Study: LNG lifecycle emissions lowering, but performance still marred by methane slip

A new lifecycle analysis of liquefied natural gas (LNG) fuel commissioned by the Society for Gas as a Marine Fuel (SGMF) has found that LNG offers variable GHG reductions compared with marine gas oil (MGO), which are heavily dependent on the types of engines burning the fuel.

The 3rd Life Cycle GHG Emission Study on the Use of LNG as Marine Fuel study, produced for SGMF by US-headquartered consultancy Sphera, shows moderate average well-to-wake (WtW) emission reductions for fossil-derived LNG and a marked variation in both comparative emissions reductions and unburned methane emissions, based on engine type.

On a WtW basis, the study finds that LNG’s performance is highly variable, with outcomes heavily influenced by how the fuel is produced and the engine technologies on board vessels.

Fossil LNG emissions performance in two-stroke, slow-speed Otto engines
According to the study, using fossil LNG reduces well-to-wake GHG emissions by 16-25% for two-stroke, slow-speed, low-pressure Otto-cycle dual-fuel engines, compared to emissions from traditional MGO.

Methane slip, the term that describes unburned methane that escapes into the atmosphere with other pollutants from an engine’s exhaust gas stream, accounts for 5-10% of the overall lifecycle WtW emissions for these engines.

The 5-10% range in methane slip emissions takes into account the variations in efficiency and emissions control technologies present across this engine type. Low-pressure exhaust gas recirculation reduced WtW GHG emissions by 9%, while using a variable compression ratio reduced emissions by an additional 2%. Per unit of fuel input, fossil LNG is also 11–16% lower than MGO due to higher engine efficiency.

At the tank-to-wake level, all LNG pathways for these engines showed the same GHG intensity, representing a 19-29% reduction compared to MGO.

Fossil LNG emissions performance in four-stroke, medium-speed Otto engines
Compared to MGO, the study shows an 11% reduction in GHG emissions for four-stroke medium-speed engines running on fossil LNG.

"Using fossil LNG results in a GHG intensity that is 11% lower than MGO, which is driven by a 14% reduction in TtW GHG emissions," the report said.

Methane slip is more pronounced in this engine type, accounting for around 12% of lifecycle WtW emissions.

As new primary TtW data was only obtained for two types of LNG engines, two-stroke slow-speed Otto dual-fuel and four-stroke medium-speed Otto dual-fuel engines, other engine types were not the focus of the primary data collection.

Upstream emissions for LNG vs MGO
Looking at upstream emissions, the study finds that well-to-tank emissions for LNG are higher than for oil-based fuels on average, largely due to the energy required for liquefaction processes, supercooling natural gas to liquefy it, as well as the potential for methane leakage across the supply chain, known as fugitive emissions. The study quantifies this difference at around 15% higher than MGO.

At the tank-to-wake level, LNG’s combustion emissions are consistently lower than MGO on average, but upstream emissions for LNG put the fuel’s performance closer to that of MGO and methane slip weighs on the decarbonisation potential for fossil LNG.

Methane slip also plays a key factor in determining LNG’s overall climate performance, as the study shows. While methane contributes a smaller share of lifecycle emissions for conventional marine fuels, it represents a significant portion of total GHG emissions in LNG pathways, particularly in low-pressure engines. As a result, methane emissions can materially reduce the net benefit of switching from MGO to fossil LNG.

The report highlights clear differences between engine technologies. The findings show that engine design choices, including combustion cycle and emissions control technologies, play a significant role in determining methane emissions and overall lifecycle performance.

This divergence means that engine choice alone can shift lifecycle emissions outcomes by a wide margin, even when using the same fuel.

Engine design improvements and after-treatment technologies are positioned in the study as key levers for improving LNG’s environmental performance.

The report points to improved outcomes for alternative LNG pathways, with significantly stronger performance for both bio-LNG and synthetic LNG (e-LNG) when compared with fossil-based LNG. By the study’s metrics, both bio and e-LNG show the potential to deliver substantially deeper lifecycle GHG reductions compared with MGO, particularly where waste-based feedstocks or carbon capture are used.

Blending renewable LNG with fossil LNG also brings improvements in LNG’s emissions profile, with the study indicating that even partial substitution can lead to incremental reductions in overall lifecycle emissions.

The study underscores that long-term decarbonisation potential lies in scaling low- and zero-carbon LNG variants and minimising methane emissions across the value chain.