How to be smarter about methane slip – right now

If your vessel uses LNG, there’s always a small amount of the fuel that doesn’t burn in the engine and escapes with the exhaust gases instead. This is known as methane slip – a challenge that engine manufacturers like Wärtsilä have been working on reducing for years. What is methane slip, why is reducing it essential and what smart solutions are on offer for methane slip reduction?

The basics – what is methane slip and why should you care about it?

LNG is predominantly methane. If your vessel’s engines burn LNG as a fuel, most of the methane will be used up in the energy conversion process. However, some of it remains unburned and escapes into the atmosphere. This is known as methane slip. If not addressed, it reduces the climate-related advantages of LNG.

When it comes to cutting the shipping industry’s emissions, liquefied natural gas (LNG) is never far from the conversation. LNG emits 25% less CO2 (carbon dioxide) than other common marine fuels for the same propulsion power, as well as lower NOx (nitrous oxide), SOx (sulphur oxide) and particulate emissions. It’s no surprise, then, that it’s often touted as shipping’s clean fuel of the future.

If you’re thinking about switching to LNG-capable engines, methane slip is probably a term you’ve already come across.

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If your vessel’s engines burn LNG as a fuel, most of the methane will be used up in the energy conversion process. However, some of it remains unburned and escapes into the atmosphere.

What is the IMO methane slip regulation?

As it stands there are no international regulations on methane emissions from maritime vessels. But, according to the Maersk Mc-Kinney Møller Center for Zero Carbon Shipping report, Reducing methane emissions onboard vessels, regulations are likely to appear soon. The report mentions the FuelEU for Maritime regulation as an example, which will include methane slip in its CO2-eq methodology.

What causes methane slip in gas engines?

Although a lean-burn LNG engine uses up most of the methane contained in the fuel in the energy conversion process, some of it remains unburned. There are three main sources of unburned hydrocarbon emissions in these types of engines:

• Flame quenching in crevices (level of methane slip does not generally vary with engine load)
• Flame quenching in the bulk charge (level of methane slip varies with engine load, with higher engines loads resulting in less methane slip and vice versa)
• Scavenging losses (level of methane slip does not generally vary with engine load)

Manufacturers have been working hard to reduce methane slip since the 1990s. For example, since 1993 Wärtsilä has cut methane slip from its dual-fuel engines by 85%.

How can you reduce methane slip?

There are various ways to tackle these different sources of methane slip in LNG-fuelled ships, including:

• Optimising the timing of gas admission – this means maximising the efficiency of scavenging and reducing the amount of gas that escapes during the process
• Reducing overlap time – this means reducing both the compression work and the combustion temperature, which makes the engine more efficient, thereby lowering emissions
• Improving the design of engine combustion chambers – this means having chambers with as few crevices as possible where unburned methane can be trapped
• Adding hydrogen to the combustion process – the more non carbon-containing fuel like hydrogen you add, the more carbon emissions you avoid
• Using a hybrid propulsion setup with batteries – during low load situations methane slip is eliminated completely because the vessel can operate solely on battery power.

Manufacturers have been working hard to reduce methane slip since the 1990s. For example, since 1993 Wärtsilä has cut methane slip from its dual-fuel engines by 85%.

“In 1993, the methane slip was 16 grams per kilowatt hour (kWh),” says Kaj Portin, General Manager, Sustainable Fuels & Decarbonisation at Wärtsilä, “By optimising engines and leakages, slip has been reduced to just two to three grams today. To put this into perspective, this represents a 4% improvement in efficiency and means that a vessel will sail almost 10% farther on the same amount of fuel,” Portin explains.

“We’ve also been developing a new combustion concept that will reduce methane slip by a further 50%, to around 1 gram per kWh. This concept will come with the added bonus of even further reductions in NOx, SOx and particulate emissions.”

Does reducing methane slip have to involve a lot of costly engineering work?

It might surprise you to learn that that addressing methane slip does not need to involve lots of time-consuming and costly engineering work. If your vessel is already powered by a Wärtsilä dual-fuel engine, a single field service technician and their laptop could be all that’s needed for a cleaner, greener machine.

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The software upgrade – which is available for Wärtsilä 20, 34 and 50 dual-fuel engines – works by marginally reducing the engine’s charge air pressure and lambda – the air-fuel ratio in the combustion chamber. It is quick and easy to install and can cut methane slip by up to 60% when your engines are operating in gas mode below 85% load.

Find out how you can optimise engine behaviour to reduce emissions including methane slip.

Editor’s note: This article was first published in October 2020. It has since been reviewed and rewritten, in February 2023.