs exist between the ability of an element to fix nitrogen and then release it as ammonia in the presence of a hydrogen donor [3].

Prior analysis has identified a small number of CLAS schemes that are theoretically capable of producing ammonia at appreciable amounts without the energetically expensive requirements for high pressure of the original Haber Bosch process. Limited experimental evidence [3] supports the hypothesis that material mixtures (usually in form of metal alloys) offer advantages in the form of bifunctional carriers that allow for increased ammonia yields. To identify metals and their alloys that maximize the theoretical yields of ammonia via low-pressure, multi-step CLAS schemes, we present a comprehensive framework for evaluation of schemes and material carriers along with the optimization of reactor conditions for these schemes at steady state. First, elements from a chemical database are paired with CLAS schemes of a recent literature review [4] and are evaluated for their thermodynamic feasibility and favorability. The feasible set is then expanded to alloy carriers. Successful CLAS processes are optimized in Aspen Plus, wherein the reaction steps are assumed to be governed by Gibbs free energy minimization constraints of the chemical and phase equilibrium. Then, the performance of optimized CLAS processes is compared and ranked against metrics that measure the energy demand, initial cost and environmental impact of the process. It is shown that combining a successful nitrogen fixing element with a successful hydrogen carrying element can increase the ammonia yield of a CLAS scheme. Additionally, the combination of elements allows for the use of low cost environmentally safe materials, which can accelerate the development of CLAS processes. This work concludes with a list of successful combinations of elements and chemical loops for ammonia synthesis at low pressure.

Acknowledgements
This work was partially sponsored by the United Technologies Corporation Institute for Advanced Systems Engineering (UTC-IASE) of the University of Connecticut. Any opinions expressed herein are those of the authors and do not represent those of the sponsor.

References

[1] Fao, World fertilizer trends and outlook to 2018; Annual Report 14; Food and Agriculture Organization of the United Nations (FAO): Rome, Italy, 2015; ISBN 978-92-5-108692-6.

[2] L. Wang et al., “Greening Ammonia toward the Solar Ammonia Refinery,” Joule, vol. 2, no. 6, pp. 1055–1074, 2018.

[3] W. Gao et al., “Production of ammonia via a chemical looping process based on metal imides as nitrogen carriers,” Nat. Energy, vol. 3, no. 12, pp. 1067–1075, Dec. 2018.

[4] L. Burrows et al., (In Review) “Feasibility of Chemical Looping Ammonia Synthesis from Metal Nitrides and Hydrides and their Alloys,” Chem. Eng. J.