Ammonia is currently being investigated as a fuel mainly for maritime transportation. Ammonia can be considered as a so called “electro-fuel” since it can be produced from nitrogen (from air) and hydrogen (through electrolysis of water). The electricity could be produced from wind, solar, or from other sustainable sources. Ammonia burns without production of carbon dioxide during combustion. However, there is almost no knowledge about the behavior of ammonia in a combustion engine. Ammonia has a relatively low calorific value, and on top of that, characteristics like low cetane number and low flame speed make it difficult to apply in combustion engines. Therefore, it is necessary to carry out investigations on how to improve the applicability of ammonia for combustion engines. Fuel cells for ammonia use are available.

Advanced Motor Fuels, one of the International Energy Agency’s (IEA) transportation related Technology Collaboration Programmes (TCP) has prepared a Special report on ammonia as motor fuel. Summary of information on ammonia as motor fuel in this report is presented here.

Engines for ammonia 

The majority of experiments on ammonia use in the literature concerned spark-ignited (SI) engines, although some have also achieved satisfactory combustion using compression ignition (CI) engines. High compression ratios, low speeds and high loads have been found preferable for ammonia fueled engines, which is primarily related to ammonia’s low flame speed. Engine tests though showed good results for an ammonia fuelled SI-engine with a small amount of hydrogen added.

Ammonia is considered interesting for marine engines, which are mostly large diesel engines today. Marine engines fueled by ammonia with pilot diesel fuel injection could be an option. Large displacement volume and operation at a constant low speed with high loads are favorable for ammonia combustion.Ammonia carriers already have experience with handling and storage of ammonia, and could thereby benefit in terms of lower CO₂ emissions and economic savings from using already on-board fuel by implementation of ammonia fueled engines.

End-use aspects of ammonia

Successful implementation of ammonia is not a question of engine technology alone. The implementation must be seen in relation to the size of the change in the infrastructure, technology and expenses.

Engine use and storage of ammonia need proper choice of materials and technology. Generally, safety issues of highly toxic ammonia are to be considered. One possible way to overcome this issue is storing of ammonia in metal amine complexes. However, emission standards for the emission of ammonia are still very strict.

Particular concern with ammonia as motor fuel is the emission of unburned ammonia, which is poisonous, and N₂O, which is a strong greenhouse gas. Ammonia slip in exhaust gas can be removed with the use of SCR aftertreatment when present in small amounts. At larger quantities it could be problematic and requires the implementation of an ammonia trap.

References (see special report)

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