Liquefied natural gas - Wikipedia, the free encyclopedia
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pipelines, LNG is transported using both road/rail truck and ship. LNG will be sometimes taken to cryogenic temperatures to reduce the mass. Recently ship-to-ship transfer (STS) transfers have been carried out by Exmar the Belgian gas tanker owner in the Gulf Of Mexico which involved the transfer of LNG from the LNG regasification vessel (LNGRV) to a conventional LNG carrier. Prior to this commercial exercise LNG had only ever been transferred between ships on a handful of occasions as a necessity following an incident.
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 470 °F (260 °C) colder. The temperature of boiling water (212 °F or 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 steam (LNG vapor boil off) is allowed to leave the tea kettle (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 −200 °F (−130 °C).
LNG, LPG, and CNG
Liquefied natural gas (LNG)
When natural gas is cooled to a temperature of approximately −260 °F (−160 °C) at atmospheric pressure it condenses to a liquid called liquefied natural gas (LNG). One volume of this liquid takes up about 1/600th the volume of natural gas at a stove burner tip. LNG is only about 45% the density of water. LNG is odorless, colorless, non-corrosive, and non-toxic. When vaporized it burns only in concentrations of 5% to 15% when mixed with air. Neither LNG, nor its vapor, can explode in an unconfined environment.
Natural gas is composed primarily of methane (typically, at least 90%), but may also contain ethane, propane and heavier hydrocarbons. Small quantities of nitrogen, oxygen, carbon dioxide, sulfur compounds, and water may also be found in "pipeline" natural gas. The liquefaction process removes the oxygen, carbon dioxide, sulfur compounds, and water. The process can also be designed to purify the LNG to almost 100% methane.
Compressed natural gas (CNG)
Compressed natural gas (CNG) is natural gas pressurized and stored in welding bottle-like tanks at pressures up to 3,600 psig (25 MPa). Typically, it is same composition of the local "pipeline" gas, with some of the water removed. CNG and LNG are both delivered to gas engines as low pressure vapor (ozf/in² to 300 psig, up to 2.1 MPa). CNG is often misrepresented as the only form natural gas can be used as vehicle fuel. LNG can be used to make CNG. This process requires much less capital intensive equipment and about 15% of the operating and maintenance costs.
Liquid petroleum gas (LPG)
Liquid petroleum gas (LPG, and sometimes called propane) is often confused with LNG and vice versa. They are not the same and the differences are significant. Varieties of LPG bought and sold include mixes that are primarily propane, mixes that are primarily butane, and mixes including propane, propylene, n-butane, butylene and iso-butane. Depending on the season—in winter more propane, in summer more butane. Vapor pressures, at 30 °C, are for commercial propane in the range 10-12 barg (1 to 1.2 MPa), for commercial butane, 2-4 barg (0.2 to 0.4 MPa). In some countries LPG is composed primarily of propane (upwards to 95%) and smaller quantities of butane. The vapor pressure of commercial butane is generally too low to release it from the top vapor space. Pumps and (hot water, steam, electricity or direct-fired) vaporizers are frequently used. An alternative to using neat butane vapor which overcomes the need for pipework heating, is to use a gas-air mixture (well outside flammability limits). Air depresses the vapor dew-point temperature. Another advantage is that the mixture can be made to "simulate" natural gas or town gas to produce the same heat release through a burner under equal supply pressures, characterized by a term known as Wobbe number or Wobbe index.
LPG compared to natural gas has a significantly higher heating value and requires a different air-to-gas mixture (propane: 24:1, butane: 30:1) for good combustion.
LPG can be stored as a liquid in tanks by applying pressure alone. While the distribution of LNG requires heavy infrastructure investments (pipelines, etc.), LPG is portable. This fact makes LPG very interesting for developing countries and rural areas. LPG (sometimes called autogas) has also been used as fuel in light duty vehicles for many years. An increasing number of petrol stations around the world offers LPG pumps as well. A final example that should not be forgotten
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