Understanding methane slip

GREEN RAY researchers recently reviewed methane slip emissions from LNG vessels in a study titled “Methane slip emissions from LNG vessels – review.” This research aimed to collect methane slip values from current literature and recent shipowner data. It contributes to understanding methane slip from the current LNG fleet and is part of the EU-funded GREEN RAY project, which seeks to minimize methane slip to promote clean waterborne transport.

The use of LNG as maritime fuel has grown but remains marginal, with less than 3% of ships using it according to the 2021 EU monitoring data. Mitigating methane slip is crucial for sustaining interest in LNG as a marine fuel, given its role in future EU mechanisms to cut greenhouse gas emissions.

The study categorized LNG marine engines into four types:

  1. Lean burn spark ignited engines (LBSI)
  2. Four-stroke low pressure dual fuel engines (LPDF 4-S)
  3. Two-stroke low pressure dual fuel engines (LPDF 2-S)
  4. Two-stroke high pressure dual fuel engines (HPDF 2-S)

Each engine type exhibits different methane slip characteristics. For example, LBSI engines showed increased methane slip at low loads. LPDF 4-S engines, common in ferries and cruise ships, displayed varied methane slip based on engine load and technological advancements. LPDF 2-S engines, prevalent in large cargo ships, showed reasonable agreement between testbed and on-board studies. HPDF engines had limited data but are considered to have low methane slip.

The STEAM Ship Traffic Emission Assessment Model was used to assess the current LNG vessel fleet, revealing most engines are LPDF 4-S or LPDF 2-S types. Emission factors varied widely, influenced by operational differences and measurement methodologies.

Continuous emissions monitoring systems for methane are recommended to help ships self-monitor and reduce emissions. The study emphasizes the importance of considering well-to-wake emissions when comparing fuel and engine options.

Originally published in Riviera Maritime Journal, May 2024.

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