LNG as marine fuel and methane slip

Stamatis Fradelos, Regional Bulk Carriers Segment Director Maritime – South East Europe, Middle East & Africa at DNV Hellas S.A., explains the key challenges of LNG as a marine fuel and issues surrounding the methane slip. Current IMO agenda includes regulating methane slip with possible application on EEDI, beginning with phase 4 and maybe on other regulations where GHG emissions play a role (e.g. IMO DCS, CII). Nevertheless, the 100-years horizon (GWP 34 instead of 86) seems to prevail while engine manufacturers are trying to implement a range of measures to reduce methane slip with significant improvements the last years.

Liquefied natural gas (LNG) consists primarily of methane and as a marine fuel has significant environmental benefit, with regards to air emissions (CO2, NOx, SOx and Particulate Matter including black carbon).

With respect to CO2 emissions, the carbon factor (CF) for LNG (i.e. 2.75) and the low specific fuel consumption (SFC) of dual fuel engines operating on LNG provide a beneficial effect on the ship’s attained Energy Efficiency Design Index and Carbon Intensity Index (CII).

Methane, like CO2, is a greenhouse gas that contributes to global warming with a global warming potential (GWP) of 28 to 34¹ according to the IPCC Fifth Assessment Report (AR5) utilizing the standard 100-year time frame (GWP100). Therefore, any emission of methane will result in a reduction of the environmental benefit from using LNG as a marine fuel.

EEDI for ships with dual fuel engines that use LNG as fuel

For EEDI the fDFgas factor is calculated as the ratio of the designed calorific value of gas fuel (LNG) and the total calorific value of liquid (HFO/MGO) and gas fuel stored onboard:

• If fDFgas ≥ 0.5, then gas fuel is regarded as the “Primary fuel”. Practically this means that the calorific value of LNG is higher than the calorific value of liquid fuels stored onboard and the SFC and CF for LNG is used in the calculations.
• If fDFgas < 0.5 then the CF and SFC is calculated as the weighted average of CF and SFC for liquid and gas mode.

The EEDI and CII calculations currently do not consider any potential methane emissions (methane slip).

Methane slip:

Methane slip is the unburned methane from engines which is caused by two main reasons:

• Due to dead volume in form of crevices between cylinder unit components
• Incomplete combustion in form of quenching at the coldest part of the combustion chamber when running lean

The methane emissions are affected by numerous factors including engine type, duty cycle, speed, load and fuel used. In the 4th IMO GHG Study the estimation of methane emissions was approached through emission factors.

Methane Global Warming Potential (GWP)

It should be noted that for 20 years’ time span the GWP of methane is 86 while for 100 years’ time span the GWP is 34². According to the recently developed Draft life cycle GHG and carbon intensity guidelines for maritime fuels (ISWG-GHG 7/5/8 by Australia, Japan, Norway, Republic of Korea and ICS), the GHG emissions are calculated as CO2-equivalents (CO2e) using the Global Warming Potential over a 100-year horizon (GWP 34).

Engine types and methane slip

As mentioned above, unburned methane can pass to the exhaust having been trapped unmixed in crevices or due to close proximity to any relatively cooler spaces found in the combustion chamber. Key parameters affecting methane slip include:

• Engine size: A large cylinder has fewer crevices leading to lower methane slip.
• Timing of the gas admission and valve overlap duration. Minimizing overlap results in a decrease in methane emissions.

Two-stroke low-pressure dual-fuel (LPDF) engines present higher methane slip compared with high-pressure dual-fuel (HPDF) engines which may cause higher CO2 equivalent emissions³.

Furthermore medium-speed dual fuel four-stroke engines present relatively higher methane slip than the slow speed two-stroke engines:

Engine manufacturers investigate options to reduce methane slip either through exhaust gas aftertreatment (e.g. by means of an oxidation catalyst) or by improving the combustion process.

Methane slip regulatory developments

Addressing emissions of methane has been included in the list of candidate short term measures in the initial IMO GHG reduction strategy (Resolution MEPC.304(72)). Further consideration of concrete proposals to reduce methane slip has been included in the agenda of the 7th meeting of the Intersessional Working Group on Reduction of GHG Emissions from Ships with three follow up proposals submitted:

1. Clean Shipping Coalition et al (ISWG-GHG 7/3), proposing to include all relevant GHGs, including methane, in future phases of the EEDI, beginning with phase 4.
2. Society for Gas as a Marine Fuel (ISWG-GHG 7/3/1) proposing methane slip to be measured with standardized methods (NOx Technical Code and ISO 8178-1:2017) during the parent engine certification and the CO2 emission be complemented with the methane slip as equivalent CO2, using the appropriate factor. The CO2-equivalent should be used in EEDI calculations and any other future regulation where GHG emissions play a role.
3. FOEI et al (MEPC 75/7/10) proposing to include all greenhouse gases emitted from ships in future phases of EEDI, beginning with Phase 4.

However consideration of the above proposals has not been completed since IMO is working in reduced capacity due to COVID restrictions and has been focused on the finalisation and approval of Guidelines associated with the implementation of the Energy Efficiency Existing Ship Index (EEXI) and the operational Carbon Intensity Indicator (CII), including the rating scheme (A to E), and enhanced SEEMP.

The views presented hereabove are only those of the author and do not necessarily those of  SAFETY4SEA and are for information sharing and discussion  purposes only.