Around 80–90% of the world’s goods is transported by ship, and shipping accounts for nearly 3% of global GHG emissions. The Organisation for Economic Co-operation and Development (OECD) estimates that in 2022 global shipping generated 858mn t/y of CO₂ emissions, compared with 739mn t/y of CO₂ from air transport.

In 2023 the International Maritime Organization (IMO) committed to reducing the carbon intensity of international shipping by at least 40% by 2030, with zero or near-zero GHG emission technologies representing at least 5% of the energy used. The long-term goal is for the global shipping sector to become carbon-neutral by 2050. It’s a big ambition.

A surprising contender has turned up as a way of reducing carbon emissions from shipping: using ammonia as a fuel. Ammonia (NH₃) is currently a widely-used industrial chemical, where most of the 235mn t/y manufactured goes to making fertiliser, despite significant health hazards.

Looking at that chemical formula – NH₃. Ammonia is composed of nitrogen (an inert gas which makes up 78% of the atmosphere) and hydrogen (which accounts for about 18% by weight). What’s more, ammonia contains no carbon, so burning it produces no CO₂.

Effectively, ammonia is a storage medium for hydrogen, which can be produced from renewable sources. ‘Green’ or e-ammonia has the potential to reduce GHG emissions by 90–100% compared with fossil fuels, and can be synthesised using the electrical Haber-Bosch process. According to a 2021 paper by Oxford University: ‘By 2050, this approach is likely to be the cheapest way to make ammonia (even without carbon pricing), and… is the only option which is entirely carbon-neutral.’

What are the principal fuels for shipping today?  
Most commercial shipping is powered by diesel engines running on heavy fuel oil (HFO). These are very efficient, but a single container ship can still produce over 50,000 t/y of CO₂. Nitrogen oxides (NO_x) are another pollutant, and a particular issue with HFO is sulphur oxides (SO_x). Sulphur content in HFO – now very low sulphur fuel oil (VLSFO) – has been cut dramatically, but it remains a concern.

Some large ships run on liquefied natural gas (LNG), which reduces overall carbon emissions and NO_x and SO_x, but – unless the LNG (methane) comes from a renewable source such as biomass – is not a ‘green’ fuel.

Similarly, liquefied petroleum gas (LPG) offers good SO_x and NO_x emissions, but is usually fossil-derived. It is best suited to fuelling LPG carriers themselves. LPG must be kept liquid at an ultra-low temperature by ‘bleeding-off’ some of it as gas, which is then used as fuel.

Hydrogen (H₂) seems the ideal fuel. In theory, a hydrogen-fuelled internal combustion engine should produce only water vapour from the exhaust. In practice, NO_x is also emitted, but this can be treated. However, hydrogen storage and transfer is rather challenging. Hydrogen is so light that that achieving adequate energy density requires storage either as a gas under high pressure (at 700 bar or more) or as a liquid at cryogenic temperatures (below −253°C).

Other storage methods have been suggested, but hydrogen’s energy density remains poor.

How can ammonia be used as a fuel?  
So, we come to ammonia. As an alternative maritime fuel, we are talking about ‘anhydrous ammonia’ – the pure, usually gaseous form of the compound, rather than as a solution in water. Pure ammonia can be stored as a liquid at near-ambient temperature, for a volumetric energy density of 11.5 MJ/l. This is not as energy dense as the 38.6MJ/l of diesel fuel, but is acceptable.

Ammonia can power ships in two ways: in fuel cells which generate electricity to drive motors, or used as a fuel in an internal combustion engine.

The most established fuel cell technology uses hydrogen as the fuel. Ammonia can be converted back to hydrogen to power these fuel cells, although this heat-intensive process reduces overall efficiency substantially – sometimes by as much as 25%.

US firm Amogy is converting a 1957 tugboat to electric power, supplied by its self-contained modular ‘Powerpack’, which uses a reactor to crack ammonia into hydrogen to feed the fuel cell.

A more efficient alternative is the high-temperature (500–1,000°C) solid oxide fuel cell (SOFC) which runs directly on ammonia. This is still in development, but the technology is moving fast. Norway’s Alma Clean Power demonstrated the first 6 kW unit in 2023, and in August 2024 announced a 100 kW module. But the next step is another order of magnitude larger.

The European Union (EU) funded ShipFC project will convert the LNG-fuelled offshore supply vessel Viking Energy to electric drive, with a 2 MW SOFC system from Alma using green ammonia from a pilot plant operated by chemicals supplier Yara’s pilot plant. It aims to sail solely on ammonia for up to 3,000 hours annually, ‘demonstrating that long-range zero-carbon emission voyages with high power on larger ships are possible’, according to Yara.

In July 2024, the Norwegian Maritime Authority approved the fuel cell system, which emits a certain amount of excess hydrogen and ammonia. Burning these would produce NO_x However, a catalytic converter turns them into nitrogen and water.

The project aims ‘to address existing challenges, such as incentivising green ammonia production through a certification scheme and developing solutions for maritime bunkering’, along with ‘development of viable business models… including the build-up of a robust infrastructure and value chain’. It also involves studies on other vessel types, including cargo ships with power ratings of 20 MW+.

