The development of rules and regulations for the safe use of ammonia as ship fuel will take time and will need to be developed on the back of a solid program of research and development
Much of this research has already begun, says Ed Fort, Global Head of Engineering Systems at Lloyd’s Register (LR). In the meantime, LR will be publishing guidance on conducting HAZID studies to support the use of hydrogen and ammonia as fuels. The organisation is also working with MAN Energy Systems on HAZID studies for its ammonia engine currently under development.
Fort notes similarities with LNG: Both hydrogen and ammonia, like natural gas, are low flashpoint fuels existing as gases under ambient conditions, and as such, they were prohibited for use as a ship fuel by SOLAS regulations until relatively recently. However, it must be noted that due to the characteristics of hydrogen and ammonia, increased and additional safety hazards need to be addressed compared to natural gas.
Ammonia is a highly toxic gas under ambient conditions, and exposure to even relatively low concentrations in air can be harmful, potentially fatal, compared to natural gas which is generally considered non-toxic, says Fort. Exposure to ammonia at concentrations of 2,500 ppm or 0.25 percent in air can result in fatalities after 15 minutes, while exposure to ammonia at concentrations of 5,000 ppm or 0.5 percent in air can be fatal within just a few minutes.
“While it is recognised that ammonia has been widely and safely used as a refrigerant within the confines of ships for many years, such refrigeration systems are typically sealed or closed loop systems, and as such the likelihood of an ammonia release within the confines of the ship are expected to be significantly lower compared with its use as a ship fuel,” says Fort.
Hydrogen and ammonia, like natural gas, will generally be compressed and/or liquefied for onboard storage. The boiling point of ammonia is -33.4 °C, and it is typically stored as a liquid either refrigerated or pressurised at ambient temperature.
In Hydrogen and Ammonia Infrastructure Safety and Risk Information and Guidance, a report for the Ocean Hyway Cluster released in May this year, Lloyd’s Register noted key differences between the hazards of liquified and pressurised ammonia. Author Olav Roald Hansen, now founder of HYEX Safety, highlights that a release of pressurised liquid ammonia would likely form a denser than air, very cold, mist cloud, and modelling suggests more severe consequences than for a release of refrigerated liquified ammonia, LNG or liquid hydrogen.
If ammonia is stored at room temperature at around 10 bar over-pressure, a leak would be pushed by a much higher vapour pressure than refrigerated ammonia which would leak by gravity. Around eight percent of the liquid ammonia released at pressure would immediately evaporate on leaving the tank, its rapid expansion would also crush the remaining liquid ammonia into a denser than air aerosol fog. The ammonia fog in air would be cooled to the saturation temperature of the liquid at the reduced pressure (down towards -70oC) and evaporate when further diluted in air. In this case, if escaping due to emergency venting, the released ammonia plume may fall down to deck or the sea at dangerous concentrations.
In contrast, if ammonia is stored at its boiling point, there will not be spontaneous boiling when it leaves containment. Leak rates would be moderate, a pool will be formed, and evaporation would mainly be due to heat from the floor. Rate will be limited and can be controlled by design, says Hansen. In an engineroom setting, the ammonia could evaporate leading to potentially dangerous concentrations, but this can be contained relatively easily and when ventilated to a gas mast the ammonia vapour will be buoyant and seek to go upwards.
“If you consider a refrigerated release in an engineroom, you would smell it immediately, and you would have some time,” says Hansen. “If you get out and close the door, it should be possible to make the situation safe. If you compare it to other flammable fuels, ammonia is only moderately flammable (concentrations above 17 percent are required compared to five percent for natural gas), so it is not necessarily as big a challenge to fight an ammonia fire. For other fuels, such as LNG and hydrogen, it is quite dangerous to be in the room when it releases as flame exposure may be fatal.”
Due to its high heat of vaporisation and strong expansion when boiling hydraulic shocks may be a concern for ammonia. A land-based accident in the US releasing 14 tons ammonia was caused by cold liquid ammonia being injected into pipes containing warm gaseous ammonia. This led to fast condensation and pressure drop which broke the pipes. In a bunkering context, there is a potential risk of hydraulic shock if residual ammonia is left in piping systems.
Experience with ammonia in selective catalytic reduction (SCR) systems has led to the use of double-walled pipes to minimise the risk of leaks. MAN Energy Systems notes that the fuel pipes on its ammonia engine currently under development will be double-walled, with the outer pipe preventing escape to machinery spaces in case of a rupture of the inner pipe. A ventilation system with the capacity for 30 air changes per hour vents any gas in the space, including around valves and flanges.
Ammonia is highly corrosive to a range of materials including zinc, copper, plastic and brass when it is mixed with water. Stainless steel and iron are relatively immune within normal operational temperature ranges. In developing its ammonia combustion engine, MAN Energy Systems has therefore, for example, determined that sealing rings will be made with Teflon.
Currently there are no detailed technical requirements prescribed in rules and regulations for the use of hydrogen and ammonia as fuel onboard ships. Detailed regulations will take several years to develop, but there is still time if prioritised by the IMO, says Fort. “Until such time as technical requirements are prescribed in the rules and regulations, the use of hydrogen and ammonia will be permitted on the basis of a satisfactory engineering analysis, essentially a risk assessment, carried out on a case by case, ship by ship basis.”
Risk assessments for new fuels are becoming more commonplace but still present challenges during ship construction, says Fort, so it is vital that all stakeholders fully recognise the importance of a thorough, rigorous process and recognise the associated impact on resources, schedule and costs. “It should be recognised that the use of hydrogen and ammonia as marine fuels is not business as usual. The potential prize of zero emission vessels is huge, but it will come at a significant cost to the industry which needs to be recognised and accounted for at the outset.”