Liquefied Natural Gas’s Hazardous Cryogenic and Flammable Properties

Unlike all cryogenic commodities commonly transported in bulk by rail in the United States, with the exception of ethylene,45 LNG combines the hazards of a cryogen with the hazards of a flammable gas. Being flammable, LNG vapor may ignite when released to reach concentrations in air of 5 to 15 percent. In addition, LNG’s combustion in a pool fire will create high flame temperatures and high heat flux to surrounding materials. LNG’s heat flux is three to five times higher than that of other commonly transported hydrocarbons, including LPG; hence, its combustion will result in a larger region of thermal damage. LNG’s cryogenic temperatures can mean that the material’s inadvertent release from containment can be in the form of a very cold liquid, exposure to which can cause the embrittlement of materials. When LNG is released in sufficient volume, the liquid state may be maintained to form a vapor–liquid pool that can cause cryogenic burns and asphyxiation by people exposed. Because most cryogenic commodities (i.e., argon, nitrogen, and oxygen) have lower boiling points than LNG, they must be transported at even lower temperatures. As a result, these cryogens pose an elevated risk for cryogenic burns and material embrittlement if inadvertently released to cause exposure. An exception is ethylene, the only other cryogen that is also flammable. Its higher boiling point allows it to be transported at higher temperatures that pose lower risk of embrittlement to materials if released. © 2022 National Research Council. All rights reserved.