(617a) Comparative Technoeconomic Analysis of Conventional and Absorbent-Enhanced Ammonia Synthesis
Recently, alternative ammonia synthesis processes have been proposed with a view of reducing the GHG intensity of centralized production and/or reducing capital cost to enable distributed production to be economically competitive despite the inherent losses in economies of scale. One promising alternative is to replace the condenser used in the conventional process with a bed of ammonia-selective absorbent . This absorbent-enhanced process allows for removal of ammonia at lower pressure (approximately 30 bar) and with less reactor effluent cooling than in the conventional process. We have previously created a mathematical model of this process. We used this model to optimize the design of the absorbent-enhanced process, minimizing its net present cost at distributed production scales of 800 to 80,000 ton/year under the condition that the process is powered exclusively with low cost wind energy .
However, the transition to a distributed, electrolysis-based ammonia production infrastructure will not be immediate, and retrofitting current natural-gas based production facilities with the absorbent-enhanced process may still reduce production costs or increase energy efficiency. To this end, in this work we extend the absorbent-enhanced process model to consider larger production scales, as well as to consider natural-gas based hydrogen production. We also develop a model of a conventional, condenser-based ammonia synthesis process. This conventional process model allows for validation of design assumptions (i.e. process configuration and unit capital cost correlations) based on the literature for technoeconomic analysis of conventional ammonia synthesis. These design assumptions can then be applied to the absorbent-enhanced process model to provide a consistent basis for comparison of the two processes.
Using these models, we determine and compare the levelized cost of ammonia production, total capital investment, energy use and GHG emissions of conventional and absorbent-enhanced process flowsheets, which have been optimized to minimize levelized cost. We consider both distributed (<500,000 ton/year) and large-scale/industrial (500,000 ton/year to 1,500,000 ton/year) ammonia production scales. For each of these production scale ranges, we consider hydrogen sourced from both natural gas and electrolysis and perform sensitivity analysis to natural gas (hydrogen) and electricity prices for these respective means of hydrogen production. This comprehensive technoeconomic analysis allows us to identify the production scales and market conditions for which the absorbent-enhanced process is best suited. Additionally, it reveals environmental policy considerations such as an emissions tax or âgreenâ production credit necessary to enable renewable-powered electrolysis-based hydrogen production to be economically competitive.
 Alexandratos & Bruinsma. World agriculture towards 2030/2050. Food and Agriculture Organization of the United Nations (FAO), Rome, Italy, 2015.
 Swaminathan & Sukalac. Technology transfer and mitigation of climate change: The fertilizer industry perspective. IPCC Expert Meeting on Industrial Technology Development, Transfer and Diffusion, Tokyo, Japan, 2004.
 Morgan, Manwell, & McGowan. Sustainable ammonia production from U.S. offshore wind farms: A techno-economic review. ACS Sustainable Chem. Eng., 2017, 5 (11), 9554â9567.
 Pfromm. Towards sustainable agriculture: Fossil-free ammonia. J. Ren. Sust. Energy, 2017, 9, 034702.
 Demirhan, Tso, Powell, & Pistikopoulos. Sustainable ammonia production through process synthesis and global optimization. AIChE J., 2018, DOI: 10.1002/aic.16498.
 Malmali, Wei, McCormick, & Cussler. Ammonia synthesis at reduced pressure via reactive separation. Ind. Eng. Chem. Res., 2016, 55, 8922-8932.
 Palys, McCormick, Cussler, & Daoutidis. Modeling and optimal design of absorbent enhanced ammonia synthesis. Processes 6, 91.