Ammonia is one of several alternative fuels being evaluated by the maritime industry as a future decarbonisation solution, thanks to its potential as a zero-carbon fuel and a carrier for hydrogen fuel.

Already a globally traded commodity, much of the ammonia shipped today is consumed in agricultural applications. To date, approximately 50 ammonia-fuelled vessels, including LPG carriers, bulk carriers, tugs and containerships, have been ordered for delivery over the next three years.

Zero-carbon fuels like ammonia have great potential to lower the carbon footprint of shipping. However, to realise this potential, ammonia must be produced from renewable electricity instead of its traditional feedstocks of coal or natural gas. It is possible to reduce carbon emissions from the ammonia lifecycle using carbon capture systems in the production process, but zero emission well-to-wake solutions have yet to be developed at scale.

Among the challenges of adopting ammonia as a maritime fuel solution are its lower energy content compared to conventional fuel oils, at about a third that of diesel, and high toxicity. However, the long track record of selecting appropriate materials for storing and carrying ammonia suggests that this will not be a significant challenge.

In addition, existing supply chains and availability of infrastructure and bunkering reinforce its suitability for propulsion applications.

A further advantage is that the production of renewable ammonia is not dependent on the availability of sustainable biogenic carbon, unlike other renewable carbon-based fuels. Already a scarce resource, sustainable biogenic carbon is expected to rise in price, making other renewable fuels more costly compared to ammonia.

It is possible to reduce carbon emissions from the ammonia lifecycle using carbon capture systems in the production process, but zero emission well-to-wake solutions have yet to be developed at scale.

What are the benefits?
Ammonia has the potential to enter the global marine fuel market relatively quickly and ultimately serve as one of a basket of fuels that could help the global fleet meet the greenhouse gas (GHG) reduction target of net zero by 2050 set by the International Maritime Organization (IMO).

Ammonia, NH₃, has no CO₂ emissions when consumed in combustion engines (not including those from the required pilot fuel) and has a low flammability risk. Because it does not contain sulphur, emissions from combustion are also free of sulphur oxides (SO_x).

Ammonia can also act as a carrier for hydrogen, alleviating some of the storage and transportation challenges associated with hydrogen’s low energy density.

For ammonia fuelled ships – and according to the IMO’s interim regulatory guideline – ammonia will need to be stored at approximately 1 bar as a liquid and at a maximum temperature of –30°C, only slightly above its boiling point of –33°C.

Storing ammonia at –30°C is a stricter requirement than the comparable IGC Code storage temperature for cargo and to maintain this temperature will require means to maintain tank pressure. A reliquefaction system – either redundant or possibly combined with a gas combustion unit (GCU) – could be seen as a mandatory requirement to deal with the resulting boil off gas.

Marine engines are currently being developed which apply existing dual fuel technologies, modified with additional equipment to make them capable of burning ammonia. The first four-stroke engine from IHI Niigata is already operating on ammonia, in the Sakigake, a dual-fuelled LNG/ammonia tugboat operated by NYK Line of Japan. Wärtsilä’s W25DF-A genset will be installed on the supply vessel Viking Energy under contract to energy major Equinor for operation from the beginning of 2026.

Production 
Ammonia is typically created by combining nitrogen with hydrogen in the Haber-Bosch process, an exothermic process which releases energy. Currently, most of the hydrogen produced globally is through the processing of coal, natural gas or other fossil fuels, ultimately carbon-loading the lifecycle emissions of ammonia fuel. However, ammonia can be produced in ‘blue’ form using recycled CO₂ and in ‘green’ from fully renewable energy sources, creating the required hydrogen feedstock.

Around 200mn tonnes of ammonia are produced annually, and global production capacity is expected to increase to around 289mn tonnes by 2030. This production increase is mainly expected to be in the form of green and blue ammonia, with the largest portion of this volume destined to be used in the energy sector, with ammonia replacing coal in power plants.

The majority of today’s ammonia production is used as a feedstock for fertiliser manufacture, which is not expected to diminish. As a result, ammonia production would need to continue to expand to support both growing food production as well as new demand from the energy sector.

Carbon emissions created during the production of ammonia will need to be considered when ammonia is used as a marine fuel. In particular the IMO’s greenhouse gas (GHG) fuel intensity (GFI) methodology will consider carbon emissions on a well-to-wake basis. Even small improvements in well-to-wake carbon intensity can make a huge difference to operating costs, as penalties will be levied at $380/t of CO₂ emitted above the base target for the GFI.  

Bunkering and safety considerations
As a globally-traded commodity, ammonia is commonly transported by ship and is frequently loaded and unloaded to and from gas terminals, providing the industry with a relatively mature infrastructure.

The operation of transferring ammonia to and from ships at ports does not differ greatly from established bunkering processes for other gas fuels. There are three key areas to consider when transporting and storing ammonia.

Ammonia is toxic to humans and aquatic life, and direct exposure must be limited to permissible limits for the safety of personnel and the environment. In low concentrations, ammonia can be irritating to the eyes, lungs and skin, while high concentrations or direct contact can be life-threatening.

Ammonia is incompatible with various industrial materials. In the presence of moisture, it reacts with and corrodes copper, brass, zinc and various alloys. Materials used for the storage and loading and unloading of ammonia should be carefully selected to limit the potential of leaks. Iron, steel and specific non-ferrous alloys resistant to ammonia should be used for tanks, pipelines and structural components where ammonia is used.

Ammonia has a relatively low fire risk due to its narrow flammability range (15.15–27.35% in dry air), relatively high ignition energy (2–3 orders of magnitude greater than common hydrocarbons) and low laminar burning speed rate (less than a quarter of methane). However, there is still potential for ammonia fires in the right conditions, so ammonia should be isolated from any ignition sources. The fire risks of ammonia when mixed with other fuels and lubricating oils are still being investigated.

