Ammonia

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Abstract: Ammonia does not include carbon and is therefore considered a potential zero-carbon fuel option. Ammonia is produced commercially from natural gas or coal via the Haber-Bosch process, which involves relatively high GHG emissions. Lower life cycle emissions are possible using power-to-ammonia production processes. As a fuel, ammonia can be burned in an engine, typically using a dual fuel strategy such as ignition with a diesel pilot. Challenges of using ammonia as fuel include emissions of unburned ammonia, NOx and N₂O, the latter a potent greenhouse gas.

Introduction

Ammonia (NH₃), composed of hydrogen and nitrogen, is potentially interesting for power production because it contains no carbon. If carbon emissions during its production can be eliminated, ammonia is widely believed to be a zero-carbon “fuel” option. However, this view ignores the fact that ammonia combustion produces another powerful greenhouse gas, N₂O.

Ammonia is classified as a secondary fuel or energy carrier because unlike primary energy sources, it does not occur naturally in sufficient quantities to be extracted and then used for energy production on a large scale. As such, it requires industrial production and requires higher energy input than is contained in the final product. Its production is most attractive in locations with abundant and relatively low-cost energy.

One attractive feature of ammonia is that it is already produced in large quantities using technology that was developed over a century ago. Worldwide production in 2020 was on the order of 185 Mt with about 70% of that used as agricultural fertilizer [5384]. Ammonia production occurs in multiple locations around the world and increases in capacity to accommodate power production would be possible.

Another attractive feature of ammonia is its storage characteristics. Temperature and pressure characteristics are similar to LPG and it can be stored as a liquid at ambient temperatures under relatively modest pressure or at ambient pressure at about -33°C.

In addition to its direct use as a fuel, ammonia can also be used as a hydrogen carrier where the hydrogen is separated from the ammonia before use. Its use for hydrogen storage is superior to most other options based on volumetric and gravimetric density as well as temperature and pressure, Figure 1 [5368].

Significant interest in ammonia has been generated by the marine sector where it is viewed as one possible option to meet the IMO’s GHG commitment. Most production and storage facilities already have marine access. A number of papers discussing its use as a marine fuel have been produced [5367][5371][5369][5382][4644]. Also, there are numerous pilot projects exploring its use in vessels [5373]. For ammonia to supply an estimated 30% of marine fuel market by 2050, an additional 150 Mt/y would be required [5371]. This would be a sizeable amount, equivalent to about 80% of gloabl 2020 ammonia production.

In 2021, the World Bank concluded ‘green’ ammonia, closely followed by ‘green’ hydrogen, has the most advantageous balance of features among a range of different zero-carbon candidate bunker fuels. These include lifecycle GHG emissions, broader environmental factors, scalability, economic viability, and the technical and safety implications of the use of these fuels. The ease of ammonia storage and handling relative to hydrogen gives it a distinct advantage [5383].

Downsides of ammonia include its toxicity, fire safety related to its low flash point and its corrosivity that requires the use of special materials and precautions.