LNG could worsen shipping’s climate impact

By: Christer Ågren

A recent analysis by the International Council on Clean Transportation (ICCT) has compared the life-cycle greenhouse gas emissions of liquefied natural gas (LNG), with those from traditional marine fuel oils.

LNG has become increasingly popular as a ship fuel as it significantly reduces emissions of air pollutants, such as sulphur dioxide, nitrogen oxides and particles. Moreover, LNG is cheaper than marine gas oil (MGO), and is – over the longer term – expected to be less expensive than both heavy fuel oil (HVO) and the blended 0.5-per-cent very low sulphur fuel oil (VLSFO).

In 2019, there were more than 750 LNG-powered ships, most of them in the offshore and ferry segments, but new tankers, container ships and cruise ships are also being built with LNG engines.

Two years ago, in April 2018, the IMO adopted an initial strategy with the stated levels of ambition that greenhouse gas emissions from international shipping should peak as soon as possible and to reduce emissions by at least 50 per cent by 2050 compared to 2008, while, at the same time, pursuing efforts towards phasing them out entirely.

Achieving these objectives will inevitably require a transition to zero-carbon fuels or propulsion systems. While LNG contains less carbon per unit of energy than conventional marine fuels, which means that burning it emits less carbon dioxide (CO₂), there are also other GHGs to consider.

LNG consists mainly of methane, which over a 100-year time period traps 36 

times more heat than the same amount of CO₂, i.e. its global warming potential (GWP100) is 36. If we look instead at a 20-year time period (GWP20), methane traps 86 times more heat.

The study compared the life-cycle greenhouse gas emissions of LNG, marine gas oil (MGO), very low sulphur fuel oil (VLSFO), and heavy fuel oil (HFO) when used in engines suitable for international shipping, including cruise ships. The analysis included upstream emissions, combustion emissions, and unburned methane (methane slip), and evaluates the climate impacts using 100-year and 20-year global warming potentials.

It was found that over a 100-year time frame, the maximum life-cycle GHG benefit of LNG is a 15 per cent reduction compared with MGO, and this is only if ships use a high-pressure injection dual fuel (HPDF) engine and upstream methane emissions are well controlled.

The authors point out, however, that the latter might prove difficult as more LNG production shifts to shale gas, and given recent evidence that upstream methane leakage could be higher than previously expected. Additionally, only 90 of the more than 750 LNG-fuelled ships in service or on order use HPDF engines.

The most popular LNG engine technology is low-pressure dual fuel, four-stroke, medium-speed, which is used on at least 300 ships and is especially popular with LNG-fuelled cruise ships. Using GWP100, this technology emitted 8 per cent more lifecycle GHGs when it used LNG instead of MGO and 16 per cent more than using MGO in a comparable medium-speed diesel engine.

Using GWP20 instead, low-pressure dual fuel, four-stroke, medium-speed technology emitted 70 per cent more lifecycle GHGs when it used LNG instead of MGO and 82 per cent more than using MGO in a comparable medium-speed diesel engine.

Dr. Elizabeth Lindstad, Chief Scientist at SINTEF Ocean, Maritime Transport, commented that: “The report shows the need for adopting policies that can reduce the broader GHG emissions of shipping instead of CO₂ only, including the well-to-tank emissions of ship fuels. If we fail to include all GHGs and focus only on CO₂, we might end up with a large number of ships fulfilling all efficiency requirements, but where the GHG savings are on paper only.”

The ICCT concludes that the results show that LNG does not deliver the GHG emissions reductions demanded by the IMO’s initial GHG strategy and that using it might actually worsen shipping’s climate impacts. Given this, it is fair to question continued investments in LNG infrastructure on ships and on shore, as these could make it harder to transition to low- and zero-carbon fuels. Investing instead in energy-saving technologies, wind-assisted propulsion, zero-emission fuels, batteries, and fuel cells would deliver both air quality and climate benefits.

Christer Ågren

The study “The climate implications of using LNG as a marine fuel” is available at: https://theicct.org/ sites/default/files/publications/Climate_implications_LNG_marinefuel_01282020.pdf

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