Discussion and perspectivation

Ammonias fuel properties are causing difficulties when used in ICE’s. These properties, are summarized in Table 1.

Some of the property related issues have, in some investigations, been solved by mechanical means. Minimum ignition energy has been proved to be possible to overcome by implementing of a new ignition system and/or adding of a second fuel. Properties like flame velocity has only been possible to change to a proper degree with the addition of a combustion promoter. Thereby not making an engine fueled by ammonia alone favorable. Hydrogen has showed excellent characteristics in improving combustion of ammonia. Other combustion promoters like diesel and gasoline have also showed promising results, though at higher quantities, and not in full operating ranges. It is also seen that engine modification to a certain degree is a necessity, when using fuel-blends. 

The vast majority of experiments found in the literature tested SI-engines. However, some have also managed to achieve satisfactory combustion using CI-engines. In terms of compression ratio, speed and load a general tendency from the literature is observed for both SI and CI. High compression ratios, low speeds and high loads are preferable for ammonia fueled engines. All those tendencies are primarily due to ammonia’s low flame speed.

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.

Passenger vehicles is a large implementation area, which need a large re-structuring of the present fuel supply system for the technology to be a success. Additional cons for this area is ammonias low energy density, not promoting it as a first choice for a small vehicle. Engine tests though showed good results for an ammonia fueled SI-engine with small amount of hydrogen. However, also big variation in speeds and loads are present in a passenger vehicle. At idle, very low speeds and at high speeds, hydrogen or another power resource would be needed. Thereby making the addition of a hydrogen tank and/or advanced cracker equipment necessary onboard. Safety issues of onboard ammonia might be avoided with the use of metal amine complexes, but still very strict emission standards for the emission of ammonia exist. Ammonia slip can, however, be removed with the use of SCR after treatment when present in small amounts. At larger quantities it could be problematic and requires the implementation of an ammonia trap.

Marine engines fueled by ammonia with pilot fuel injection of diesel or other high-cetane fuels, could be a feasible application in the near future. Such engines have a large displacement volume and are operated at a constant low speed with high loads (supercharged), making them favorable for ammonia combustion. Additionally, dual-fuel marine-engines fueled by alternatives fuels like LPG already exist [39], and since ammonias physical properties are somewhat similar with LPG, experience from these engines could help accelerate the implementation of ammonia in marine engines. 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. As described in section 2 in case of an ammonia slip, ammonia will quickly evaporate and ascend into the atmosphere and the toxicity may therefore not be that large of a concern in this sector and safety could be easier to obtain.

References

[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

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[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

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