Mind the methane gap

LNG could cut the shipping industry’s CO2 emissions by 25%, but could its methane emissions undermine its climate advantages? Addressing this ‘methane slip’ is vital to LNG’s viability as a clean fuel solution.

When it comes to cutting the shipping industry’s emissions, liquefied natural gas (LNG) is never far from the conversation. Often touted as shipping’s clean fuel of the future, 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. However, LNG releases methane into the atmosphere, which is roughly 30 times more potent as a greenhouse gas than CO2.

With the International Maritime Organization (IMO) aiming to halve greenhouse gas emissions from international shipping by 2050, tackling this methane slip is therefore vital to LNG’s future as a clean solution for merchant shipping.

What’s with the slip?

LNG is predominantly methane. When LNG is burned as fuel, most of the methane is consumed in the energy conversion process, but the unburned methane escapes into the atmosphere, thereby reducing the climate advantages of using LNG. A recent report by the International Council on Clean Transportation (ICCT) even claimed there is “no climate benefit from using LNG”.

Yet, the way LNG is measured affects its viability. Matteo Natali, General Manager of Business Intelligence at Wärtsilä,says “the methane emission levels used in the study do not reflect the latest gas engine technology. Moreover, methane’s impact is calculated based on a 20-year timeframe, whereas most scientific studies have adopted a 100-year view, as does all relevant legislation.”

He adds that “considering appropriate methane slip values and a 100-year timescale, a four-stroke low pressure gas engine proves to be remarkably less polluting than a diesel engine.”

Kaj Portin, Wärtsilä’s General Manager of Fuel and Operational Flexibility, agrees. He notes that the LNG measurement process is uniform – “you simply measure the total hydrocarbons in the exhaust and then calculate what the methane part is” – but timescale is important as the global warming potential (GWP) of LNG falls considerably when looking at a 100-year period (28 GWP) compared to a 20-year-period (84-86 GWP).

“For me, it’s best if you look as far ahead in the future as you can and still be accurate. For the emissions you put out today, it’s better to think about their impact after many years, so the longer the better.”

Source: Wärtsilä

Reducing the methane slip

Measurement preferences are inherently subjective, but there is a simple solution: reduce the methane slip. From getting the timing of gas admission and reducing overlap time to improving the design of engine combustion chambers, and adding hydrogen to the combustion process, there are various ways to reduce the amount of methane released by LNG-fuelled ships, which has decreased ever since the issue was first addressed in the 1990s.

Wärtsilä, for example, has cut the methane slip from its dual-fuel engines by 85% since 1993. “In 1993, the methane slip was 16 grams [per kilowatt hour],” Portin says, “but by optimising engines and leakages, we have been able to take that slip down from 16 to between two and three grams today.”

He adds that Wärtsilä is currently working on a range of methane-reducing solutions, which include figuring out “how to get gas into the cylinder; how to design the cylinder so there is no place for the gas to hide; how to optimise the combustion room so that combustion is fast and complete; how to improve fuel injection so the pilot fuel is stable and spread around in the best way; and having the right air-fuel ratio.”

With these innovations, Portin is confident Wärtsilä’s next combustion concept can take down the methane slip by more than 50% to roughly 1 gram per kwh, while keeping the NOx, SOx and other particulates at the same level.

Preparing for the future

Optimising LNG-fuel engines and the combustion process not only cuts the methane slip, but also improves the efficiency of the vessel. What’s more, engines and fuel supply systems designed for LNG can be adapted to burn a wide range of alternative fuels.

“Wärtsilä engine technology is extremely flexible and will allow operators to transition to any of the sustainable fuels expected to become available over the coming years, with a fairly limited exchange of components,” says Natali.

“There are uncertainties around future fuels, including when and where they will be available and at what price. Building fuel flexibility into new vessels offers a hedge against these risks.”

Portin also sees the switch to LNG as a matter of posterity. “By changing to LNG now, you can immediately impact CO2emissions with our engines. By setting up the gas infrastructure now, it’s much easier in the future to take in decent new fuels. If you go to gas now, you will be future-proofed.”
Source: Wärtsilä

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