Shipping ammonia fuel could reduce the sector’s greenhouse gas emissions, but air pollution controls would be needed to limit health impacts

The International Maritime Organization has outlined a goal of reducing greenhouse gas (GHG) emissions from international shipping by at least 40% by 2030, compared to 2008 levels. This would require the use of alternative fuels or other energy solutions, such as electrification. The European Commission has contributed €10 million to establish a global network of Maritime Technology Cooperation Centres in developing countries which will help these countries limit and reduce GHS emissions from shipping through technical assistance and capacity building.  

Ammonia (NH₃) has been proposed as a possible candidate for an alternative maritime fuel, with the potential to be manufactured in a low-carbon way using carbon capture or renewable energy. However, its combustion¹ may lead to a negative impact on air quality through the production of nitrous oxide (N₂O), nitrogen oxides (NOx) and fine particulate matter (PM_2.5) caused by chemical reactions of ammonia emissions in the atmosphere. 

The new study modelled the air quality and climate impacts of transitioning from fossil fuels to ammonia as the major shipping fuel. It assessed six scenarios which varied according to two types of technology:  

• Pure ammonia engines, with low NOx and high ammonia (NH₃) emissions with unburned NH₃ resulting in high levels of PM_2.5 
• Ammonia-hydrogen engines NH₃-H₂ in which part of the ammonia is used to generate hydrogen that supports combustion, which have high Ox and low ammonia emissions, therefore less PM_2.5  

The scenarios also varied according to three policy approaches: 

• Following IMO regulations as of 2020 
• Applying scrubbing to remove ammonia from emissions (assumed to be 95% effective) in current NOx emission control areas (which, in Europe, are the Baltic Sea and the North Sea) ²  
• Extending NOx emissions compliance and ammonia scrubbing, globally 

The study used data from ammonia engine experiments and ship Automatic Identification System data to estimate the emissions of ammonia, nitrogen oxide and nitrous oxides from converting the entire global fleet to ammonia-powered ships. 

The results show ammonia has the potential to be a more climate-friendly shipping fuel. There has been concern about tailpipe nitrous oxide emissions from ammonia, but the analysis estimated that the climate impact was 5.8% of the current fossil fuel fleet (as measured by global warming potential). This is assuming that the production of ammonia is low carbon. 

However, the researchers highlight that ammonia fuel leads to nitrogen emissions that could cause ecological damage, such as soil acidification and eutrophication, and lead to additional emissions of nitrous oxide, a potent GHG, which may affect the overall greenhouse gas balance. Unburned ammonia emissions from pure ammonia engines would also lead to large increases in PM_2.5 and related global deaths. Ammonia leakages could also occur at other points in the supply chain, such as during production, distribution and fuelling. 

The public health impacts of air pollution that would arise from a switch to ammonia depend largely on the engine technology and policy choices. The ammonia-hydrogen engine appears to be the superior of the two technologies in this respect. The study results suggest that switching to this type of engine globally would avoid 16,900 deaths from PM_2.5 and 16,200 deaths from ozone (O₃) per year. 

Ammonia scrubbing within current NOx emission control areas would lead to a small reduction in death rates from PM_2.5 pollution for ammonia-hydrogen engines: 22,100 avoided deaths; but for pure ammonia engines NH3 scrubbing would give rise to 623,900 additional deaths annually compared to diesel engines to larger improvement in PM_2.5-related deaths or ammonia-hydrogen engines: 66,500 avoided deaths. For pure ammonia engines, even with worldwide NOx emissions standards and scrubbing, there would be 1,200 additional deaths resulting from PM_2.5 compared to diesel engines. 

These results show that ammonia-powered ships potentially offer a more climate-friendly solution for the shipping industry than fossil fuels, which make it easier to meet IMO targets on GHG reductions. However, to avoid negative impacts of air pollutants on health and the environment, stringent new ammonia emission regulations would be needed, particularly for pure ammonia engines scrubbing, or they would increase global deaths from PM_2.5. 

The large range of public-health outcomes shown in their study show there is reasonable doubt about emissions from ammonia engines. They conclude that transparent disclosure of emission data from the industry is crucial to eradicate such doubt from scientific communities and the public. 

Footnotes: 

1. The researchers say they endorse technologies that simultaneously minimise NOx emissions (currently regulated) and ammonia emissions (currently unregulated). 

2. This research focused on NOx emission control areas in 2020, which did not include the Mediterranean region. The Mediterranean region is, however, included in SOx ammonia emission control areas from 2025. 

Reference: 

Wong, A.Y.H., Selin, N.E., Eastham, S.D., Mounaïm-Rousselle C., Zhang Y. and Allroggen, F. (2024) Climate and air quality impact of using ammonia as an alternative shipping fuel. Environmental Research Letters 19. 084002. https://doi.org/10.1088/1748-9326/ad5d07 

To cite this article/service: 

“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol. 

Notes on content: 

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.