Another fuel alternative which is taken into consideration is Ammonia which recently has gained increased attention as a possible marine fuel that could help accelerate the decarbonization process.
Ammonia is also interesting in the context of a growing hydrogen economy because it is the cheapest way to transport hydrogen over long distances and in large quantities. It is a fundamental chemical that is traded and produced on a global scale which has been primarily used in the production of fertilizer to date.
Ammonia is a toxic, carbon-free energy carrier which can be used as a marine fuel in both internal combustion engines and fuel cells. Ammonia or ammonia mixtures combustion can result in the emission of nitrogen oxides (NOx), nitrous oxide (N2O), and direct ammonia (NH3). It is colourless gas at atmospheric conditions, lighter than the air, strong smell and caustic.
Because ammonia (sometimes called “the other hydrogen”) is a hydrogen (H2) carrier, it has gained attention in the context of a future hydrogen economy. It is a zero-carbon synthetic energy carrier that may be useful for decarbonizing a variety of sectors that require alternative energy carriers, such as hydrogen-based fuels.
Haber-Bosch, the established process for producing ammonia, revolutionized fertilizer use over a century ago. However, the process is extremely energy-intensive.
Ammonia is produced today by feeding natural gas into a steam methane reformer in order to generate hydrogen. The hydrogen is used as an input to the Haber-Bosch process, which uses a catalyst to convert it, along with nitrogen (N2) from the air, into ammonia.
To produce ‘green’ ammonia using renewable energy, the hydrogen for the Haber-Bosch process is generated using renewable electricity via water electrolysis, and the nitrogen is supplied via cryogenic air separation (Oeko-Institut 2019a).
No marine ammonia engines have been built to date, but there are projects already started and some of the shipping companies together with shipbuilders and engine manufacturers are already taking the lead in this direction.
Among the liquid ammonia’s properties is its energy density of approximately 18.6 MJ/kg, is significantly less expensive than modern marine fuels, as well as natural gas and liquefied petroleum gas. As a result, ammonia has a low energy density, but the primary advantage of ammonia is that it can be stored in liquid form at elevated temperatures and atmospheric pressure. It is a liquid that is slightly cryogenic which require –33,4 degrees at atmospheric pressure. Also requires approximately 10 bar pressure at ambient temperatures of 20 degrees.
The graph below illustrates the pressure versus boiling temperature relationship for ammonia.
Using ammonia as a fuel is quite new in marine industry, but due to its high toxicity new safer systems need to be designed. Therefore, ammonia will require bigger storage tanks which will potentially “eat” from the cargo space, all below deck piping will likely need to be double-walled, as burning ammonia will produce a lot of NOx emissions, under actual regulations an SCR (Selective Catalytic reduction) need to be installed and engine room will require new safety equipment like emergency ventilation or gas absorption in the event of a ammonia leak.
For now, the most suitable vessel to burn ammonia are the one which use to carry ammonia as a cargo, similarly with vessel experience using LNG, LPG or methanol. The only main issues will be to adapt and install a new the fuel system dedicated to ammonia and to retrofit/upgrade the engines.
For vessels which are not carrying ammonia as a cargo, a system for loading and storing it onboard, new fuel system dedicated to ammonia need to be installed and engine upgrade needs to be done. The most suitable systems for storage are the C type pressurized tanks as they can store product at the ambient temperature and does not require reliquification system, they are can be installed on deck and can be easily integrated on a commercial vessel. The only inconvenient is that they are suitable for vessel with short routes as their limit of applicability is around 2000 m3.
The flammability of ammonia is relatively limited, with high auto-ignition temperature, low flame speed and for its combustion will requires very high compression rate and temperature or a pilot fuel valve, thus dual fuel engines will be the most probably way for ammonia to enter the maritime industry as a fuel. These types of engines are expected by 2024 and MAN Diesel is working towards burning ammonia in their ME-GI engines.
The use of ammonia will lead to major changes in engine room, as the entire treatment of HFO will disappear (settling tanks, purifiers, heaters, booster pumps, viscometers, filters etc), but new system will need to be installed specific for ammonia use.
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