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.

 

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.

Engines for ammonia

Ammonia engines can be constructed for both premixed (Otto) and diffusion controlled (Diesel) combustion principles. The Otto principle is the most simple and applicable for small and medium sized 4-stroke engines, while the Diesel principle has more advantages in large dual fuel engines, including large 2-stroke marine engines.

Light duty engines

Experimental ammonia-fueled Otto engines have been demonstrated to operate most efficiently with near-stoichiometric combustion. There is a trade-off between high NOx formation at lean combustion, and increased emissions of ammonia and hydrogen at fuel rich combustion. Lean burn does not appear to be feasible with ammonia engines operating with the Otto principle, so load control must be based on the control strategies normally used in SI engines.

Ammonia has not received much attention or consideration as fuel for light duty engines. There have, so far, not been any factors motivating the development of small ammonia engines. However, in June 2023, the Chinese owned engine manufacturer GAC and Toyota presented a prototype 2.0 L, 120 kW SI engine, operating on pure ammonia[1]. The engine is built for passenger car use, as a proposed near zero carbon alternative to battery electric vehicles.

Marine engines

Due to the increasing interest in the potential for reduction in CO2 emissions in the marine sector, marine engine designers are now developing large marine engines for ammonia.

2-stroke dual fuel ammonia engines are currently being developed, building on the experience and technology developed for dual fuel solutions with direct gas and liquid fuel injection. 2-stroke propulsion ammonia DF engines are expected to be ready for delivery and installation in new ships from 2024, while retrofit solutions are expected to be available for existing ships from 2026[2]^,[3].     

Wärtsilä has recently released a 4-stroke ammonia DF engine and fuel system intended for marine power generation and propulsion[4]. The engine is equipped with low pressure port fuel injection of ammonia, with diesel pilot injection. The engine is capable of both DF and fuel oil only operation.   

Emissions

The use of ammonia as fuel generally has the potential to reduce emissions of CO2 and particulate matter, including black carbon. In marine applications, substituting marine fuels with ammonia will also reduce sulfur dioxide emissions.

Combustion of pure ammonia results in the formation of water (H2O), nitrogen (N2) and oxides of nitrogen (NO, NO2 and low ppm concentrations of N2O). In fuel rich combustion, emission of unburned ammonia and hydrogen will increase, while oxides of nitrogen decrease due to competition for oxygen.

Emissions of N2O are potentially concerning, since the GWP of N2O is 273[5] on both a 20-year and a 100-year timescale. Emissions are however expected to be less than 50 ppm on average and may furthermore be reduced by avoiding low load ammonia combustion, which promotes formation of N2O.

Emissions of hydrogen is also a growing concern. Although hydrogen does not absorb light in the infrared spectrum, it does affect the composition of the troposphere by inhibiting breakdown of methane by hydroxyl radicals, while also contributing to production of ozone. The CO2 equivalent GWP of hydrogen has recently been estimated to be 11.6 +/- 2.8 [6]. It is therefore important to be observant of hydrogen emissions when operating engines near the stoichiometric value, where hydrogen can be formed due to oxygen depletion. This is most relevant for small engines operating around the stoichiometric condition, whereas large marine engines operating with excess air will most likely not have significant formation of hydrogen.        

Upstream emissions of GHG related to the production and transportation of the ammonia must be considered as well. Ammonia is an energy carrier, and the CO2 emitted when producing, transporting, and storing it emits more than 3 tons of CO2 per ton of ammonia, which is comparable to the CO2 emitted directly when burning fossil fuels[7]. Today, ammonia is produced from methane which is steam reformed and combined with atmospheric nitrogen. For ammonia to come closer to being a zero-carbon fuel, it must be produced using energy from renewable sources.

Dual fuel engines are however designed to operate with fuel oil as ignition source. The fuel oil substitution rate effectively determines the direct emissions of CO2, as well as all other pollutants formed with fuel oil combustion.    