These challenges need to be considered in the design of ammonia carriers, bunkering vessels or vessels using ammonia as a fuel or a carrier for hydrogen fuel. In addition, dedicated safety regulations need to be developed and implemented to enable the safe application of ammonia fuel.

Regulation and risk management
Ammonia has historical precedents as a fuel. First demonstrated in a locomotive in 1822, it was subsequently used as fuel in the first engine developed by Rudolf Diesel.

The reintroduction of ammonia to diesel engines is still in its early stages. Purpose-built ammonia-fuelled engines are under development and ammonia use is also being explored in fuel cells. However, using ammonia in internal combustion engines has known drawbacks.

Ammonia typically requires a hydrocarbon pilot fuel to be injected into two-stroke diesel cycle engines, which increases carbon emissions – although these are lower than those from engines burning only traditional fuels.

Additionally, while significant amounts of nitrous oxides (NO_x) were expected when ammonia is combusted in compression ignition engines, recent test results on full-scale engines have shown that NO_x emissions are significantly reduced compared to burning fuel oil. NO_x emissions are strictly regulated by the IMO and need to be considered in future ammonia engine development.

For operations in compliance with IMO Tier III regulation, ammonia-fuelled engines will still require an emissions abatement system, most likely selective catalytic reduction (SCR). For operation to Tier II standards, a compliant NO_x emission level can easily be met.

Burning ammonia can also potentially generate ammonia slip from the combustion process, which will be released through the exhaust gas. Emitting this through the stack could pose a safety risk, but engine makers suggest using SCR as a passive means to reduce ammonia slip.

Using ammonia as fuel also generates nitrogen dioxide (N₂O) during combustion as the result of an incomplete combustion process. Recent tests have shown that in practice, N2O emissions are very low, similar to levels seen in engines burning conventional fuel oil.

Even so, N₂O is a critical component in ammonia emissions, since it is 265 times more potent than CO₂ as a GHG. Even a small amount of N₂O in the exhaust gas can add substantially to well-to-wake emissions under the GFI regulation which includes both methane and N₂O as GHGs to be counted. 

ARM systems
The onboard storage of ammonia and its use as a marine fuel are governed by multiple conventions, regulations and codes. A recent addition to these requirements is the ammonia release mitigation (ARM) system, which needs to be incorporated into the design of fuel tanks and supply systems.

This system is designed to handle the release of ammonia vapour onboard ship, reducing risks to the crew and the surrounding environment. Various designs are being developed for the ARM system, either based on dissolving the ammonia vapour in a water solution or burning the ammonia in a GCU or a boiler.

The IMO’s safety-related regulations for international shipping are applied through the International Convention for the Safety of Life at Sea (SOLAS) which has historically prohibited the use of conventional fuel oils with less than a 60°C flashpoint (ammonia’s flash point is 11°C).

To accommodate interest in using gaseous and liquid fuels with a flashpoint of less than 60°C, the IMO adopted the International Code of Safety for Ships using Gases or Other Low-Flashpoint Fuels (IGF Code) by including a new Part G to SOLAS II-1 in 2015.

Part D of the IGF Code, which covers all gases and low-flashpoint fuel applications for IGF Code ships under SOLAS, requires companies to ensure that the seafarers on board these ships have completed the training that will give them the ability to fulfill their designated duties and responsibilities.

This is applied through the IMO’s International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW). When the SOLAS amendments were adopted into the IGF Code, the STCW Convention and Code were also amended to add specific training requirements and certification for IGF-Code seafarers.

To support the application of ammonia as fuel, member states – through the offices of flag states – should develop national training and certification suitable for certification to the STCW Convention.

Supporting ammonia adoption 
ABS recognises that safe ammonia integration requires a holistic approach encompassing three interconnected pillars. First, proactive regulatory engagement is paramount. A forward-thinking approach minimises disruptions and maximises the return on investment in ammonia-powered vessels.

Secondly, a robust multifaceted safety framework is essential. ABS’ approach combines a qualitative risk assessment through hazard identification (HAZID) studies with quantitative analysis using advanced computational fluid dynamics (CFD) modelling.

HAZID meticulously identifies potential hazards throughout the ammonia life cycle on board, informing targeted mitigation strategies. CFD simulations using modern tools to quantitatively assess the risks associated simulations then quantify the potential impact of ammonia releases, enabling optimised design of ventilation, gas detection systems and emergency response protocols. This combined approach helps ensure a comprehensive understanding and mitigation of risks.

Finally, real-time monitoring and optimised emergency response complete the safety equation. By integrating cutting-edge technologies like acoustic cameras for early leak detection, the industry can minimise potential exposure and prevent escalation.

By systematically addressing these three interconnected pillars, the maritime industry can confidently embrace ammonia as a cleaner fuel source. This comprehensive strategy allows for not only environmental sustainability, aligning with global decarbonisation goals, but also the safe, reliable and efficient operation of ammonia-fuelled vessels.

This marks a significant step toward a future where sustainability and safety are not competing priorities, but rather integral components of a thriving maritime sector.

• Further reading: ‘Are ammonia-fuelled ships a viable alternative option for greener maritime operations?’ Given the significant impact of maritime operations on greenhouse gas emissions, a range of alternative fuel options are being considered, including ammonia-fuelled ships. Toby Clark considers the pros and cons of using ammonia as fuel for shipping operations, which nevertheless carries recognised safety risks.
• As shipping strives to cut greenhouse gas (GHG) emissions, a raft of regulatory changes are on the horizon. But do they fudge the complex issue of maritime emissions reduction, asks Eirik Nyhus, Vice President, Director Environment, DNV, who outlines what lies ahead.