Future Fuels: Momentum Builds for Ammonia
Strong demand is driving development but emissions issues remain, writes René Sejer Laursen, ABS Director – Fuels & Technology, Global Sustainability
The interest in ammonia stems both from its zero emissions when used as fuel and because its production isn’t dependent on biogenic carbon sources.
To realize large-scale production of green ammonia to serve new markets, its production capacity, along with that of renewable electricity and green hydrogen, will need to grow tremendously.
Current projections for the growth in global production indicate there will be enough renewable electricity to produce the volumes of green ammonia needed for the maritime fleet alone by 2040. However, because shipping will also be competing with many other industries for both the renewable electricity and green hydrogen necessary to produce ammonia, as well as with other sectors that depend on the consumption of green ammonia such as agriculture and coal fired power plants, supply is expected to be constrained.
- Propulsion Technology
First tests have been performed using ammonia as fuel in combustion engines by several of the main engine manufacturers.
Though the amount of pilot fuel and levels of NOx, NH3 slip and N2O emissions have yet to be quantified for the commercial marine engines, marine engine makers generally agree that the Diesel cycle is best suited for combustion of ammonia.
Research is ongoing for both diesel and otto cycle combustion concepts. Optimising emissions reductions is foreseen as a challenge and control of N2O and ammonia slip requires high temperature combustion which also generates high NOx levels.
Tests on two-stroke engines have shown that NOx is less of a problem using the Diesel cycle combustion principle when burning ammonia. When ammonia is injected into the combustion chamber, it expands and generates a cooling effect that removes the high peak temperatures in the combustions zones that generated the high NOx.
Pilot fuel is necessary to ignite ammonia and it is also needed to keep combustion stable. For smaller four-stroke engines, 10% pilot fuel is required once engine optimisation has been completed and after the engine is in service. For large two-stroke engines using Diesel cycles, just 5% pilot fuel is required, and some engine makers expect that this amount can be further reduced.
- Assessing Emissions
The actual amount of NH3 and N2O emissions is still to be accurately assessed, however, emissions are expected to be low, particularly for the Diesel combustion cycle. Even so, with N2O having a 20-year global warming potential (GWP) of 264 and a 100-year GWP of 265 according to IPCC 2013-ARS, the emitted levels may negate much of the CO2 benefit of using ammonia as a fuel. This remains a significant potential barrier to adoption.
Two-stroke marine engine designers have, however, found in their tests that N2O level are low - in the same range as we see for other fuels including marine diesel, LNG and methanol. Overall it seems that the Diesel combustion principle is ideal for use of ammonia since the temperature in combustion chamber hits a ‘sweet spot’ where the NOX, N2O and ammonia slip levels are recorded at a very low level. It is therefore expected that those engines will be able to operate to IMO NOx Tier II standards without any need for an abatement system.
As of Q1 2024, the main marine engine makers have the following development plans and lead times for ammonia-fueled engines:
- Two-stroke ammonia dual fuel engines covering power ranges from 5 MW to 31 MW. These engines will be available for delivery starting from Q4 2024/Q1 2025.
- Four-stroke ammonia engines as dual fuel gensets engines are also becoming available. Two engine manufacturers will launch this type of engine at the end of 2024 or beginning of 2025.
- Safety and Exhaust Treatment
Most engine designers expect that exhaust gas after-treatment will be needed to comply with the IMO NOx Tier III standard, and all of them expect to specify Selective Catalytic Reduction (SCR) as the preferred means of cleaning the exhaust gas after it has left the combustion chamber, rather than exhaust gas recirculation (EGR) which changes the combustion conditions thereby limiting NOX formation. The EGR is reducing the amount of oxygen in the intake air, and the fear is that this will have a very negative impact on the performance of ammonia combustion, but this is still to be investigated.
In addition to main engines and gensets operating on ammonia, designs are also emerging for auxiliary engines required to complete the transition to vessels running on ammonia. Boilermakers are preparing dual fuel boilers for use with ammonia as fuel to be able to generate steam and heat from burning ammonia.
Working with ammonia onboard on a day-to-day basis requires a solution to collect ammonia vapor in a safe manner. Different solutions for vapor handling are under development from several manufacturers, including water scrubber designs that can remove ammonia vapor from the purge air. In this solution, ammonia vapor is stored in dedicated tanks as a water-ammonia solution. However, this approach would require dedicated infrastructure at the port to receive and store it.