What is LNG?

Today we take a day off from future outlook, projections, uncertainties, and all that. Today, like good engineers, we look at the facts. And we start with natural gas at ambient temperature and pressure.

Natural gas is primarily methane, with small portions of heavier gases. Methane is a hydrocarbon gas with formula CH4, i.e. it is one carbon atom and 4 hydrogen atoms in the molecule. This formula is important to remember when we get to the environmental comparison to other hydrocarbons. Some properties of methane (ref Wolfram Alpha):

  • Molecular weight: 16.0425 g/mol
  • Density: 6.67151*10^-4 g/cm3
  • Boiling point: -161,48 degrees celsius
  • Vapor density: 0.55 relative to air

Since what we need the natural gas for is to produce energy, we are particularly interested in the heat of combustion values (ref Wikipedia):

  • Specific heat of combustion for natural gas 54 kJ/g
  • Specific heat of combustion for gasoline 47 kJ/g
  • Specific heat of combustion for diesel 45 kJ/g

So, clearly natural gas offers more energy per mass than the other common fuels.

Back to the molecule. The basic hydrocarbon molecules are chains of carbon atoms connected together, and hydrogen atoms connected to the free spots along the chain. Then, by simple math, we see that with only one carbon atom in the chain you get the best ratio of hydrogen atoms per carbon atoms in the molecule. This translates directly into less CO2 produced in the combustion, simply because there are fewer carbon atoms available.

The emission reductions compared to oil products will broadly speaking be as follows, but this is dependent not only on basic chemical properties, but also on engine technology, combustion characteristics, etc:

  • NOx: 85% reduction
  • SOx: ~100% reduction
  • Particles: ~100% reduction
  • CO2: 15-20% reduction

So what has LNG got to do with this? LNG is just a transportation mode, it is not a separate source of energy, it is simply natural gas made liquid for transportation phase, It is regasified before it is used. The reason why it is made liquid is that methane requires 600 times more volume in gas phase than in liquid phase. Hence it is much more efficient to transport it in liquid phase.

In order to get the methane into liquid phase it needs to be cooled down below its boiling temperature of -161 degrees celsius. The LNG is transported at this temperature and at atmospheric pressure. This means the liquid methane is continously boiling during the transportation, and as long as the vaporized gas is released from the tank the pressure will not increase. This is just like boiling water at 100 degrees celsius; the temperature will not increase above 100 degrees, but there will be less and less water left in the pan.

Finally, we better touch upon the safety related properties of LNG. It is important to understand that LNG can not explode or burn. LNG first needs to evaporate and mix with air in gas phase. So if there is a leak and the vapour cloud mixes with air it will form an ignitable cloud when the mix contains between 5 and 15 % methane. Below 5% mix will not be enough too ignite, and above 15% will too much to ignite. Also, in order to get an explosion, instead of just a fire, the vapour cloud needs to be contained in an enclosed area.

There is much more details to discuss on each of the topics above, but this post was intended to be a quick intro, so I think we stop here.

An LNG carrier transports LNG at -161 degrees celsius and atmospheric pressure. The boil-off gas is either used in the ships engines or re-liquefied and sent back to the tanks.

7 Comments , ,
  1. We would like to help you bring Lng to the world by using our specialy engineered designed plants. Lng is the fuel of the future now.

  2. Richard

    The LNG ship is sailing up the English Channel to deliver its cargo to a LNG terminal ( choose your terminal, there are several). There is sea fog, and a rather large private yacht is having difficulty ( the captain has had too many gin and tonics ?) (choose any reason that you wish) and gets his boat entangled in the rudder of the LNG ship. Forthwith the ship can not steer, and it gets stuck on a sand bank. ( You can choose any of several sandbanks), and the tide is ebbing, the ship sends out an SOS, from a position opposite the estuary of the River Thames, or in front of the French nuclear plant at Dungerness, or, (Choose any delicate point from several hundred along the coastlines of England, France, Belgium or Holland for example), as the tide ebbs the LNG ship starts to lean over to such an angle that the refrigeration motors can no longer function correctly, and the ship starts to ventilate the boiling LNG gas into the atmosphere. The ship itself is fully equipped with all the safety mesures to avoid sparks or ignition of the gas bleed-off. No harm can come, non of the crew smoke, special tropicalised tug boats (that produce no sparks) are sent to pull the boat off of the sandbank, but it was a spring tide and the ship lists even harder to starboard ( you can choose which way it leans over). After three days of pulling and pushing the ship is beyond the salvage angle, and starts slipping down the sandbank into the deeper waters of the rather restricted dregged shipping channel. The back-up refrigeration power supply is running dangerously low, so the crew have to be taken off. Slowly the ship sinks into the dregged channel. The boil-off LNG cloud starts to become visible in the atmosphere, rather like that seen before the enormous explosion on the pipeline in Belgium a few years ago, but the ship sinks into the cold waters of the channel, BUT THE LNG starts to approach -260° C
    And the volume changes by a factor of six hundred times the ship’s capacity ?
    As yet no big bang, we are lucky, BUT an enormous volume of water suddenly produces a local psunami (sorry I can’t spell that) and a tidal wave rushes up the Thames, (choose your river ) and the London flood barrier is destroyed just when it was being closed to save LONDON.
    And then disaster, a chap on the back of a Sea France ferry going from Calais to Dover lights up a Gauloise on the deck at the back of the boat, and BANG.
    The resulting fireball unfortunately kills all the people working at Dungerness nuclear power station, due to there being no oxygen left in the air. The nuclear plant closes down automatically, but then itself is hit by the tidal wave from the gas decompression. The electricity goes off in southern England, northern France & western Belgium. All gsms and telephone links go down. All the coastal holiday appartment buildings on the coast are on fire, because their windows broke in the gas decompression, and then caught fire when the curtains were hit by the passing fire-ball.
    END OF CHAPTER ONE.
    This could be made into a blockbuster film ?
    Thanks for letting me have expressed myself here,
    Yours sincerely, Richard.

    • Thanks for this entertaining read, Richard.

      The phenomenon you refer to is known as RPT – Rapid Phase Transition. It can occur, as you describe when droplets of LNG is engulfed in water, causing a quick temperature rise in the LNG and instantanous transfer from liquid to gas phase. But, to my knowledge, this phenomenon is only reproduced in experiments where LNG is in direct contact with water. For your story above, the insulation of the tanks will slow down the heat ingress from the water, and it is not a huge challenge to design pressure relief valves large enough to transport the vaporized gas out of the tanks and emitted into the atmosphere.

  3. Robin

    The scenario described in terms of initiating event is plausible, but the outcome described is not. Lars Petter provides some good reasons. Recent publications hace addressed underwater releases of LNG, as might occur if the hull is punctured and then the tanks through several barriers. These show the outcomes, while serious, are not catastrophic at any reasonable distance from the vessel (See textbook Woodward and Pitblado – LNG Risk Based Safety, Wiley Interscience, 2010).

    Have a look also at the current work Sandia is carrying out for Cascading failure as might result from terrorism attacks. http://www.narucmeetings.org/Presentations/Hightower%20NARUC%20LNG%20Cascading%20Damage%20Update%207-2010.pdf

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