LNG as an alternative fuel

The world’s long coastlines and the large number of coastal ports make maritime transport a smart, efficient and competitive alternative to land-based transport. Per tonne-kilometre, the environmental performance of maritime transport is often superior. This is especially true for large vessels, whose greenhouse gas emissions per unit of transport work are the lowest in freight transport.

LNG as a ship fuel can help significantly reduce the environmental impacts of shipping operations, most likely without increasing costs. Several gas solutions are available:

  • spark-ignited lean-burn gas engine
  • low-pressure dual-fuel engine
  • gas-diesel

Available studies point to a significant reduction of pollutant emissions in the gas mode.


Maritime shipping is estimated to be the source of approximately 3% of greenhouse gas (GHG) emissions worldwide. With international trade projected to grow significantly in the years ahead, maritime shipping emissions are expected to increase by a factor of 2-3 by 2050 if no action is taken. To curb emissions growth, IMO regulations require new-build vessels to be more fuel-efficient. LNG thus represents one of the options to reduce ships’ GHG emissions.

LNG’s contribution to GHG emission reduction is due to its lower carbon content. The well-to-propeller GHG reduction potential depends on the fuel taken as a reference. Use of LNG fuel results in 20% lower GHG emissions compared with Heavy Fuel Oil (HFO). If Marine Gas Oil (MGO) is used, the reduction of GHG emissions is more limited.


LNG can be gradually replaced by biomethane produced by anaerobic digestion and perhaps, in the future, by gasification of biomass. Bio-LNG is no different from fossil LNG, but has the potential to drastically reduce GHG emissions. The infrastructure now under development allows for a change from fossil LNG to its biological counterpart; only the “upstream route” is different.

Biomethane is currently used for power production, heating purposes and to a limited extent in transport applications, with public transport as one example. Despite its limited availability, interest from different sectors is great. This may lead to discussions about future availability of biomass sources for LNG production.

Methane slip

Loss of unburned methane is called “methane slip.” Methane slip from gas engines can be divided into two categories:

  • operational emissions
  • engine emissions

Operational emissions

Under certain operating conditions, methane may have to be vented into the atmosphere. During refuelling operations as well as during storage on land, there may also be minor methane releases.

Engine emissions

One drawback of gas engines is their proneness to methane slip due to incomplete combustion of the methane in the engine. The global warming potential of methane is a 25 times higher than that of CO2. As a result, a few grams of methane emitted per kWh add 10-15% to an LNG-fuelled vessel’s GHG emissions. Methane emissions of 4-8 g/kWh for spark-ignited and low-pressure dual-fuel engines are reported.

Methane slip does not occur in diesel-cycle mode, but only in Otto-cycle mode (dual-fuel as well as spark-ignited) and is generally higher at lower engine loads. Engine manufacturers are currently working on prevention of methane slip in order to guarantee the GHG reduction potential of LNG.


Especially when sailing close to shore, ship emissions affect the local air quality, which impacts human health. Emissions of sulphur oxides (SOx) from shipping represent about 60% of global transport SOx emissions. Emissions of nitrogen oxides (NOx) from shipping account for about 15% of global anthropogenic NOx emissions and approximately 40% of global NOx emissions from freight transport. Seventy percent of all shipping emissions are emitted within 400 km of the coast and can therefore significantly affect air quality in coastal cities and ports, as well as inland.

Long-term exposure to particulate matter (PM), nitrogen oxides and sulphur oxides has an harmful effect on human health. These pollutant emissions therefore need to be further reduced to protect human health and ecosystems and improve local air quality in port areas and neighbouring residential districts.

Reduction potential of LNG per pollutant
Air pollutant emission (in gas mode) Reduction potential
NOx 85-90%
SOx ~100%
PM ~100%

The exact air pollutant emission reduction potential depends on the type of engine used.

Further reading