Ammonia as a marine fuel is making the leap from theory to practice. Engines have been developed, ports are testing ammonia bunkering, and ships are being built or retrofitted. The sector is entering a new phase: ‘Ammonia is now where LNG was fifteen years ago.’
This article was writte by Mels Dees, freelance maritime journalist, mels.dees@me.com.
At an event dedicated to ammonia held in Rotterdam in early June, extensive attention was given to the use of ammonia as a marine fuel. The emerging picture is that of a supply chain being readied for real-world implementation — from fuel supply and engine development to ship design and safety systems.
Ports are already preparing for the bunkering of ships running on ammonia. ‘As a port, you need to be ready when the first ammonia-powered vessels arrive — which is expected within two years,’ says Duna Uribe, Senior Business Manager at the Port of Rotterdam. With that ambition in mind, the Port Authority organised the first ship-to-ship ammonia bunkering within a port area in April 2025.
For this milestone, 500 tonnes of grey ammonia were transferred from a bunker vessel under strict safety measures and in close cooperation with partners such as OCI, APM Terminals, and Trammo. ‘We worked on this for a year and a half,’ says Uribe. Éverything had to be right: safety, procedures, and coordination with partners. There were zero emissions during the trial — even during the flushing of the pipes. That’s an achievement we’re proud of.’
The pilot is part of a broader strategy in which the port is working towards regular ammonia bunkering using its own Port Readiness Level methodology. ‘This test was a system-level validation,’ Uribe explains. The project was made possible in part through European funding and was carried out in collaboration with, among others, Erasmus University.
Also read: VIDEO: Port of Rotterdam trials ammonia bunkering
Suitable engines
At the heart of using ammonia as a marine fuel lies, of course, the availability of suitable engines. Manufacturer Wärtsilä began developing ammonia-powered engines in 2020. Now, the first commercial sale of the so-called 25-engine – an Otto-cycle engine for ammonia – is a reality. ‘The principle is similar to our gas engines,’ explains René Zuidam, Senior Sales Manager Marine New Build at Wärtsilä. ‘It’s a dual-fuel engine. If there’s a failure in the gas supply section, it automatically switches to 100 per cent diesel. Safety is built in.’
The complete solution includes, in addition to the engine, the ammonia fuel gas supply system, the Wärtsilä Ammonia Release Mitigation System (WARMS), and a Wärtsilä NOx Reducer (SCR) for optimal exhaust gas aftertreatment.
The engine is available in power outputs ranging from 1.7 to 3.4 MW, making it particularly suitable for smaller vessels or as part of hybrid propulsion systems. The first prototypes are already running in Finland. ‘We’ve achieved over 1000 operating hours — without leaks and without the smell of ammonia,’ reports Zuidam.
Also read: Wärtsilä ammonia engine cuts GHG emissions by 90%
Risks and reluctance remain
There are certainly risks. The toxicity of ammonia requires strict safety protocols. ‘Crew members will be required to wear personal protective equipment when in the same space as an operating ammonia engine.’ Wärtsilä further supports the safe and smooth introduction of ammonia as a fuel for crew members through dedicated training and 24/7 global support.
In terms of performance, Wärtsilä reports that the efficiency of ammonia engines is comparable to that of LNG engines. ‘There’s a slight drop on paper, but in practice the engine runs steadily. And we’re already working on power upgrades,’ says Zuidam. At the same time, the company is developing multiple dual-fuel options, so that methanol, LNG or even hydrogen can also be used.
Still, Zuidam acknowledges that there is hesitancy in the market. He identifies three main barriers: acceptance by crew and shipowners, the higher cost of ammonia, and the strict requirements around safety and operational readiness.
Scaling up and retrofit pilot
In addition to choosing a suitable engine, the design and construction of the vessel itself play a crucial role when switching to ammonia. Niels de Vries, Head of Energy at C-Job Naval Architects, highlights the bigger picture: ‘The engine and fuel system often get all the attention, but integrating the auxiliary systems is also a real challenge. Think ventilation, piping, storage tanks, fire safety, and emissions treatment.’
