Liquefied Natural Gas (LNG) as ship fuel?


To be honest: being a Graduated Engineer for Ship Operation Technology of the “old school” and for around 18 years up to now a specialized shipping journalist with a particular focus on combustion engines, I’m pretty much, let me say it politely: confused!

Wherever you read an article about gas fuelled engines everybody speaks about LNG-fuelled engines! Lately I read an article in the pretty much well-known daily German newspaper THB (Täglicher Hafenbericht=Daily Port Report) where the following is mentioned: “…Both ferries (Stavangerfjord and Bergensfjord) will be driven solely with liquefied gas (LNG) instead with heavy fuel”. But is this correct?

Most important question: what is LNG and why do we have LNG?

Natural gas (NG) is a mixture of different gases consisting primarily of methane. The higher the methane content (more than 90%) the better. Because the high methane number (MN) influences positively the combustion process. It is usually found in association with fossil fuels and in the past represented a by-product of oil production. Unwanted natural gas was burned off at the well site or pumped back into the oil reservoir with an “injection well”. Today, natural gas is traded around the world by pipeline (in its gaseous form) or by sea in its liquid condition (Liquefied Natural Gas, LNG). The advantage of transporting NG in LNG condition is clear: a defined volume of LNG contains approximately 600 times more energy than the same volume of NG. The phase transformation from gaseous to liquid implies that cryogenic temperatures (cryogenic=producing, or related to, low temperatures) are reached. Liquefied Natural Gas is the most advantageous way for gas transportation and onboard storage, but a change in mind regarding fuel utilization is required. Equipment related to gas storage, handling and utilization should be designed in order to continuously operate in cryogenic conditions, thereby ensuring the safety and reliability of the installation.

At a temperature of minus 162 degrees centigrade the natural gas changes its gaseous condition and gets liquid – now one speaks of liquefied natural gas, widely to be known as LNG. Reason for this liquefaction is the storage capacity, because once the gas changes its condition and gets liquefied it also reduces enormously its volume - by a factor of 600!

In other words: One cubic meter gas fits into a thermos bottle with a content of 1.5 liter!

But liquefied gas cannot be burned in an engine – it must be returned to gaseous condition by a regasification unit at a temperature of around 30 to 40 degree centigrade. In this gaseous condition the gas passes the gas train into the engine under little more than atmospheric pressure.

So, the correct description is: a natural gas driven (or fuelled) combustion engine!

If technical oriented magazines / shipping magazines would use the correct terminus technicus the reader would better understand the terms “natural gas” and “liquefied natural gas”.

Source from : Maritime Propulsion