Liquefied Natural Gas (LNG) as engine fuel?

By Peter Pospiech at June 18, 2013 04:16
Filed Under: Fuels & Lubes, LNG 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.

Natural Gas in Transport

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”.

Exhaust emissions in comparison


graphs: Wiener Motorensymposium

Comments (4) -

Of course you are correct - gas is the fuel in the combustion chamber. LNG cannot burn.

But the designation "LNG Fuelled" does convey valuable information about the powerplant. It tells us that the fuel is bunkered and stored on board as LNG, which is quite different from another common fuel storage strategy, ie., compressed gas. So I think it's legitimate to refer to such a powerplant as "LNG fuelled".

Of course, I'm a wheelhouse guy, not an engineer! So maybe I should defer to you.

Reid Sprague |     6/19/2013 2:15:23 PM #

I have a small technical firm active in ship design and now with special interest in LNG operation, dual fuel arrangement, diesel electric type.. The name of my firm is Sea Technology AG, residual in Saltsjobaden, close to Stockholm. An article about us was published in the magazine "World Cargo News" in November 2012. We have mothership transporting over the oceans "satellite vessels" via float on float off. The satellites operate close to the coasts. We focus now on a design with a large depot of LNG tanks installed on the mothership supplying the satellites with gas. We would appreciate receiving info about tank designs. 200 m3 - 5000 m3. And all other info is of interest. Regards Bengt Lundquist

Bengt Lundquist |     6/20/2013 4:59:29 AM #

Dear Bengt,

read your request with much interest. Please send me your contact data. I have access to a number of LNG-tank purveyors.

regards, Peter

PePo |     6/21/2013 6:15:23 AM #

You're right. As an engineer, the first thing I thought of when I saw the banner "LNG fueled" was; how can they compress a liquid, or for that matter, ignite it? Both are vital parts of the 2 and 4 stroke internal combustion engine cycles. But I guess the best way to describe a fuel system that cryogenically stores the fuel as a liquid, then restores it to a gas to use as a fuel would be 'LNG Powered', at least until we come up with a new name for it.

Michael Dean |     6/20/2013 5:13:21 PM #

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