Global warming is linked strongly to increased greenhouse gas (GHG) concentration in the earth’s atmosphere. The maritime cluster plays a critical role in reducing GHG emissions by replacing fuels of fossil origin with low-carbon or carbon-free alternatives. As a carbon-free fuel, anhydrous ammonia has received much attention recently due to its established production technology, distribution infrastructure, and satisfactory energy density as a marine fuel.
As one of the largest bunkering ports in the world, Singapore will foresee opportunities arising from adopting alternative marine fuels, and the bunkering of ammonia can be one of the candidates. Currently, there is no established ammonia bunkering infrastructure or guidelines in Singapore. The other essential part – the safety study around the operational or accidental release during the ammonia bunkering process, is still awaiting a comprehensive investigation. Ammonia bunkering deserves a timely study in Singapore to prepare for its potential adoption in the future. The study led by MESD, together with ASTI, ABS and industry partners, aims to provide a timely report to the marine community that includes ammonia production and supply, hypothetical ammonia bunkering process, and impact analysis of ammonia release from various scenarios.
The study started in September 2020 and concluded in September 2021, aiming at audiences ranging from port operators, bunker suppliers, ship owners and port authorities to other relevant stakeholders considering ammonia as the next generation low carbon fuel.
The ammonia value chain includes production, storage, transportation/distribution, bunkering and onboard energy conversion. Ammonia can be produced from fossil-based feedstock or renewables by the Haber Bosch process or electrochemical process. Given that 80% of global ammonia production is consumed as fertiliser and only 1% is spared for energy-related use, ammonia production capacity has to be upscaled to meet the rising energy demand. Meanwhile, the development of green ammonia plants with cost reduction will play a decisive role in entering the marine community, where business continuity must be achieved along with GHG emissions reduction from the future perspective. When ammonia attracts more recognition from the maritime industry, the entire value chain shall be developed to suit the needs of various types
of vessels and bunkering configurations. Although ammonia bunkering standards and guidelines are not established yet, there have been increasing discussions and studies on ammonia bunkering in recent years.
Examples include the work on provisional guidelines for ammonia fuelled ships and bunkering operations being developed by the International Association of Classification Societies (IACS).
The ammonia bunkering process shall be designed based on the fact that anhydrous ammonia is an ambient saturated liquid. Unlike normal ambient liquid fuels such as diesel or residual oil, ammonia relies on refrigeration or pressurisation to maintain the liquid phase. The operating temperature to store and transfer ammonia is from -33°C to ambient temperature (25°C), corresponding to a pressure range from 1 to 10 bar. Boil-off gas (BOG) can be generated at constant pressure by the addition of heat to saturated liquid and involves significant enthalpy change. In contrast, flash gas is generated when saturated liquid undergoes a reduction in pressure with no associated enthalpy change. This study proposes 33 ammonia bunkering configurations generated from the combination of 4 main bunkering supply modes (truck-to-
ship, ship-to-ship, shore/terminal-pipeline-to-ship and cassette bunkering), 3 bunker receiving modes and 3 storage conditions (fully refrigerated, semi-refrigerated or non-refrigerated tanks). The study also presents hypothetical processes for ammonia bunker transfer under different storage conditions consisting of 8 main steps, namely 1) initial precooling, 2) bunker hose connecting, 3) 1st inerting, 4) purging, 5) transferring, 6) stripping, 7) 2nd inerting and 8) bunker hose disconnecting. The precooling process is not required if the bunker transfers from a non-refrigerated tank to a non-refrigerated tank. Similar to conventional marine fuel bunkering, the quality of ammonia bunker shall be regulated when anti-corrosion or combustion-supporting
agents are added.
After comparing the threshold concentrations of ammonia to induce fire or harm to personnel, we conclude that its toxicity is of the utmost concern during ammonia bunkering. This study takes reference from the well-established AEGL (Acute Exposure Guideline Level) limits to gauge the toxic impact on humans during an ammonia release. Based on literature review and consultation with industry partners, leakage from the rupture of connecting hoses or pull-away incidents is one of the most common likely causes of the loss of containment for ammonia bunkering. When a catastrophic hose rupture happens, ammonia will release rapidly into the environment, causing danger to the personnel in the surroundings. The safety analysis carried out in this study is focused on the release of ammonia from the hose rupture scenario. Due to the limit of the study, there is no physical set-up for actual ammonia release and monitoring. The release of ammonia is studied by simulating the dispersion pattern using the simulation software PHAST (Process Hazard Analysis Software Tool) to predict the corresponding consequences of the release. Simulations are performed for various bunkering modes: shore-to-ship, truck-to-ship, ship-to-ship and simultaneous operations (SIMOPS), which include four loss-of-containment scenarios. The 3% lethality footprint is used as an indicator.
• Scenario A Shore-to-Ship bunkering: A total of 17,040 kg of ammonia is released in 1 min from
the refrigerated storage with a rainout rate of more than 80%. The 3% lethality footprints reach a
maximum downwind distance of 370 m during the day and 400 m at night.
• Scenario B Truck-to-Ship bunkering: 198 kg of ammonia is released in 1 min from the pressurised
storage condition, with no rainout, and the vapour cloud forms a puff right after the end of the
release. The ammonia cloud concentration falls below the AEGL-2 level by about 4 min during
the day and 7 min at night. Although the maximum cloud footprint has reached approximately
800 m, the 3% lethality footprint is less than 100 m from the source of release for both day and
night conditions.
• Scenario C Ship-to-Ship bunkering: A total of 17,040 kg of ammonia is released in 1 min with
a rainout rate of approximately 80%, of which approximately 60% of this rainout will eventually
dissolve in seawater. The 3% lethality footprints reached a maximum distance of about 1.3 km
during the day and 700 m at night. The maximum cloud and lethality footprints are significantly
larger during the day than at night.
• Scenario D SIMOPS: 17,040 kg of ammonia is released in 1 min. The dispersion pattern of ammonia over the sea is the same as that in scenario C. For the dispersion of ammonia over land, the 3% lethality footprints reached a maximum distance of 310 m during the day and 340 m at night.
A preliminary review of mitigation measures was conducted at the end of the study. Water curtains, absorbent spray, and membrane separation are commonly considered by other industries where there is a potential ammonia leak. However, these technologies have not found ways to meet the requirement of future ammonia bunkering operations. It is expected that various types of mitigation measures can be applied together to enhance performance. Further studies with physical validation are indispensable, as this is the decisive way to provide quantitative and qualitative proof of the mentioned mitigation measures.
In Singapore, effective mitigation measures will help overcome challenges encountered during ammonia bunkering under land and sea space scarcity.