Ammonia Fueled Engines

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Abstract: As an engine fuel, ammonia presents several challenges related to its poor combustion characteristics and emissions. Several combustion concepts are possible, such as compression ignition with a diesel pilot, as well as spark ignition strategies, typically with the addition of hydrogen to facilitate ignition. Commercial four- and two-stroke ammonia engines have been developed, primarily focused on marine applications.

Ammonia as Engine Fuel

As an engine fuel, ammonia has seen sporadic use for well over a century [5368]. Due to its high auto-ignition temperature, high heat of vaporization, high minimum ignition energy and narrow flammability range, a dual fuel CI strategy such as ignition with a diesel pilot is typically used, Figure 1. While SI strategies are also possible, hydrogen is typically added to ammonia to facilitate ignition. This hydrogen can be supplied as an external gas stream or else it can be generated through ammonia decomposition. Spark ignition of neat ammonia is possible under some conditions.

As is the case with natural gas engines, ammonia engines can operate on different cycles depending on how fuel is introduced and ignition is achieved. While spark ignition engines in Figure 1 operate on the Otto cycle, the dual fuel engines can represent Otto cycle, diesel cycle or a dual fuel mixed cycle depending on when and how ammonia is introduced and the quantity of the pilot fuel injected. Premixing ammonia with air in the port or an early direct injection and using a relatively small pilot injection for ignition would represent Otto cycle operation while ignition with a large pilot would be dual fuel mixed cycle operation. Late direct injection of ammonia ignited by a diesel pilot would represent diesel cycle operation.

Emissions

Emission Characterization

While there are no direct carbon emissions from ammonia combustion, the use of hydrocarbons as the pilot fuel would result in CO₂ emissions. Lifecycle GHG emissions from ammonia depend strongly on feedstock and manufacturing process details.

Emissions of unburned ammonia and N₂O are a concern with ammonia fueled engines and levels in the exhaust could potentially be several times higher than engines burning conventional fuels. Ammonia emissions are a concern from a toxicity perspective while N₂O is a powerful greenhouse gas [5367]. Emission levels of 1.9 g/kWh of N₂O would have the same climate warming effect as CO₂ emissions from diesel combustion [5728].

Emissions of unburned ammonia and N₂O are significant in Otto cycle and dual fuel mixed cycle operation where ammonia and air are premixed well before combustion. One contributor to this challenge is the presence of combustion chamber crevices. Ammonia trapped in crevices during combustion and released during expansion is a significant source of unburned ammonia. It may also be an important source of N₂O emissions as the unburned ammonia released from the crevices is only partially oxidized [5915]. Flame quenching near combustion chamber surfaces is another potential source of unburned ammonia.

Diesel cycle operation shows the potential of low-speed two-stroke dual fuel engines using late cycle injection of ammonia to have low emissions of ammonia and N₂O. However, ammonia leakage from the fuel injector can be a significant source of unburned ammonia but can be mitigated with suitable injector design [6462][6471].

The emission challenges of using ammonia as a fuel in Otto cycle and dual fuel mixed cycle engines is apparent in a number of studies in which a dual fuel approach is used with a premixed ammonia charge and a diesel pilot for ignition [5395][4672][5230][5370][5396][5716][6477]. These studies were carried out with diesel engines ranging in size from about 1 L/cylinder to 2.4 L/cylinder and in all of them, ammonia was introduced into the intake manifold which allowed the ammonia and air to premix. While increasing ammonia fraction tended to decrease NOx emissions, in studies that also measured N₂O emissions, the lower NOx emissions were often associated with increased N₂O emissions. N₂O is produced almost entirely from the decomposition of ammonia. Unburned ammonia emissions also tended to be high with values as high as 35 g/kWh and up to 15% of the supplied ammonia escaping combustion. Advancing the diesel injection timing typically resulted in lower N₂O emissions and improved thermal efficiency but increased NOx emissions. Richer ammonia air ratios also lower N₂O emissions.

Considering GHG emissions, safety issues with ammonia and other criteria, the Mærsk Mc-Kinney Møller shipping industry consortium proposed emission target levels of 10-30 ppm for NH₃ and 0.06 g/kWh for N₂O [5728].

Ammonia engines are also capable of producing particulate emissions not only from lubricating oil but from other sources as well. Sulfate from sulfur in lubricating oil and nitrate salts with ammonium have been identified as potential sources of engine-out particulate emissions [6522][6521]. If an SCR catalyst is used, ammonium nitrate and urea derived particles can also be formed in the aftertreatment system.

Emission Aftertreatment

An optimal—albeit not yet developed—aftertreatment system that has been suggested to achieve low emission levels is an SCR catalyst optimized to reduce both NOx and N₂O, followed by an NH₃ slip catalyst [5728]. The ammonia that escapes combustion in the engine could contribute to NOx reduction over the SCR catalyst but depending on the NH₃/NOx ratio, further urea or ammonia injection upstream of the SCR catalyst may be required to achieve NH₃/NOx ~1 required to maximize NOx reduction.

Ammonia slip catalysts are currently used in many diesel applications equipped with an SCR system for NOx reduction. They could also be applied to ammonia engines downstream of an SCR catalyst. Ammonia oxidation activity on these catalysts is often a trade-off with selectivity towards NOx and N₂O—i.e., catalysts with a higher ammonia oxidation activity and lower light-off temperature tend to oxidize the ammonia to NOx and N₂O instead of N₂. Thus, these catalysts tend to have lower precious metal loadings (usually Pt) and a higher light-off temperature (around 250°C) to ensure ammonia is oxidized to N₂. A dual layer catalyst with an SCR catalyst layer over the oxidation catalyst is commonly used to further improve selectivity towards N₂.

In cases where the unburned ammonia emissions from the engine are too high for the SCR/ASC combination, an ammonia oxidation catalyst (AMOX) placed upstream of the SCR catalyst could be used. Ammonia is partly oxidized on the AMOX (e.g., a Pt/Pd oxidation catalyst) to NOx, which reacts with NH₃ in the SCR catalyst [6519].

In some applications, N₂O decomposition catalysts may be required. Unlike N₂O reduction on an SCR catalyst, N₂O decomposition catalysts do not require a reductant. Decomposition of N₂O can be achieved with noble metal catalysts, non-precious metal oxides, and molecular sieve catalysts [6513][6514][6515][6516].