A Theoretical Study on the Economic Feasibility of Proton Exchange Membrane Electrolysis Cells for Nitrogen Fixation

Abstract

The Haber-Bosch process for the thermochemical synthesis of ammonia transformed the fertilizer industry by allowing for ammonia production in industrial scales. Since its invention in the early 1900s, the process has enabled the continuous expansion of the global population. The process operates at high temperatures (700 K) and pressures (100 bar) and takes advantage of economies of scale to produce around 150 million tons of ammonia per year at an efficiency of up to 70%. However, the operating conditions and the massive scale of the process lead to high levels of energy consumption and elevated fertilizer prices due to additional transportation costs from the highly centralized production facilities to the decentralized locations of use. In addition, the process relies on methane reforming as a hydrogen source, making ammonia the commodity chemical with the most significant carbon footprint. One possible strategy to overcome the shortcomings of the Haber-Bosch process is the decentralized production of ammonia-based fertilizers by a proton exchange membrane (PEM) electrolysis plant. Electrochemical ammonia synthesis by a PEM electrolysis cell offers renewable and clean production of ammonia and, consequently, has attracted interest in the scientific community. This technology would be coupled with renewable energy sources to harness nitrogen and water from the air to produce fertilizers near the location of use. This proves advantageous for the agricultural industry because the inexpensive feedstock materials and reduced transportation costs may improve access to fertilizers in the developing regions. However, the efficiencies reported for this technology are still too low to be competitive with the Haber-Bosch process. A detailed theoretical study on the economic feasibility of PEM electrolysis cells for nitrogen fixation is nonexistent, which hinders the development of this technology. A detailed economic analysis is valuable to determine the cost of the technology and the efficiency levels that have to be achieved before the technology is competitive. This work presents a comprehensive economic analysis to investigate the efficiency threshold that would make the technology viable. First, a thermodynamic energy analysis is used to investigate the thermodynamic characteristics of an ammonia production facility while also incorporating the electrochemical characteristics of a PEM electrolysis cell. The model developed is coupled with experimental data to determine the system losses, energy inputs, and the system required size. The system losses and energy inputs are then used to calculate the systems energy and operation costs. Then, a detailed manufacturing cost analysis is used to determine the system capital cost. The model serves to define the optimal manufacturing process to reduce cost. Finally, a life cycle analysis is performed to calculate the systems social and environmental costs. The system performance gathered from the thermodynamic, manufacturing, and life cycle analyses are compared to the current ammonia fertilizer method (Haber-Bosch process) to determine the technology competitive advantage. This work offers a better understanding of the current state of the use of PEM electrolysis cells for ammonia production. In addition, this work highlights the systems threshold values that have to be achieved to provide a competitive advantage of the Haber-Bosch process.