While LNG has been among the most-discussed solutions to shipping industry’s decarbonization pathway, the often-unplanned methane slip as a result from LNG combustion is the main argument of “LNG as a marine fuel” opponents. What does “methane slip” exactly refer to?
“Greenhouse gas emissions” are a major topic of discussion in shipping, with CO2 being the most regularly encountered issues. However, CO2 is not the only greenhouse gas that is driving global climate change. There are a number of others which have contributed a significant amount of warming to date, including nitrous oxide and methane. Methane is 80 times worse a greenhouse gas than CO2, which makes the attention on its reduction vital.
Methane emissions in numbers
- In 2018, methane contributed 17.3% of total world’s emissions.
- In 2020, methane (CH4) accounted for about 11% of all U.S. GHG from human activities, which include leaks from natural gas systems.
- GHG emissions —including CO2, methane, and nitrous oxide— of total shipping (international, domestic and fishing) have increased from 977 million tonnes in 2012 to 1,076 million tonnes in 2018 (9.6% increase), IMO data show.
What is “methane slip”?
Equipment that uses natural gas as a fuel is generally designed to have at least 98% combustion efficiency – which means at least 98% of the gas to be burned. This also means that the other 2% of methane is released as unburned gas. This is known as methane slip.
In simpler terms, methane slip is the unburned fuel that is not fully combusted in ships’ engines. Although methane slip accounts for a generally a small percentage of the fuel used, in operations that use a significant amount of energy, methane slip can be a major source of emissions.
What causes methane slip?
The methane emissions are affected by numerous factors including engine type, duty cycle, speed, load and fuel used. Stamatis Fradelos from DNV Hellas S.A. has earlier explained to SAFETY4SEA that methane slip is caused by two main reasons:
- Due to dead volume in form of crevices between cylinder unit components; and
- Due to incomplete combustion in form of quenching at the coldest part of the combustion chamber when running lean.
Either by leakage through piston rings, or as a result of insufficient combustion, methane slip occurs when gas is emitted unburned from the engine.
Methane slip: The case against LNG as a marine fuel
The main advantage of LNG as a marine fuel is its mature technology compared to other fuel options, enabling the start of shipping decarbonization journey now. With LNG offering a 23% cut in greenhouse gas emissions over oil-based marine fuels today and given the urgency for action on shipping emissions reduction today, there is no need to wait a decade or longer for other, untried and unproven fuels, argues industry coalition SEA LNG. Reflecting these benefits, the Shell LNG Outlook 2022 showed that LNG is increasingly being adopted to address emissions from shipping, with 30% of new large ship orders (in gross tonnage) being for LNG-fueled vessels in 2021.
Despite its immediate availability, LNG has been facing criticism for its role in the shipping sector’s ambitious decarbonization plans, with industry stakeholders recognizing that it has to tackle the disadvantage of methane emissions. More specifically, criticizers of LNG as a shipping fuel stress that emissions reductions of the magnitude mentioned above apply only to two-stroke, high-pressure engines, and that methane emissions from LNG-fueled vessels are potentially equal to those from marine conventional fuel powered ships.
As explained, the 85%-95% of LNG is methane, a greenhouse gas that contributes to global warming. Therefore, any emission of methane would result in a reduction of the environmental benefit from using LNG as a marine fuel. In line with this, a World Bank report last year found that LNG could not form a large proportion of the bunker fuel mix in 2050 due to its carbon intensity.
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Methane slip: Latest developments
Earlier in 2022, lobby group Transport & Environment, which has long-ago expressed its opposition to the use of LNG as an environmentally friendly fuel, revealed the findings of an investigation on a ship in Port of Rotterdam, Europe’s largest port. Using an advanced camera with a special filter to detect hydrocarbon gases, the images unveiled significant amounts of unburned methane being released into the atmosphere.
Roughly 80% of Europe’s LNG used by ships today are worse for the climate than the fuels they replace, due to the release of this potent gas which is over 80 times more warming than carbon dioxide
…the group explains, noting that, even when companies may advertise their LNG ships as reducing CO2 emissions, the lack of information on methane emissions can be considered as greenwashing.
In May 2019, the IMO tabled “further consider concrete proposals to reduce methane slip” as one of the issues to be discussed as part of its working sessions, but no regulatory requirements have yet been imposed on either operators or manufacturers that provide them with a suitable standards and incentive to mitigate methane slip, informs Mr. Panagiotis Mitrou, Global Gas Director, LR.
How can methane slip be prevented?
When natural gas must be used as a fuel, improving the efficiency of the combustion engines is critical, according to Methane Guiding Principles. Indeed, engine makers are often seen promoting their advanced engine designs aimed at reducing methane slip. This can be done either by adding advanced technology, such as exhaust gas recirculation, or by redesigning their engines to be more environmentally friendly.
In the short-term, operators have the ability to alleviate some of the issues associated with LNG via a combination of process and technology. The use of fixed-arm connections during bunkering is more efficient than hoses, for instance, and capacity advancements could help to reduce leakage by reducing the frequency with which refuelling occurs
…suggests Mr. Mitrou.
However, the general low awareness on methane emissions -often accompanied by a misleading report on CO2 emissions reductions- as well as the lack of regulatory impetus as highlighted above -let us take into account that EEDI and CII calculations currently do not consider any potential methane slip– shipping operators have no compelling reason to explore and invest in methane emissions reduction technologies.