‘Green ammonia is the most promising fuel for decarbonisation of shipping.’ – Professor Jim Hall, programme lead, the Environmental Change Institute (ECI) at the University of Oxford

Can ammonia power diesel engines?  
Ammonia can be used to fuel gas turbines and, more commonly, internal combustion engines, generally two- or four-stroke diesels. However, a small proportion (10% or less) of pilot fuel (usually oil) is needed to start combustion. This comprises the ‘zero carbon’ nature of the engine, unless the pilot fuel is sustainable. Exhaust treatment such as selective catalytic reduction (SCR) is also needed to cut NO_x output and catch any unburnt ammonia.

Rather than creating clean-sheet designs, manufacturers such as MAN and Wärtsilä are developing ammonia engines from existing oil- and LPG-fuelled models. MAN said recently: ‘In the short term, we expect a fast uptake of ammonia-fuelled engines towards the end of the decade, as we obtain positive seagoing experience from the first engines. In the long term, we expect ammonia to comprise around 35% of fuel used onboard large merchant-marine vessels by 2050 due to lower production cost compared to other e-fuels.’

MAN’s 7S60ME-ammonia engine is being installed in a 200,000 dwt bulk carrier by Imabari Shipbuilding. Similarly, WinGD and CMB.Tech aim to install ammonia power in ten 210,000 dwt bulk carriers by 2026. This summer (2024), Japanese shipping giant NYK operated the ammonia-fuelled A-Tug, and in 2026 plans to operate an ammonia-fuelled ammonia gas carrier (AFAGC), which resembles an LPG carrier and will use gas vapourised from the cargo as fuel.

There are clear risks to using ammonia. It is not especially flammable compared with other fuels, but is a severe hazard to unprotected skin and if inhaled is potentially fatal. Even a tiny concentration discharged into the sea is environmentally damaging. ‘Ammonia slip’ is uncontrolled release from the fuel system or exhaust into the atmosphere. While ammonia is not a GHG, it is easily converted in nature to nitrous oxide (N<em>2</em>O), which has an exceedingly high global warming potential (GWP) of 265 times CO₂ on a 100-year basis.

Although no port is currently approved to bunker ammonia as a fuel, there are decades of experience and protocols in the storage, transfer and handling of the chemical. Antwerp is likely to be the first to handle ammonia, from 2025. While Singapore’s Maritime Energy Training Facility (METF) is expected to train around 10,000 maritime personnel in handling ‘clean’ marine fuels such as ammonia by the 2030s.

A 2023 report from the European Maritime Safety Agency says: ‘Experience with the carriage of ammonia in liquefied-gas carriers, and the specific requirements for storage, distribution, PPE, etc in the [IGC Code], provide some of the statutory requirements [for] ammonia-fuelled ships.’ But this will make ship designs more complex and may prove ‘a more appropriate solution for deepsea cargo ships rather than short-sea, passenger or inland waterway craft’.

What is the next step towards ammonia becoming a shipping fuel?  
The technology to produce and use green ammonia seems achievable, but the supply chain needs to be developed.

A recent study from the Environmental Change Institute (ECI) at the University of Oxford models a possible global infrastructure, and makes the optimistic suggestion that green ammonia could ‘fulfil the fuel demands of over 60% of global shipping by targeting just the top 10 regional fuel ports’. The study employed a modelling framework of scenarios, with a fuel demand model, future trade scenarios and a spatial optimisation model for fuel production, storage and transport.

The study concludes that potential production costs are similar to VLSFO. Programme Lead Professor Jim Hall says: ‘Green ammonia is the most promising fuel for decarbonisation of shipping.’ The report predicts that the ammonia fuel supply would be regional, with large producers being those with good solar resources and close to major shipping hubs. The report suggests ‘locations… in Northern and Western Australia, Chile, Western Africa, India and the southern Arabian Peninsula’.

Another report from the Global Maritime Forum and RMI points out that ‘the cost of renewable energy impacts the delivered cost of e-ammonia… significantly more than the cost of transport’. But it adds that ‘many ports have favourable conditions to produce e-ammonia. In the medium to long-term, local production will be the most economical option for these ports’.

The overall costs are daunting though. ECI report lead author Dr Jasper Verschuur says: ‘Investments up to $2tn are required to build new infrastructure, with a large share of this in developing countries. Here, green finance is key to support the transition to hydrogen and green ammonia production.’

• Further reading: ‘Maritime decarbonisation: ports and green shipping corridors’. The Port of Huelva is assisting in the development of both alternative maritime fuels and green shipping corridors, thanks to its geographical position in the southern part of Spain. Director of the Port Authority of Huelva, Ignacio Álvarez-Ossorio Ramos, explains.
• The decarbonisation of international shipping will require global and regional cooperation and work to bring together often segregated stakeholders across the energy and shipping value chains. Find out more about the ways in which the decarbonisation picture is shaping up around Singapore and the wider Asia-Pacific region, as well as the lessons that can be learnt from regional initiatives like the Silk Alliance.