Emission aftertreatment options

Ammonia combustion can create high emissions of NOx, which is harmful to both humans and the environment. NOx can however be reduced to very low concentrations with SCR catalysts, which use ammonia as reductant. This allows for optimizing the engine emissions with respect to both NOx formation and ammonia slip, such that these can react together in the SCR and result in low emissions of both.  

Leading catalyst developers in the chemical industry have made catalytic materials for SCR solutions that are also capable of reducing N2O, which is not reduced with conventional catalyst materials.

Large slips of ammonia from the fuel system or through defective fuel injectors into the exhaust is a safety hazard to ship crew and passengers. The most effective way of neutralizing this hazard is by absorbing the ammonia in spray towers. SOx scrubbers will be effective in the exhaust stream and may be used for cleaning air purged from contaminated areas as well.   

 

 

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)

[1] – W. L. Ahlgren “The Dual-Fuel Strategy: An Energy Transition Plan” IEEE No. 11, November 2012 | Proceedings of the IEEE DOI: 10.1109/JPROC.2012.2192469

[12] – C.S. Mørch, A. Bjerre, M.P. Gøttrup, S.C. Sorenson, J. Schramm “Ammonia/hydrogen mixtures in an SI-engine: Engine performance and analysis of a proposed fuel system” Fuel — 2011, Volume 90, Issue 2, pp. 854-864

[23] - J. W. Hodgson. “Is ammonia a transport fuel for the future?” Asme Pap — 1973, Issue 73

[41] – EES using fundamental equation from: Tillner-Roth, Harms-Watzenberg, and Baehr, "Eine neue Fundamentalgleichung fur Ammoniak", DKV-Tagungsbericht 20:167-181, 1993.

[42] – EES using fundamental equation from: J. W. Leachman, R. T Jacobsen, S. G. Penoncello, and E. W. Lemmon J. “Fundamental Equations of State for Parahydrogen, Normal Hydrogen, and Orthohydrogen” Phys. Chem. Ref. Data 38, 721 (2009)

[43] – EES using fundamental equation from: Span, R. and Wagner, W. "Equations of State for Technical Applications: II Results for Non-Polar Fluids" Int. J. of Thermophysics, Vol. 24, No. 1, Jan. 2003

[44] - EES using fundamental equation from: Lemmon, E.W., Huber, M.L., "Thermodynamic Properties of n-dodecane", Energy and Fuels, Vol. 18, No. 4, pp. 960-967, 2004

[45] – EES using fundamental equation from: "Fundamental Equations of State", Shaker, Verlag, Aachan, 1998.

[46] – EES using fundamental equation from: J. A. Schroeder, S. G. Penoncello, and J. S. Schroeder "A Fundamental Equation of State for Ethanol" Journal of Physical and Chemical Reference Data 43, 043102 (2014)

[47] – K. Mazloomi, C. Gomes. “Hydrogen as an energy carrier: Prospects and challenges” Renewable and Sustainable Energy Reviews — 2012, Volume 16, Issue 5, pp. 3024-3033

[48] – M. Eyidogan, A. N. Ozsezen, C. Mustafa, A. Turkcan. “Impact of alcohol-gasoline fuel blends on the performance and combustion characteristics of an SI engine”  Fuel — 2010, Volume 89, Issue 10, pp. 2713-2720

[49] – C. T. Chong, S. Hochgreb. “Measurements of laminar flame speeds of liquid fuels: Jet-A1, diesel, palm methyl esters and blends using particle imaging velocimetry (PIV)” Proceedings of the Combustion Institute — 2011, Volume 33, Issue 1, pp. 979-986

[50] – S. Frigo, R. Gentili, F. De Angelis. ” Further Insight into the Possibility to Fuel a SI Engine with Ammonia plus Hydrogen” Sae Technical Paper Series — 2014

[51] – H. Stokes “Alcohol Fuels (Ethanol and Methanol): Safety” Project Gaia Jan 2005 ethoscon.com/pdf/ETHOS/ETHOS2005/pdf/stokes_paper.pdf

[52] - Safety Management Services, Inc.(1999) Data Guides http://www.smsenergetics.com/wp-content/uploads/2015/11/Data_Guides.pdf