Principal Investigators
At a Glance
Ammonia (NH3) is an attractive solution to transport and store hydrogen (H2). The fertilizer industry has developed a mature and robust ammonia infrastructure over the last century, H2 conversion to NH3 has a low energy penalty, and ammonia can either be converted back to H2 through cracking or burned as a low-carbon fuel. However, ammonia energy adoption nonetheless faces challenges. The potential emission of reactive nitrogen species (NH3 , NOx , and N2O), negatively impacts air quality, the environment, human health, and climate. In a multidisciplinary effort, Porporato’s research group and collaborators have quantified these potential emissions from worst-to-best-case scenarios. This work highlights the need for proactive engineering practices and policies to reduce environmental concerns.
Research Highlight
Hydrogen (H2 ) has the largest potential to be the low-carbon fuel of the future because of its versatility and scalability. Large-scale international H2 production projects are underway, with many anticipating the growth of a robust international H2 trade between renewable-rich regions and demand hubs. However, H2 direct transport faces challenges due to its low energy density, requiring either extremely low-temperature liquefaction (<-253 C) or high-pressure compression (300-700 bar). Both operations are technologically and economically demanding. They are also prone to risky leakage, with obvious drawbacks linked to economic losses, safety risks, and even climate impacts due to H2’s indirect greenhouse gas (GHG) effect.
Converting hydrogen into ammonia (NH3) through the HaberBosch process (N2 + 3 H2 ⟶ 2 NH3) is arguably the most promising strategy for H2 long-distance transport. This industrial process is already applied at scale (180 Mt NH3 /y), mostly to produce agricultural fertilizers. Ammonia would offer advantages like storage at more reasonable conditions, mature transport infrastructure, and the ability to be converted back into hydrogen or burned as a fuel. Overall, this strategy holds promise for developing an ammonia-based economy, with ongoing projects exploring NH3 use in vessels and power plants. Yet, researchers and industry need to address the concerns about ammonia emissions and environmental impacts.
Ammonia (NH3) is a corrosive and toxic gas that can cause air and water pollution and harm ecosystems and human health. Despite mature infrastructure and regulations, satellite observations reveal that industrial NH3 production plants are hotspots of ammonia emissions (Figure 1.1). In the ammonia economy, emissions from pipelines, distribution and storage systems, fuel stations, and combustion and cracking sources may also occur. Additionally, undesired emissions of nitrogen oxides (NOx) and nitrous oxide (N2O) could occur during unabated or improper ammonia combustion. Together, these emissions would add another significant perturbation to the nitrogen cycle, a crucial aspect of Earth’s ecosystems that agricultural activities have already disrupted.
Satellites reveal ammonia leaks from the largest (≈4 Mt/y) production facility in the United States (Donaldsonville, Louisiana).
The potential emissions of nitrogen compounds will depend on the amount of ammonia produced and the losses due to leakages and undesired byproducts during combustion. For example, if ammonia fuel achieves a market penetration of around 5% of the current global primary energy demand (≈30 EJ/yr), ammonia production would need to increase to 1,600 Mt NH3 /y, around ten times the current level. If only a small percentage (0.5 to 5%) of the nitrogen in ammonia is lost due to leakages or undesired emissions during combustion, the resulting perturbation of the global nitrogen cycle could be between 6 and 65 Mt N/y. The upper limit is around 50% of the global impact of fertilizers (≈120 Mt N/y).
The efficacy of ammonia as a mitigation solution depends on the potential emissions of nitrous oxide (N2O). N2O is a greenhouse gas (GHG) around 300 times more potent than CO2 and is the leading anthropogenic contributor to stratospheric ozone depletion. Emissions could occur due to unwanted reactions during ammonia combustion. With a 1% nitrogen conversion of ammonia into N2O, ammonia combustion would have a GHG footprint worse than coal (Figure 1.2). Therefore, it would cause more climate damage than conventional fossil fuels. Though high-temperature combustion generally leads to much lower final N2O levels, challenges like local quenching during off-design conditions may affect emissions, requiring tradeoffs with other system performance metrics.
GHG footprint of ammonia compared to other sources for electricity generation. Upstream emissions of ammonia production are taken from the IRENA report (2022), with blue ammonia derived from a range of fossil fuels with carbon capture and storage. Values for the other energy sources are from the IPCC Report, with black diamonds standing for median values (Schlomer et al., 2014).
To maximize the benefit of ammonia adoption in the energy sector, it will be necessary to address the environmental challenges through proactive engineering measures before implementation. Identifying worst-case scenarios for ammonia systems can highlight areas of concern during development and optimization. Alternative combustion strategies, ammonia cracking, and existing technologies for converting emissions back into nitrogen offer potential solutions. Early evaluation and learning from past mistakes are crucial for a smooth transition to a more ammonia-based energy system.
References
Bertagni, M. B., et al., 2023. Minimizing the impacts of the ammonia economy on the nitrogen cycle and climate. Proceedings of the National Academy of Sciences 120(46):e2311728120. (https://doi.org/10.1073/pnas.2311728120).
IRENA. Innovation Outlook: Renewable Ammonia. International Renewable Energy Agency, 2022. (https://www.irena.org/publications/2022/May/Innovation-Outlook-Renewable-Ammonia).
J. Rockstrom et al., 2009. A safe operating space for humanity. Nature 461:472–475. (https://doi.org/10.1038/461472a).
Schlomer, S. et al., Annex III: Technology-specific cost and performance parameters, in Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, O. Edenhofer et al., Eds. (Cambridge University Press, Cambridge, United Kingdom and New York, NY, 2014). (https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf).