(706c) Feasibility Analysis of Chemical Looping Ammonia Synthesis from Stranded Natural Gas.

Stranded Natural Gas (SNG) flaring is intermittent flaring of natural gas at a volume or location which makes it uneconomical to recover by traditional methods (compression to liquid natural gas, or combustion for electricity). SNG flaring releases a substantial amount of greenhouse gas emissions, prompting public and private entities to focus efforts on developing technologies to convert SNG to a transportable value-added product.[1] Ammonia production from SNG is a practical solution, considering it is industrially produced from natural gas today through the Haber Bosch process (1).

N₂+H₂NH₃, T=450°C, P=200atm

While the high temperature and pressure of the Haber Bosch process is needed to push the reaction equilibrium forward at an industrially acceptable rate, it makes the process uneconomical at small scales [2]. Recently attention has been given to chemical looping for ammonia synthesis (CLAS), because of its ability to produce ammonia at atmospheric pressures and favorable small-scale economics [2]. A literature review identified previously proposed chemical loops for ammonia synthesis with varying levels of technical success. Therefore, the objective of this work is to provide a techno-economic assessment of ammonia production from SNG using chemical looping synthesis.

Chemical looping is capable of circumventing the high pressures of the Haber Bosch process by separating the reaction into individual steps mediated by a material carrier. Generally, in CLAS a material carrier is used to fix nitrogen, then a hydrogen source is used to reduce the carrier and evolve ammonia leaving the material carrier to complete the chemical loop. Recently, a chemical loop using alkali metals as the material carrier was demonstrated to produce ammonia at atmospheric pressures and 100 [3]:

xN₂ + 4AH_x 2xA_2/xNH + xH₂

2xA_2/xNH +2xH₂xNH₃+2AH_x

(A denotes an alkali metal and x is the valence of A)

A different chemical loop was proposed in [4] using water as the hydrogen source mediated by a calcium carrier. Although, the hydrolysis of calcium nitride is thermodynamically favored to produce ammonia it also produces a stable calcium oxide. This results in an energy intensive process to regenerate calcium and complete the chemical loop:

3Ca +N₂ Ca₃N₂

Ca₃N₂ +3H₂O 2NH₃ + 3CaO

CaO +R Ca+ RO

(R denotes suitable reducing agent)

Here, to evaluate previously proposed chemical loops for mild condition synthesis of ammonia, focus is given to their thermodynamic feasibility. Thermodynamic feasibility is determined as a negative Gibbs free energy of reaction at a temperature in which the constituents of the reaction do not phase change. Along with thermodynamic feasibility, process safety, reaction yield and other criteria are used to create a decision tree to rationally evaluate chemical loops presented in this work. For each material carrier and chemical loop combination, the temperature that minimizes the Gibbs free energy of reaction at a constant pressure of 1 atm was found, assuming ideal states. Aspen plus was used to simulate the equilibrium reactions and to assess economic viability of the process using its economic analyzer. From this work it was discovered that modifying a previously proposed unsuccessful chemical loop, creates a new feasible chemical loop, shown in Fig.1. This new chemical loop along with other techno-economically feasible chemical loops are studied and presented in this work. Further studies are recommended to study the kinetic feasibility of the proposed chemical loops for industrial applications.

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] Natural Gas Flaring and Venting: State and Federal Regulatory Overview, Trends, and Impacts, 2019.

[2] R. Michalsky, B.J. Parman, V. Amanor-Boadu, P.H. Pfromm, Solar thermochemical production of ammonia from water, air and sunlight: Thermodynamic and economic analyses, Energy. 42 (2012) 251–260. doi:10.1016/j.energy.2012.03.062.

[3] W. Gao, J. Guo, P. Wang, Q. Wang, F. Chang, Q. Pei, W. Zhang, L. Liu, P. Chen, Production of ammonia via a chemical looping process based on metal imides as nitrogen carriers, Nat. Energy. 3 (2018) 1067–1075. doi:10.1038/s41560-018-0268-z.

[4] R. Michalsky, Thermochemical production of ammonia using sunligth, air, water and biomass, Kansas State University, 2012.