As part of the Ammonia2-4 project, C-Job is working on both four-stroke and two-stroke solutions together with Wärtsilä. ‘The four-stroke engines have already achieved more than eighty per cent CO2 equivalent reduction. That configuration is now being further scaled up and adapted for maritime conditions. At the same time, work is underway on a two-stroke conversion kit for existing ships, with multi-fuel injection systems.’
Eventually, a 300-metre-plus container ship belonging to shipping company MSC will be fitted with this retrofit package. ‘Not a lab test, but a real-world application at scale,’ De Vries emphasises. ‘The development involves a system that supports LNG, but is also ready for ammonia. This avoids having to break things down again during ammonia conversion after LNG conversion.’
Also read: 1957-built tug now operates carbon-free on ammonia
Creative integration
Implementing ammonia as fuel requires clear decisions. Storage is done in type C tanks, which are cost-effective, but take up significant space. ‘On container ships, every bit of lost volume means lost cargo capacity,’ says De Vries. ‘So you need to be creative with integration.’ Piping, ventilation, and fire safety all demand a tailored approach. For example, ammonia spaces are kept under negative pressure relative to adjacent zones: ‘As soon as you open a door, air flows in and not out for the purpose of containment.’
The Ammonia Mitigation System is an important safety mechanism. ‘In case of leakage, you need to be able to handle both liquid and gaseous ammonia,’ explains De Vries. ‘We use a “knockout drum” for the liquid ammonia and a water washer that absorbs the gaseous ammonia in the ventilation air. The system does not run continuously, but only activates upon detection.’
De Vries stresses that ammonia systems require a separate drainage system. ‘You don’t want mixing with your existing bilge system to minimise the risk of incidents.’ The choice to use ammonia as a fuel depends heavily on the type of ship, among other things.
‘For cruise ships, it is not a suitable option at the moment. Thousands of untrained passengers and ammonia as fuel is simply not a suitable combination. For deep-sea cargo, on the other hand, there are possibilities with trained crew. Furthermore, the factor of fuel costs carries enough weight to justify allocating space for ammonia storage.’ In practice, De Vries expects that initially it will be ammonia tankers that will run on this fuel alongside other large cargo ships.
A logical step after LNG
Like other energy carriers, the transport and storage of ammonia require specialised containers and tanks. Can Murtezaoğlu, International Business Development and Strategy at GTT – a company specialising in containment systems – sees ammonia as a logical next step after LNG. ‘We believe ammonia will play a key role in the decarbonisation of the maritime sector,’ he states. GTT, known for its LNG membrane technologies used in storage tanks, is now also focusing on ammonia.
‘With LNG, we saw that once production scaled up, there was demand for large volumes. That helped lower the cost per molecule. We want the same for ammonia,’ says Murtezaoğlu. ‘We believe ammonia is now where LNG was fifteen years ago: on the verge of significant growth.’
Also read: ABS publishes safety insights for ammonia as a marine fuel
Foundation is being laid
GTT is drawing on its experience with liquefied gases. ‘There are around 450 LNG carriers operating worldwide that are equipped with our LNG tanks. Together, they account for 13,000 operational years without incident. We aim to carry that same level of reliability over to ammonia,’ says Murtezaoğlu.
The outlines of a functioning system are beginning to take shape. With engines now on the market, shipbuilders revising designs, ports preparing for bunkering, and suppliers focusing on safe containment systems, the foundation is being laid for ammonia-powered shipping.
‘If ammonia were to supply one-third of the global maritime market, we’d be looking at 200 million tonnes of ammonia per year. That’s not wishful thinking — it’s a realistic outlook,’ Murtezaoğlu concludes.
Picture (top): René Zuidam, Senior Sales Manager Marine New Build at Wärtsilä: ‘We’ve achieved over 1000 operating hours — without leaks and without the smell of ammonia’ (photo by Studio Flitsend).
Also read: Hapag-Lloyd orders 24 ammonia-ready container ships
