LNG - an environmentally friendly ship fuel?

Perjantai 5.11.2021 klo 15.08 - Mikko Nikinmaa

Ships started using Liquefied Natural Gas (LNG) as a fuel, because there was a grave need to decrease the sulphur and nitrogen oxide pollution caused by ship transport. Also, particle emissions are significantly smaller than for diesel-fuelled ships. Thus, the introduction of motors using LNG was considered to be a significant environmental win.

However, recent findings cast a doubt on this. In Environmental Science & Technology, Grönholm et al. (Environ. Sci. Technol. 2021, 55, 13677−13686) have measured the emissions from LNG-fuelled ships in the Baltic. Although also the carbon dioxide emissions decrease about 20 %, there is a marked methane emission. Since methane is about 30 times worse greenhouse gas than carbon dioxide, a ship using LNG instead of diesel oil may have at least 30-fold climate effect. This is because of the LNG motors are commonly similar to the two-stroke motors in common use in e.g. leaf blowers. These motors emit a lot of unburned fuel in the environment. There are also LNG motors without this problem, the high pressure dual fuel engines. However, they have not become common for the simple reason that their nitrogen oxide emissions are higher than those of low pressure dual fuel engines and International Maritime Organization has setlimits to nitrogen oxide emissions. In contrast, there is no limit for methane emissions. The nitrogen oxides could be removed from the exhaust fumes by catalytic converters, but that would increase the cost of the motor, and as long as the methane emission is unregulated it is unlikely that the LNG-fuelled motors would change.

The simple solution would be to set emission limits for methane in boats. Since ship traffic accounts for approximately 3 % of world’s greenhouse gas emissions, without limits to methane emissions, conversion of ships to LNG-fuelled ones could increase this proportion far above 10 %. In order to combat climate change such a simple solution should be implemented until more environmentally friendly fuels become mainstream.