Ship fuels and LNG

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Ship fuels and LNG

Ship fuels cannot be compared using a single metric. Sulfur oxides and particulate matter are among the air pollutants from ship exhaust; when assessing climate impact, methane and emissions from fuel production and supply also matter, in addition to CO₂. Liquefied natural gas (LNG) should therefore not be assessed solely by its fuel name: the engine, its operation and the upstream supply chain all need to be considered.

Sulfur oxides, particulate matter and nitrogen oxides

Since 1 January 2020, MARPOL Annex VI has generally limited the sulfur content of ship fuel to 0.50 percent by mass outside designated emission control areas; in certain areas, the limit is 0.10 percent. The requirement is based on the sulfur content of the fuel. According to the IMO, a ship may use compliant fuel or an approved exhaust gas cleaning system as an alternative means of compliance.

This sulfur rule is not a complete assessment of all air pollutants or of climate impact. Nitrogen oxides are subject to separate requirements and depend on the engine’s technical design and operation. A single sulfur value therefore describes neither the full exhaust composition nor a ship’s energy consumption.

Climate impact: consider CO₂ and methane separately

For LNG, the climate assessment needs to account for the greenhouse gases produced across the entire fuel chain. The IMO’s life-cycle assessment guidelines, adopted in 2024, consider emissions from fuel extraction and supply through to use on board. They cover CO₂, methane and nitrous oxide, and distinguish the upstream phase (“well-to-tank”) from use on the ship (“tank-to-wake”). Together, these two stages make up the life-cycle assessment (“well-to-wake”).

This also explains why comparing only CO₂ emissions at the exhaust can be incomplete. Methane is itself a greenhouse gas; losses in the supply chain or unburned methane during operation belong in a comprehensive assessment. The actual impact depends on the emissions factors and the specific installation. The LNG label alone does not establish a blanket climate benefit.

What a fuel comparison says about cruise ships

For a specific ship, the fuel, engine system and onboard energy, exhaust cleaning, energy demand and operating profile all matter. Shore power in port is a separate operational aspect: it can supply electricity while the ship is berthed, while the questions of fuel and propulsion remain relevant at sea. Design and infrastructure determine which options a ship can use.

A meaningful comparison therefore specifies whether it considers sulfur limits, individual air pollutants or greenhouse gases over the life cycle. These measures answer different questions and should not be reduced to a single label such as “clean” or “climate-friendly.”