Liquefied natural gas (LNG)

Liquefied natural gas, or LNG, is natural gas in its liquid form. When natural gas is cooled to minus 259 degrees Fahrenheit (-161 degrees Celsius), it becomes a clear, colorless, odorless liquid. LNG is neither corrosive nor toxic. Natural gas is primarily methane, with low concentrations of other hydrocarbons, water, carbon dioxide, nitrogen, oxygen and some sulfur compounds. During the process known as liquefaction, natural gas is cooled below its boiling point, removing most of these compounds. The remaining natural gas is primarily methane with only small amounts of other hydrocarbons. LNG weighs less than half the weight of water so it will float if spilled on water.

The most common use of LNG in the U.S. is for “peakshaving,” a way local electric power and gas companies store gas for peak demand that cannot be met via their typical pipeline source. This can occur during the winter heating season when cold fronts move through or when more natural gas is needed to generate electric power for air conditioning in the summer months. The utility companies liquefy pipeline gas when it is abundant and available at off- peak prices, or they purchase LNG from import terminals supplied from overseas liquefaction facilities. When gas demand increases, the stored LNG is converted from its liquefied state back to its gaseous state, to supplement the utility’s pipeline supplies. LNG is also currently being used as an alternative transportation fuel in public transit and in vehicle fleets such as those operated by many local natural gas utilities companies for maintenance and emergencies.

LNG is transported in double-hulled ships specifically designed to handle the low temperature of LNG. These carriers are insulated to limit the amount of LNG that boils off or evaporates. This boil off gas is sometimes used to supplement fuel for the carriers. LNG carriers are up to 1000 feet long, and require a minimum water depth of 40 feet when fully loaded. In 2007 there were about 136 ships which transport more than 120 million metric tons of LNG every year.

When LNG is received at most terminals, it is transferred to insulated storage tanks that are built to specifically hold LNG. These tanks can be found above or below ground and keep the liquid at a low temperature to minimize the amount of evaporation. If LNG vapors are not released, the pressure and temperature within the tank will continue to rise. LNG is characterized as a cryogen, a liquefied gas kept in its liquid state at very low temperatures. The temperature within the tank will remain constant if the pressure is kept constant by allowing the boil off gas to escape from the tank. This is known as auto-refrigeration. The boil-off gas is collected and used as a fuel source in the facility or on the tanker transporting it. When natural gas is needed, the LNG is warmed to a point where it converts back to its gaseous state. This is accomplished using a regasification process involving heat exchangers.

When cold LNG comes in contact with warmer air, it becomes a visible vapor cloud. As it continues to get warmer, the vapor cloud becomes lighter than air and rises. When LNG vapor mixes with air it is only flammable if it's within 5%-15% natural gas in air. If it's less than five percent natural gas in air, there is not enough natural gas in the air to burn. If it's more than 15 percent natural gas in air, there is too much gas in the air and not enough oxygen for it to burn.

LNG hazards result from three of its properties: cryogenic temperatures, dispersion characteristics, and flammability characteristics. The extremely cold LNG can directly cause injury or damage. A vapor cloud, formed by an LNG spill, could drift downwind into populated areas. It can ignite if the concentration of natural gas is between five and 15 percent in air and it encounters an ignition source. An LNG fire gives off a tremendous amount of heat.

Serious accidents involving or related to LNG include:

  • October 1944, Cleveland, Ohio: At a Cleveland peak-shaving plant a tank failed and spilled its contents into the street and storm sewer system. The resulting explosion and fire killed 131 people. The tank was built with a steel alloy that had low-nickel content, which made the alloy brittle when exposed to the extreme cold of LNG.
  • 1977, Azrew, Algeria: Aluminum valve failure on contact with temperatures at a LNG liquefaction facility due to a wrong aluminum alloy replacement valve. LNG released, but no ignition. One worker frozen to death.
  • 1979, Cove Point Maryland: An explosion occurred within an electrical substation at an LNG terminal. LNG leaked through LNG pump electrical penetration seal, vaporized, passed through 200 feet of underground electrical conduit, and entered the substation. Since natural gas was never expected in this building, there were no gas detectors installed in the building. The normal arcing contacts of a circuit breaker ignited the natural gas-air mixture, resulting in an explosion that killed one person.

No death or serious accident involving an LNG facility has occurred in the United States since the accident in 1979 at the Cove Point receiving terminal. The global LNG industry has a very good safety record. From 1959 to 2007, seaborne LNG transport tankers (including their loading terminals) did not have had a major accident, although there have been several minor incidents with LNG carriers.