Switching ships from diesel to ammonia fuel poses potential health risks

As colossal container ships cross the oceans to deliver cargo, their huge diesel engines emit large quantities of air pollutants that drive climate change and have human health impacts. With maritime shipping responsible for almost 3% of global carbon dioxide emissions and causing approximately 100,000 premature deaths annually due to its negative impact on air quality, decarbonizing the industry is imperative.

To achieve this, the International Maritime Organization, a U.N. regulatory body, aims to transition the global shipping fleet from fossil fuels to sustainable alternatives like ammonia.

While ammonia has the potential to be nearly carbon-free when considering its production and use, a recent study by a diverse team of researchers from MIT and other institutions raises a red flag. They warn that using ammonia as maritime fuel without stringent emissions regulations could worsen air quality further and lead to devastating public health impacts.

Ammonia combustion produces nitrous oxide (N₂O), a greenhouse gas that is 300 times more potent than carbon dioxide. It also releases nitrogen oxides (NO and NO₂, known as NO_x) and unburnt ammonia, which can form fine particulate matter in the atmosphere. These particles pose serious health risks, including heart attacks, strokes, and asthma.

According to a new study, current legislation regarding the global switch to ammonia fuel could result in up to 600,000 additional premature deaths annually. However, by implementing stricter regulations and adopting cleaner engine technology, the switch could potentially prevent around 66,000 premature deaths caused by maritime shipping emissions while also having a significantly lower impact on global warming.

“Not all climate solutions are created equal. There is almost always some price to pay. We have to take a more holistic approach and consider all the costs and benefits of different climate solutions, rather than just their potential to decarbonize,” says Anthony Wong, a postdoc in the MIT Center for Global Change Science and lead author of the study.

Ammonia production has traditionally been associated with a large carbon footprint, but there is hope for a greener alternative. The maritime shipping industry is exploring the use of “green ammonia,” which is produced using renewable energy to make hydrogen via electrolysis and to generate heat. However, even green ammonia can still generate harmful emissions such as nitrous oxide, nitrogen oxides, and ammonia, which can have detrimental effects on the environment and human health.

The researchers aimed to provide a comprehensive understanding of the environmental and public health implications associated with transitioning the global fleet to ammonia fuel. They developed various scenarios to assess how changes in technology and policies would impact pollutant emissions.

Technologically, they examined two types of ship engines. The first utilizes pure ammonia, resulting in higher unburnt ammonia levels but lower nitrogen oxide emissions. The second involves a combination of ammonia and hydrogen to enhance combustion, optimizing the catalytic converter’s performance to control both nitrogen oxides and unburnt ammonia pollution.

In terms of policy, three scenarios were considered: current regulations, additional limits on ammonia emissions in North America and Western Europe, and global restrictions on both ammonia and nitrogen oxide emissions.

Using a ship track model, the researchers calculated the changes in pollutant emissions for each scenario and incorporated the results into an air quality model to evaluate the impact of ship emissions on particulate matter and ozone pollution. They then estimated the potential effects on global public health.

One of the main challenges stemmed from the lack of real-world data, as there are currently no ammonia-powered ships navigating the seas. Therefore, the researchers relied on experimental ammonia combustion data from their collaborators to construct their model.

Switching the entire shipping fleet to engines that burn pure ammonia, with no new regulations, could result in 681,000 additional premature deaths annually. This highlights the potential danger of ammonia emissions, as they are currently unregulated.

However, using cleaner engine technology, even without new regulations, could reduce the number of premature deaths to about 80,000. With stronger global regulations and cleaner engine technology, the number of deaths from shipping-related air pollution could be reduced by about 66,000.

“The results of this study show the importance of developing policies alongside new technologies,” says Noelle Selin, an MIT professor in the Institute for Data, Systems, and Society and the Department of Earth, Atmospheric and Planetary Sciences (EAPS). “There is a potential for ammonia in shipping to be beneficial for both climate and air quality, but that requires that regulations be designed to address the entire range of potential impacts, including both climate and air quality.”

The impact of ammonia on air quality varies across the globe, with East Asia being the most affected due to less stringent air quality regulations and higher levels of existing air pollution. This leads to the formation of more particulate matter from ammonia emissions, compounded by the region’s greater shipping volume.

In the future, the researchers aim to refine their analysis and urge the marine industry to share engine data to better evaluate air quality and climate impacts. They also seek to raise awareness among policymakers about the urgency of updating shipping emission regulations.

Journal reference:

1. Anthony Y H Wong, Noelle E Selin, Sebastian D Eastham, Christine Mounaïm-Rousselle, Yiqi Zhang and Florian Allroggen. Climate and air quality impact of using ammonia as an alternative shipping fuel. Environmental Research Letters, 2024; DOI: 10.1088/1748-9326/ad5d07