Liquefied Natural Gas (LNG)

A natural gas that can be converted temporarily to liquid form for easiness of transport and storage is known as Liquefied Natural Gas (LNG). LNG can be predominantly methane (CH₄). LNG has about 1/600^th the volume of natural gas in the gaseous state. It is colorless, odourless, non-toxic and non-corrosive. LNG is highly inflammable. Asphyxia ids one of the hazards caused by LNG. In the liquefaction process of LNG several components like dust, acid gases, and helium, water and heavy hydrocarbons are removed. By cooling the gas to -162 degree Celsius it is condensed into a liquid close to atmospheric pressure. For the easiness in transporting the gas to places where pipelines do not exist, reduction in volume is the option used. Moving natural gas by pipelines is practically impossible and it is not economically possible. To transport LNG cryogenic sea vessels or cryogenic road tankers are used.

Basic facts about LNG

Normally LNG is used for transporting natural gas to markets. In these markets the gas will be regasified and distributed through pipelines. LNG is used in natural gas vehicles. It is more common nowadays to design vehicles to use compressed natural gas. It is not much used for commercial applications. The reason is its production cost is very high and for its storage expensive cryogenic tanks are required. The density of LNG is roughly 0.41 kg/L to 0.5 kg/L. The density of LNG changes according to temperature, pressure and composition. The heat value of the gas is based on the gas source used and the process used for liquefying. The higher heating value of LNG is 24MJ/L. Its lower heating value is estimated to be 21 MJ/l. the energy density of LNG is estimates as  2.4 times greater than that of CNG. Its energy density can be compared to that of propane and butane.

Production of LNG

The first step involved in the production of LNG is the supply of natural gas into LNG plant. The gas is then treated to remove water, hydrogen sulphide and carbon dioxide.  The gas is treated under low temperature. LNG contains more than 90 % of methane and small amounts of ethane, propane, butane and some heavier alkanes. When cold LNG mixes with water a risk of Rapid Phase Transition can happen. An LNG plant with one or more LNG trains is the most important infrastructure necessary for LNG production. Each LNG train used should be an independent unit for gas liquefaction.

Questions:

• Define LNG?
• What are the characteristics of LNG?