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Liquefied natural gas (LNG) is created by removing contaminants and refrigerating natural gas to -260°F, reducing its volume by 1/600. LNG is transported in cryogenic vessels and stored in purpose-built terminals. Safety concerns include flammability and environmental impact.
Liquefied natural gas (LNG) is natural gas that has been converted into a liquid. This process is done because the substance is more easily transported or stored in liquid form than in gaseous form. Natural gas is converted to liquefied natural gas in a process that includes contaminant removal followed by refrigeration. The liquefaction process is carried out in a plant that contains process machinery called an LNG train. A single plant usually contains several LNG trains, each capable of carrying out all the phases related to the conversion of natural gas into liquid.
Natural gas is composed primarily of methane, but also contains small traces of carbon dioxide, water, mercury and hydrogen sulfide, as well as dust and other organic compounds. To convert the gas into a liquid, all of these contaminants must be removed, as they complicate the liquefaction process.
After the contaminants have been removed, the gas is gradually refrigerated to convert it into a liquid. At -260 degrees Fahrenheit (-126.6 degrees Celsius), the gas condenses into liquefied natural gas. At this temperature the liquid condenses to about 1/600 of the volume of the gas. This significant reduction in volume means that it is much more cost effective to transport the gas when it is in liquid form. This is especially true in places where there are no pipeline networks.
Liquefied natural gas is transported in cryogenic tankers or cryogenic seagoing vessels. The term cryogenic refers to the fact that these vessels are equipped to maintain the temperature which keeps the gas liquid. Vessels carrying the liquefied gas are built with double hulls to reduce the risk of damage to the hull which could lead to leaks. Vessels dock at LNG terminals, which have been purpose-built to cope with vessels carrying liquefied natural gas. There are terminals in Africa, Asia, North and South America, Australia and a number of European countries.
There are several safety and environmental issues associated with the conversion, storage and transportation of natural gas in liquid form. One of the most pressing is that large amounts of carbon dioxide are released when natural gas is extracted and when it is converted into liquid. LNG plants also release nitrogen oxide and particulate matter, which aggravate the symptoms of respiratory diseases such as asthma and bronchitis.
Another problem is that natural gas is flammable, even in liquid form. The liquid itself cannot burn, but if the liquid begins to vaporize and mix with air, the resulting gas is flammable. While there has never been a major LNG-related accident aboard a carrier vessel, there have been a number of accidents at LNG facilities, including in the United States and Algeria. Many of these accidents occur due to the explosion of gas or liquid gas that evaporates during the conversion process.
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