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| Fig. 1: A high-pressure steel reactor (1921) for the production of ammonia via the Haber-Bosch process shows the original production method of ammonia. (Source: Wikimedia Commons) |
Ammonia (NH3) has emerged as a promising vector in the ongoing energy transition, primarily by supporting the reduction of CO2 emissions across several sectors. As the world continues to seek scalable substitutes for fossil fuels to mitigate climate change, experts have looked to ammonia for its distinct physical, chemical, and logistical advantages that can enable cleaner energy systems with fewer greenhouse gas emissions.
Globally, ammonia production currently accounts for approximately 1.4% of all energy-related CO2 emissions. [1] Conventional ammonia is produced via the Haber-Bosch process (See Fig. 1), which relies extensively on natural gas both for its hydrogen source and as process heat. This type of ammonia is categorized as either brown (uses fossil-fuel-derived hydrogen) or grey (uses methane-derived hydrogen). If it implements carbon capture and sequestration (CCS), it is categorized as blue ammonia. However, a potential alternative is green ammonia, which uses renewable energy (solar, wind, etc.) instead of fossil fuels for hydrogen production via water hydrolysis. [2]
The transition to green ammonia could decrease or eliminate nearly all process- related CO2 emissions. If widely adopted, this could help decarbonize large segments of the fertilizer, shipping, and power generation industries.
Ammonia synthesis is energy-intensive, requiring about 812 MWh (megawatt-hours) of energy per tonne of NH3 produced. When synthesized via water electrolysis and renewable electricity, the primary emissions come from the carbon intensity of the power source. If the electricity used is below 200 grams of CO2 per kWh, net CO2 emissions from green ammonia production and use in shipping or industrial fuel become substantially lower than current fossil alternatives. [3,4]
Two different types of ammonia have the potential to reduce CO2 emissions:
E-ammonia made from wind or solar (with minimal hydrogen/ammonia emissions) can achieve approximately 80% lower lifecycle greenhouse gas (GHG) emissions compared to very low sulfur fuel oil (VLSFO) for shipping. [5]
Blue ammonia, which uses carbon capture, can achieve up to 0.1-0.2 tCO2/tNH3 reductions when capture rates are high, but real-world results depend heavily on effective sequestration and low methane/hydrogen leakage. [2,5]
Table 1 summarizes energy and emissions values for ammonia production routes:
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| Table 1: Energy consumption and greenhouse gas emissions for various ammonia types. [7] Indirect CO2 emissions are calculated using world average electricity types and do not account for additional renewable energy inclusion. |
Ammonia's versatility is reflected in its potential for shipping fuels, electricity generation, and as a hydrogen carrier. In maritime transport, ammonia can displace bunker fuel, drastically cutting emissions if nitrogen management is implemented to control N2O (a potent greenhouse gas) formation. In power generation, ammonia can be combusted directly or cracked into hydrogen for use in turbines, helping to balance renewable grids and improve energy storage options.
As a hydrogen carrier, ammonia offers a practical solution compared to pure hydrogen, which is difficult to store and transport due to its low volumetric energy density and propensity to leak. Ammonia, with a higher energy density and the ability to be liquefied under relatively mild conditions, can efficiently deliver hydrogen to end users. [6]
Despite its promise, ammonia's environmental impact must be carefully managed. The emission of reactive nitrogen compounds, such as N2O, during combustion or leakage can negate climate benefits and cause ecological harm. Technical measures, such as combustion tuning, catalytic converters, and robust leak prevention, are essential to ensure that ammonia use remains a net positive for decarbonization.
Furthermore, the production of ammonia is not 100% efficient. Therefore, some of the energy that is put into the manufacture of ammonia is not recoverable from the product due to the production of waste heat. Improving production efficiency will be a technological challenge to overcome in order to minimize wasted energy.
Ammonia is poised to play a major role in a lower-carbon future. Green ammonia from renewable power can eliminate nearly all process-related CO2 emissions. It is especially promising in decarbonizing agriculture, shipping, and electricity generation thanks to high energy density, existing infrastructure, and multifaceted utility. Continued innovation and strict controls are needed to unlock the full climate benefits while minimizing ecosystem risks.
© Hannah McCollum. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Capdevila-Cortada, "Electrifying the Haber-Bosch," Nat. Catal. 2, 1055 (2019).
[2] M. C. Hatzell, "The Colors of Ammonia," ACS Energy Lett. 9, 2920 (2024).
[3] "Mission Possible: Reaching Net-Zero Carbon Emissions From Harder-to-Abate Sectors," Energy Transitions Commission, November 2018.
[4] U. Jafar et al., "A Review on Green Ammonia as a Potential CO2 Free Fuel," Int. J. Hydrog. Energy 71, 857 (2024).
[5] J. Schmidt et al., "Life Cycle Assessment of Ammonia Fuel," LCA Consultants, February 2025.
[6] Y. Kojima, "Safety of Ammonia as a Hydrogen Energy Carrier," Int. J. Hydrog. Energy 50A,732 (2024).
[7] C. Rufer, "Ammonia - Fuel For Net-Zero," MAN Energy Solutions, Oct 2024.
