What is LNG?

March 23, 2014 By

March 23, 2014

by Mark Pilcher

LNG has been in the news a lot lately.  Political opponents have been waging a propaganda war over the economic benefits and the environmental impact of LNG exports from the U.S.  LNG has been described as a clean transportation fuel alternative to diesel, and as an environmental Trojan Horse.  More recently, LNG has been touted as a geopolitical tool in a revived Cold War.  LNG is not a household term, nor is it widely understood outside of the small international community of technical and commercial experts who produce, store, market and transport it around the world.  This is the first in a series of articles in which we will discuss LNG:

  • What is it?
  • How is it produced?
  • Where is it produced?
  • How is it stored and transported?
  • How and where is it consumed?
  • How is it sold?
  • What is the future of LNG as a transport fuel?

LNG is an acronym for Liquefied Natural Gas, and should not be confused with NGLs, or Natural Gas Liquids.  LNG is simply natural gas which has been cooled to −260°F (−162°C) to convert it to liquid for ease of storage and transport at close to atmospheric pressure (maximum transport pressure set at around 25 kPa (4 psi)).  The volume of LNG is about 1/600th the volume of natural gas at standard conditions.  Per unit volume, the energy of LNGis 2.4 times greater than that of compressed natural gas (CNG), but only 60 percent of that of diesel fuel.

Natural gas is a gas mixture consisting primarily of methane (CH4), but commonly including varying amounts of higher alkanes such as ethane (C2H6), propane (C3H8), butane (C4H10) and lesser percentages of carbon dioxide (CO2), nitrogen (N2) and hydrogen sulfide (H2S).  Natur

al gas can also contain small percentages of higher molecular weight hydrocarbons (e.g. pentanes), water vapor (H2O), helium (He), and trace amounts of mercury (Hg) and other impurities.  The composition of natural gas can vary dramatically by source, but typically methane comprises 80 – 90% of the natural gas mixture, with the percentages of ethane, propane, butane and other components in the single digits.

Before converting natural gas to LNG, the gas is treated to remove H2O, H2S and CO2and other components that would damage the liquefaction plant or freeze at the temperature of LNG.  Therefore, the composition of LNG is very similar to that of the natural gas from which it is produced, albeit with proportionally higher concentrations of the hydrocarbon components.  LNG, when converted back to the gas phase, has the same properties and uses as natural gas that has not been liquefied.  Most LNG is transported long distances across oceans in special purpose ships to large gas consuming markets where it is re-gasified and distributed in gas pipelines, although small scale LNG projects are constructed for a variety of reasons.  LNG use in transportation fuels is growing, a topic we will discuss in a future installment of this series on LNG.

In our next installment, we will describe how LNG is produced.