Emilian Buksak, VPS Decarbonisation Advisor of marine fuels testing company VPS, on Wednesday (27 February) dived into how methane slip can be reduced when liquefied natural gas (LNG) is used as a marine fuel in a ship’s engine as LNG emerges as a key transition fuel in the maritime industry: 

As an industry the world’s shipping fleet consumes over 230 million metric tons of fuel per year, which in turn produces 716 million metric tons of CO2-equivalent emissions. However, shipping is in the midst of a major transition to reduce its emissions and become a far more sustainable industry.

There are many options and opportunities to achieve this, including the development and configuration of ships engines, plus consideration of numerous low-to-zero carbon fuels. According to Clarksons Research, nearly 30% of all vessels on order are designed to use alternative fuels, with another 14% carrying “alternative-fuel ready” notations—clear evidence that greener shipping solutions are on the rise. Regarding marine fuels, we see an increase in the uptake of sustainable biofuels and Renewable Liquid & Gaseous Fuels of Non-biological Origin (RFNBO, e.g., methanol) but this is still limited. Therefore, LNG has become a leading transition fuel for the maritime sector.

Since 2021, the number of LNG-fuelled vessels has tripled, now surpassing 1,200 ships according to the DNV Alternative Fuels Insight (AFI) platform. This fleet includes about 452 container ships, 242 tankers, 208 car carriers, 73 bulk carriers, and 49 cruise vessels, underscoring LNG’s accelerating adoption as a transitional fuel. All those, in addition to 751 LNG carriers.

But why use LNG as a marine fuel? Well, LNG is primarily methane (85-95%) with a high energy content of 55.5MJ/Kg and offers around 30% lower CO₂ emissions (TTW) than traditional bunker fuel. However, a major negative impact to using LNG is that methane has a global warming potential (GWP) 29.8 times higher than CO₂ over 100 years—and more than 80 times higher over 20 years. This makes reducing any methane emissions across the entire LNG lifecycle critical.

When used as the fuel in a ship’s engine, a certain percentage of the methane present remains unburnt and can escape to atmosphere. This is known as “methane-slip”. Our data shows that double-digit slip (in g /kWh) can and does occur—often due to engine malfunctions or low-load operation. Beyond combustion, fugitive emissions (leakage) and emergency releases also add to the total methane footprint.

Efforts to mitigate methane slip vary widely—from proactive collaborations with engine manufacturers that achieve 50–70% reductions, to a “do nothing” approach relying solely on EU MRV default slip factors. Under these regulations, default slip values range from 3.1% for dual-fuel medium-speed engines to 0.2% for dual-fuel slow-speed engines. Notably, four-stroke LNG engines make up nearly 30% of all LNG-capable vessels.

While the industry considers future fuels such as, biofuels, renewable bio-methane and e-methanol, these introductions alone will not resolve methane slip in existing methane-fuelled operations. Consequently, equipment manufacturers, shipowners, operators, and regulators are focusing on practical, near-term measures to reduce onboard methane slip at the vessel level—an essential step toward lowering the climate impact of LNG-fuelled shipping.

Across international shipping, there are still no dedicated regulations specifically targeting methane slip. Within the EU’s Fit for 55 package we have the FuelEU Maritime, which includes well-to-wake (WtW) methane slip in its compliance framework. This requires operators to account for the CO₂-equivalent impact of methane across the entire fuel life cycle. We also have the Emissions Trading System (ETS), which already mandates tank-to-wake (TtW) methane slip reporting in CO₂-equivalent terms. Starting in 2026, methane emissions will directly influence ETS allowance requirements, linking methane slip more tightly to trading costs.

Meanwhile, the Global Methane Pledge calls for rapid methane reductions to keep the 1.5°C climate target within reach. Furthermore, the IMO’s 2024 Guidelines on Life Cycle GHG Intensity of Marine Fuels may soon drive stricter measures relating to methane slip. Taken together, these developments signal a future with tighter regulations, stronger enforcement of methane accounting in CO₂-equivalence terms, and a faster pace of innovation to reduce methane slip in shipping.

So, how can methane slip be reduced? There are four key areas starting with:

Engine Design Improvements

Reduce Crevice Volumes: 

• Redesign piston crowns, cylinder heads, and valve seats to minimize gaps.
• Use advanced manufacturing tolerances and materials for tighter seals.
• Exhaust Gas Reduction: Moderates combustion temperatures, improving overall efficiency and reducing unburned fuel.
• Lambda Control: Real-time monitoring of oxygen levels for precise air-fuel ratio control, preventing incomplete combustion.

Transition to Diesel-Cycle Gas Engines with High-Pressure Gas Injection:

• Direct High-Pressure Injection: Adopt high-pressure gas injection (HPGI) systems that inject LNG directly into the combustion chamber at high pressures and near the end of the compression stroke.
• Upgrade engine components to support higher compression ratios and improved ignition control, optimized for HPGI.

Optimize Low-Pressure Dual-Fuel Engines (Otto-Cycle)    

• Use updated software and hardware to fine-tune gas admission timing and pilot-fuel injection (if applicable).
• Align ignition timing with real-time load conditions to improve combustion completeness.
• Adapt piston and cylinder designs specifically for low-pressure gas engines.
• Incorporate design features—like shaped combustion chambers—to minimize fuel trapping.
• Employ sensors and electronic controls to continuously monitor combustion quality.
• Adjust the air-fuel ratio and pilot-fuel quantity in real time to maintain efficient ignition and reduce slip.

After-Treatment Solutions

Install Methane Oxidation Catalysts:

• Incorporate catalysts into the exhaust system of newbuild ships.
• Ensure catalysts are maintained and replaced as needed to sustain up to 80% reduction in methane slip.
• Monitor exhaust temperatures and adjust as necessary to keep catalysts within optimal operating ranges.

Operational Adjustments

Avoid Prolonged Low-Load Gas Operation:

• Plan voyages and engine use to maintain higher loads when running on LNG.
• Use operational strategies such as scheduling heavier loads or adjusting speeds to minimize low-load scenarios.

Switch Between Gas and Fuel Oil at Low Loads:

• Utilize the dual-fuel capability to switch to fuel oil when LNG combustion efficiency drops at low loads.
• Establish clear operational thresholds for switching based on load conditions and engine performance data.
• Note that dual-fuel engines often run with a higher pilot fuel percentage to prevent maintenance issues with fuel injectors.

Ensure Proper Maintenance and Engine Optimization:

• Implement routine inspections and cleaning of fuel injectors, valves, and sensors.
• Regularly calibrate and tune engine settings such as injection timing and air-fuel ratios for optimal combustion.
• Train crew in best practices for engine management and maintenance procedures.

Hybrid or Battery Integration

Integrate Hybrid Systems or Batteries:

• Install batteries or hybrid power systems to assist during periods of low power demand.
• Use energy storage to maintain engine operation at higher, more efficient loads around 95%.
• Design the ship’s power management system for seamless interaction between engine and battery, allowing for consistent high-load operation.
• Plan system integration to account for space, weight, and cost considerations while optimizing fuel and emission performance.

Note: The full article by VPS on methane slip read be found here.

Photo credit: VPS
Published: 27 February, 2025