What is LNG?
Liquefied natural gas (LNG) is the liquid form of the same natural gas millions of people use in their homes for heating, cooking and for generating electricity from gas-fired power stations. Natural gas is primarily methane, nature's simplest and most abundant hydrocarbon fuel.
Why is Natural Gas Liquified?
LNG is about 1/614th the volume of natural gas at standard temperature and pressure (STP), making it much more cost-efficient to transport over long distances where pipelines do not exist. Where moving natural gas by pipelines is not possible or economical, it can be transported by LNG vessels, where the most common tank types are membrane(prismatic), Moss Rosenberg (spheres) or Self-Supporting Prismathic Type.
When natural gas is extracted from underground reservoirs, it often contains other materials and components, such as those listed below, that must be removed before the gas can be liquefied and used by the consumer.
- Sulfur, carbon dioxide and mercury, which are corrosive to equipment.
- Water, which would naturally freeze and cause equipment blockage if not removed.
- Heavier hydrocarbons and carbon dioxide which can freeze (like water).
To convert the natural gas to a liquid, it is cooled to the temperature at which the main component, methane, will form a liquid (approximately -160°C). The liquefaction process works much like a typical refrigerator: cold liquid refrigerants such as propane and ethlyene are reduced in pressure and evaporated as heat is exchanged with the natural gas steam. As this happens, it cools the natural gas to the point where it turns into a liquid. Once the gas has been liquefied, it is sent to storage to await shipping.
The insulation, as efficient as it is, will not keep the temperature of LNG cold by itself. LNG is stored as a "boiling cryogen," that is, it is a very cold liquid at its boiling point for the pressure it is being stored.
Stored LNG is analogous to boiling water, only 260°C colder. The temperature of boiling water (100 °C) does not change, even with increased heat, as it is cooled by evaporation (steam generation).
In much the same way, LNG will stay at near constant temperature if kept at constant pressure. This phenomenon is called "autorefrigeration". As long as the LNG vapor boil off is allowed to leave the tank, the temperature will remain constant. If the vapor is not drawn off, then the pressure and temperature inside the vessel will rise. However, even at 1000 psig (7 MPa), the LNG temperature will still be only about -130 °C.
LNG General Facts:
- Colorless, odorless and non-toxic.
- Less than half the density of water.
- Liquefies at -160°C.
- No contaminants.
- Transported and handled similar to other liquid gases such as oxygen and nitrogen.
- For statistics on LNG production and demand, please visit the statistics section.