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Methane number determination in gaseous liquefied natural gas

08 November 2022

Stringent legislation on air pollution and greenhouse gas emission and conflicts in countries with oil reserves all drive the shipping industry to focus on alternatives to heavy fuel oil.

Figure 1: Launching ceremony of the LNG cruise ship built in Northern Europe
Figure 1: Launching ceremony of the LNG cruise ship built in Northern Europe

Small ferries can be powered purely by electricity, but large cruise and transport ships usually rely on LNG. Boil-off gas from an LNG tank can be very different from the gas that is pumped from the bottom of the tank and vaporized, e.g. to supply a ship’s engine. In addition, the composition varies depending on where the LNG was bunkered.

Additionally, there is a lack of skilled workers in the sector who can operate the complex and very sensitive process gas chromatographs. Therefore, operators must resort to alternative measurement techniques.

A gasQS™ static had to prove itself in two sea trials at the beginning of this year. The challenge here was to be able to integrate additional measuring technology into an almost finished ship. The measuring device had to be installed within a few days and the workers on site had to be trained accordingly.

The successful measurements in Figure 2 show the variations in the methane number. The transitions between gas from the boil-off phase and from the liquid phase (after evaporation of LNG) are interesting: The boil-off gas with a high methane content has a higher methane number than that from the liquid phase.

Since the higher hydrocarbons can condense out in the feed line from the liquid phase, the LNG from this feed line briefly contains an increased proportion of higher hydrocarbons after switching from boil-off gas, resulting in a lower methane number. Measuring the gas quality will be necessary for gas engine manufacturers in the future.

Figure 2 (below): Variation of methane number in sea trial

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