Home - Background: Fuel Types - LNG
Liquefied natural gas for vehicles comes from the same sources as compressed natural gas (CNG), or for that matter as the gas that cooks your dinner. Unlike liquefied petroleum gas (LPG), which is changed from a vapor to a liquid at room temperature by application of pressure, LNG has to be cooled to very low temperatures in order to cause it to liquefy; this makes it hard (though not impossible) to transport via tanker, and it is usually liquefied at the dispensing station.
As with CNG, LNG benefits from decades of infrastructure development because of heavy domestic, industrial, and utility use of natural gas.
LNG is somewhat behind CNG in retail availability, because of the added complexity of a cryogenic (ultra-cold) liquefying station compared to a compressor station. However, there are projects being undertaken (like the Clean Cities Corridors program) to construct enough refueling stations along major trucking highways to allow LNG-powered long-haul trucks to replace diesels along some routes. Also, individual local delivery fleets are building their own stations for central refueling so that they don't have to worry about publicly-available facilities.
LNG has all the emissions advantages I bragged about for CNG. In addition, the liquefaction process amounts to a distillation, so the fuel is essentially pure methane (CNG can contain up to 12% of heavier molecules in California, like ethane and propane, and even more elsewhere), which prevents variations in fuel quality that I'm told can occur for CNG (or gasoline, for that matter). Also, LNG is a somewhat less bulky and heavy way to store natural gas than as CNG in high-pressure tanks.
Though LNG tanks are less bulky and heavy than CNG tanks, they are still more so than tanks for liquid fuels like gasoline, diesel, or alcohols. They are also more complex and expensive because they have to insulate the fuel very well in order to prevent it from warming up and boiling off too fast. Even with modern, rocket-science (literally!) insulation materials and techniques, a LNG tank will begin venting fuel if left to sit for several days, so the fuel is best used in high-duty-cycle applications like delivery trucks.
new 11 July 1998, revised 15 September 1998