Operating experience is vital, says ExxonMobil

“Unlike other hydrocarbon-based fuels, such as LNG and methanol, where there are years of field experience, the impact of ammonia on lubrication is relatively unknown”, he commented to The Motorship.

This uncertainty is linked to a lack of operational experience, he said, because the technology needed for engines to burn ammonia for marine vessel propulsion is still evolving. Because of this, it is even too early “to determine what, if any, lubrication challenges exist due to the combustion of ammonia”, he added.

As a result, “the impact of ammonia and its combustion by-products on the lubrication of pistons and liners is not yet very well understood”.

Asked whether lube oils that have been granted Category II status by MAN Energy Solutions for use with low-sulphur fuels would therefore be suitable for sulphur-free ammonia, he said that this had not yet been evaluated, for the same reason.

ExxonMobil’s Mobilgard 540 AC is such a fuel but since ammonia as a marine fuel is in the early stages of development, “this new lubricant technology has not yet been evaluated in an ammonia-powered engine”, he said.

One potential challenge is ammonia’s high pH of 11.6, making it a strong alkaline. Lubes are also alkaline and “it is not yet clear what impact prolonged operation with highly alkaline fuel and lube will have on pistons and liners”, Mr Givens said.

A bigger challenge, however, arises because ammonia is more difficult to ignite than hydrocarbon fuels so its pH “is not necessarily the feature that determines its combustion characteristics”, he suggested.

Like sulphur in traditional fuels, the nitrogen in ammonia will lead to acidic emission compounds, but it is “too early to tell if new chemistries will be needed” to neutralise these, Mt Givens said. “The kinetics [of the acid formation] are not yet fully understood and if countermeasures are needed, such as lubricant reformulations, they will have to be developed subsequently”.

“It is likely that the initial lubrication of ammonia engines will be based on current lubricant technology”, he predicted, with ‘optimisation opportunities’ being identified as engine technology evolves and operational experience is gained.

ExxonMobil has initiated laboratory studies to explore the effects of ammonia/oil blends on lubricant characteristics and performance, he said. And although ammonia’s effect on engine lubrication is not yet clear, ammonia itself “is a very mature industrial chemical and its chemical and physical characteristics are well understood”.

For example, ammonia is corrosive to metals such as copper and some nickel alloys. It also has a much lower flame temperature, and its heat of vaporisation is much higher, than hydrocarbon-based fuels and hydrogen.

Because of this, he believes “it is a reasonable assumption that corrosion and emulsification could be factors to consider in addressing lubrication needs” but he expects that engine OEMs will address these issues when designing their fuel handling systems.

He also expects that four-stroke and two-stroke ammonia engines will require different lubrication regimes, as at present. “The key difference of viscosity (SAE 30/40 for 4T and SAE 50 for 2T cylinder oil) is expected to persist even when ammonia is the propulsion fuel”, he said. But even here, experience-building is still needed to “confirm how different the lubricant requirements will be”.