Discussions Emerge About the Green Potential of Ammonia

By Surbhi JainAug 24 2022Reviewed by Susha Cheriyedath, M.Sc.

In an article recently published in the journal ACS Energy Letters, researchers discussed the routes to a future of green ammonia.

Study: Pathways to a Green Ammonia Future. Image Credit: Evgenii Panov/Shutterstock.com

Background

Ammonia (NH₃) produced by industrial synthesis has been recently identified as a promising carbon-free H₂ carrier for energy storage and transmission.

The standard Haber-Bosch NH₃ synthesis method, which produces H₂ by steam methane reforming (SMR), accounts for 1.2% of the world's annual energy consumption and generates around 235 million tons of CO₂ annually. Due to the direct CO₂ emissions, this process is challenging to decarbonize.

In contrast, using low-carbon electrical sources, green H₂ synthesis from water electrolysis only yields H₂ and O₂. Therefore, significant decarbonization of NH₃ synthesis might be achieved by replacing SMR with water electrolysis powered by renewable energy sources. A variety of water electrolyzer types have been taken into consideration for the modified Haber-Bosch process that uses power produced from renewable sources to synthesize green NH₃. These include alkaline water electrolysis (AWE), solid oxide water electrolysis (SOE), and polymer electrolyte membrane water electrolysis (PEM WE).

For each type of water electrolyzer used, a techno-economic analysis (TEA) is necessary to assess the viability of producing green NH₃ based on potential changes in the price of green H₂. Green NH₃ generation using the main electrolyzer technologies has not yet been examined in terms of economic estimates or an analysis of the environmental impact.

About the Study

In this study, the authors presented the TEA for large-scale green synthesis of NH₃ using PEM WE, AWE, and SOE with electricity produced by solar photovoltaics. They estimated the costs of producing NH₃ through the year 2050 while taking into account the learning-by-doing effect, which could lower capital expenditure (CAPEX) through cumulative production and real-world user experience. They also considered variations in the Levelized cost of electricity (LCOE) for solar photovoltaics, assuming a variety of water electrolyzer efficiency levels.

The team carried out a life cycle assessment (LCA) to compare the environmental effects of the traditional Haber-Bosch process with green NH₃ synthesis utilizing several kinds of water electrolyzers. In order to determine whether green NH₃ production using PEM WE, AWE, and SOE could become cost-competitive with the commercial Haber-Bosch process and attain the green NH₃ target prices, the LCA was included in the TEA via a carbon tax.

The researchers detailed all processes for calculating the unit production costs and the CO₂ emissions for the synthesis of green NH₃. With a unit energy price of 0.06 USD per kWh^-1, the impact of system efficiency and learning rate on the cost of producing green H₂ was examined for PEM WE, AWE, and SOE.

Observations

The parity years for AWE, SOE, and PEM WE were 2035, 2035, and 2050, respectively, with the CO₂ tax, and 2050 and 2045, respectively, without the CO₂ tax. In the best-case scenario, AWE, PEM WE, and SOE would have costs of 0.42 USD, 0.60 USD, and 0.35 USD kg_NH3^-1 with the highest CO₂ tax. In the worst-case scenario, these costs would be 0.78 USD, 0.92 USD, and 0.69 USD kg_NH3^-1; respectively.

In the best-case scenario, the CO₂ tax increased the cost of producing green NH₃ by 37.6%, 23.2%, and 37.6% for AWE, PEM WE, and SOE, respectively. Prices for H₂ for AWE, SOE, and PEM WE in 2020 under the baseline scenario were 10.79, 14.31, and 13.35 USD kg_H2^-1, respectively.

When taking into account increases in electrolyzer energy efficiency, drops in the price of water electrolyzers, and decreases in the LCOE for renewable energy, the results implied that green NH₃ production could be an economical and environmentally friendly alternative to the traditional Haber-Bosch process. One of the most promising technologies for decarbonizing NH₃ synthesis, scenario-based cost projective analysis, could help determine the research and technology development priorities needed to enable green NH₃ synthesis utilizing H₂ from water electrolysis.

In a CO₂ tax scenario, green NH₃ production could also be economically viable and may even be considerably less expensive than the traditional Haber-Bosch process, which could be significant given the growing concern for environmental issues around the world.

Conclusions

In conclusion, this study discussed the forecasted H₂ pricing for AWE and SOE in 2035 and 2036, respectively, which, in the best case scenario, were expected to be less than 2.16 USD kg_H2^-1. On the basis of the combined TEA and LCA results, the authors presented the possible benefits of SOE for green NH₃ production. They showed that SOE combined with the Haber-Bosch process might be the most viable electrolyzer type. They also presented numerical findings about the impact of CO₂ price strategies on achieving green NH₃ production. The effect of the presence of a CO₂ tax on the competitiveness of green NH₃ production was demonstrated.

The authors mentioned that the NH₃ economy roadmap and environmental policies could be guided by this integrated techno-economic and environmental study of green NH₃ production for the optimistic, median, and pessimistic scenarios. They believe that the findings of this study shed light on the viability of green NH₃ synthesis and the variables that are anticipated to influence modifications to its techno-economic and environmental profile in the ensuing decades.

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References

Lee, B., Winter, L. R., Lee, H., et al. Pathways to a Green Ammonia Future. ACS Energy Letters (7) 3032-3038 (2022). https://pubs.acs.org/doi/10.1021/acsenergylett.2c01615

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