The results of a new analysis by the International Council on Clean Transportation (ICCT) show that, when combined with a trend toward higher methane leakage and combustion slip, there is no climate benefit from using liquefied natural gas (LNG) as a marine fuel—regardless of the engine technology.

LNG is becoming popular for several reasons. First, it contains very little sulfur. Additionally, LNG engines are tuned to either emit low nitrogen oxide (NO_x) emissions—at the cost of higher methane emissions in some cases—or to incorporate NO_x reduction technologies such as exhaust gas recirculation (EGR) or selective catalytic reduction (SCR). These low sulfur oxide (SO_x) and NO_x emissions make LNG an attractive fuel for ships that operate in Emission Control Areas (ECAs), where ships must comply with more stringent air quality standards.

Second, LNG is, and has been, less expensive than MGO and is now in some regions cheaper than heavy fuel oil (HFO). Moreover, to comply with IMO’s 2020 global sulfur limit, ships must either switch from HFO to more expensive very low sulfur fuel oil (VLSFO) or use HFO with a scrubber, if they have one installed. However, the use of scrubbers has recently come under intense scrutiny and is undergoing regulatory review due to concerns about both the reliability of these systems to deliver consistent air pollution compliance and because of the water pollution produced by open-loop operations. Increasing numbers of national, sub-national, and local regions have disallowed the use of open-loop scrubbers, including Malaysia, China, Singapore, and the port of Fujairah; the Panama Canal also recently banned the use of open-loop scrubbers. California disallows the use of all scrubbers—open-loop or closed-loop—because of evidence that suggests that scrubbers may not result in equivalent emissions reductions as ECA-compliant fuels.

As of November 2019, VLSFO was nearly as expensive as MGO. LNG will likely remain less expensive than VLSFO in the future and might be less expensive than HFO, depending on how the price of HFO responds to the IMO’s 2020 global sulfur limit (CE Delft et al., 2016). Therefore, some ship owners are finding that it makes economic sense to invest in an LNG-fueled ship.

—Pavlenko et al.

The shipping industry increasingly is also looking to LNG as a way to meet the International Maritime Organization’s (IMO) climate goals. LNG emits approximately 25% less CO₂ than conventional marine fuels in providing the same amount of propulsion power.

However, LNG is mostly methane, a potent greenhouse gas (GHG) that traps 86 times more heat in the atmosphere than the same amount of CO₂ over a 20-year time period.

Multiple ship engine technologies can use natural gas; each of these engine technologies emits unburned methane (methane slip). Unburned methane arises primarily from incomplete combustion and fuel concealed in crevices in the combustion chamber during compression. Lean mixtures injected at low pressure are associated with more methane slip than high-pressure injection.

The most popular LNG marine engine—low-pressure dual fuel (LPDF), medium-speed, four-stroke—is also the leakiest. Using LNG, this technology emitted 70% to 82% more life-cycle GHGs than MGO.

In addition, the authors noted:

Methane slip varies as a function of engine load, with higher slip at lower loads. Our analysis relied on weighted methane slip emission factors that represent the IMO’s E3 or E2 test cycles. We know from Olmer et al. (2017) that actual ship operations are different. It is possible to model the methane slip from each LNG-fueled ship hour-by-hour by modifying the approach of Olmer et al. (2017). This would likely show that the real-world consequences of using LNG as a marine fuel are worse, from a climate perspective, than we find here. In the wake of the global financial crisis, many ships have responded by sailing slower, operating at low engine loads. Olmer et al. (2017) found that container ships, in particular, operated at very low engine loads in the years 2013, 2014, and 2015. As more container ships opt for LNG, their methane slip emissions could be higher than we estimate here.

—Pavlenko et al.

To better understand the full climate impacts of using LNG as a marine fuel, the ICCT team compared the life-cycle GHG emissions from LNG, including upstream emissions from methane leakage during extraction, processing, and transport and downstream emissions from combustion and unburned methane (slip), to those of heavy fuel oil (HFO), very low sulfur fuel oil (VLSFO), and marine gas oil (MGO).

Life-cycle GHG emissions by engine and fuel type, 20-year GWP, higher methane scenario. Credit: The ICCT

The authors used a representative life-cycle emission factor as well as the 100-year and 20-year global warming potentials (GWPs) for methane included in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report. The 20-year GWP better reflects the need to reduce GHGs quickly.

The IMO has signaled that it will regulate GHGs under its initial GHG strategy. Continued investment in LNG infrastructure on ships and on shore risks making it harder to transition to zero-emission vessels in the future.

The ICCT said that investments should instead be focused on technologies that reduce total life-cycle GHG emissions, including energy-saving technologies, wind-assisted propulsion, zero-emission fuels, batteries, and fuel cells.

Resources

• Nikita Pavlenko, Bryan Comer, Yuanrong Zhou, Nigel Clark, Dan Rutherford (2020) “The climate implications of using LNG as a marine fuel”