(320c) General Techno-Economic Analysis of Electrochemical Ammonia Production Processes with Various Nitrogen Sources

Ammonia (NH₃) is one of the most important industrial chemicals, which has been widely used in agricultural and industrial fields as a feedstock for fertilizer, refrigerant, explosives, etc.¹ The global NH₃ market in 2020 was US$63.7 billion per year, and it is expected to reach US$ 89.6 billion per year by growing at 6.9% of the compound annual growth rate.² Also, the NH₃ composed of nitrogen and hydrogen could be a promising hydrogen carrier without concerns of carbon dioxide emission, which incorporates it as a promising energy storage material.³

The NH₃ has been commercially produced by Haber-Bosch (HB) process reacting nitrogen and hydrogen at high temperature (400–500°C) and high pressure (20–30 MPa) over Fe-based catalysts.⁴ Although the HB process enables large-scale ammonia production, a large amount of energy and resource is required for the harsh operating conditions. Consequently, the global HB process consumed more than 1% of global energy demands, which contributed to a significant portion of greenhouse gas (GHG) emissions.⁵

To overcome this limitation, we focused on developing an electrochemical NH₃ production process based on renewable power. The electrochemical NH₃ production is based on the reduction of nitrogen sources (N₂, NO_x, etc.) obtained from air or wastewater, and it could be free from GHG emission and resources depletion by using renewable electricity.⁶ Although previous experimental studies reported the potential of electrochemical NH₃ production via nitrogen reduction reaction (NRR)⁶ or nitrate reduction reaction (NO₃RR)⁷, few studies have been conducted on the system-level of techno-economic analysis and life cycle assessment for the electrochemical NH₃ production process.⁸ In this study, we analyzed the electrochemical NH₃ production processes as a potential alternative to the HB process based on the techno-economic feasibility and environmental impact. Furthermore, we present a one-step system for the production of NH₃ from NO₃ in a cascade structure in which reaction, N₂ is oxidized to NO₃ ( N₂OR ) at the anode of the electrolyzer, and the NO₃ is supplied to the cathode to produce NH₃. Here, we focus on the conceptual process designs based on three electrocatalytic Nitrogen reductions reactions (NRR and NO₃RR, and N₂OR) in the NH₃ production process. First, we proposed an integrated system for the production of NH₃ based on three types of nitrogen reduction, which consist of nitrogen-source reduction (electrolyzer) and NH₃ separation (flash and distillation columns). Second, a techno-economic analysis was conducted on the three proposed processes. An automated process simulation and evaluation framework were developed to evaluate the impacts of various parameters of electrochemical reduction on the economics of electrochemical NH₃ production.

The automated framework of techno-economic analysis with rigorous process simulation managed by the central platform (Matlab) was employed to perform extensive sensitivity analysis. The process simulation is automatically performed by running commands in the Matlab code, and the Aspen Plus simulation results are transmitted back to the techno-economic evaluation code to determine the MSP of NH₃ of each process. Through global sensitivity analysis, the importance of significant parameters of electrochemical reactions such as current density, Faraday efficiency, and overpotential was analyzed to evaluate the performance of the design variables of the electrochemical process. The analysis was also conducted on several scenarios in which electrochemically produced NH₃ can compete in price with NH₃ of the existing HB process. To evaluate the environmental impact of the electrochemical ammonia production process, cradle-to-gate life cycle assessments for the electrochemical process and HB process were performed using SimaPro software. The two systems using NO3RR show the price of $ 4-6 /kg at 2000 mA/cm², 1 V overpotential, and 90% of Faray efficiency, which suggests that it is possible to compete with the HB process if the power generation cost of renewable energy and the installation cost of the electrolyzer system is reduced through technological improvement in the future. The minimum selling price of the one-step system (N₂-NO₃-NH₃) was $ 2.5/kg, which showed the lowest price compared to the N₂RR and NO₃RR systems in the future-based scenario. In this study, it is crucial that the environmental impact of the electrochemical process are significantly dependent on the carbon intensity of the electricity used. The operation of an electrochemical system with renewable energy would result in low climate change impact than HB process.

Finally, we proposed the bottlenecks of electrochemical NH₃ production and a feasible range of electrochemical reduction performance to replace the HB process. We hope that the present study will guide the electrochemical ammonia production to target overcoming the technological goals to make electrochemical reduction economically and environmentally compelling compared to the HB process.

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2. Global Ammonia Industry: Global Industry Analysts; 2022; 2020.
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