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 Introduction to LNG

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Appendix 1: Other Fuel Terminologies

LNG is often confused with other terminologies such as Natural Gas Liquids (NGLs), Compressed Natural Gas (CNG), Liquefied Petroleum Gas (LPG), Gas-to-Liquids (GTL).

LNG is made up of mostly methane as shown in the figure below. The liquefaction process requires the removal of the non-methane components like carbon dioxide, water, butane, pentane and heavier components from the produced natural gas. LNG is odorless, colorless, non-corrosive, and non-toxic. When vaporized it burns only in concentrations of 5% to 15% when mixed with air.

Natural gas liquids (NGLs) are made up mostly of molecules that are heavier than methane. These molecules liquefy more readily than methane. NGLs are the hydrocarbon molecules that begin with ethane and increase in size as additional carbon atoms are added.
In the U.S. NGLs are typically extracted during the processing of natural gas for industrial uses and in order for the gas to meet the pipeline specification. LNG shipped to the U.S. generally must meet pipeline heating value specifications, that is, it must contain only moderate quantities of NGLs. If LNG is shipped with NGLs, the NGLs must be removed upon receipt or blended with lean gas or nitrogen before the natural gas can enter the U.S. pipeline system. Few locations (only the Lake Charles, Louisiana receiving terminal in the U.S., for instance) are near processing facilities that can take LNG cargos that are “rich” with NGLs.

However, the LNG heat content specification in Japan, Korea and other Asian countries is higher than in the U.S. or Europe. For these countries, NGLs are left in the LNG and, in some circumstances, LPG is added to the vaporized LNG at the receiving terminal to increase the heat content.

LNG is not the same as Liquefied Petroleum Gas (LPG). LPG is often incorrectly called propane. In fact, LPG is predominantly a mixture of propane and butane in a liquid state at room temperatures when under moderate pressures of less than 200 psig (pounds per square inch gauge (psig) is a common measure of pressure). The common interchanging of the terms LPG and propane is explained by the fact that in the U.S. and Canada LPG consists primarily of propane. In many European countries, however, the propane content in LPG can be lower than 50 per cent.

In Europe, LPG has been used as fuel in light duty vehicles for many years. Many petrol or gasoline stations have LPG pumps as well as pumps to distribute gasoline.

LPG is highly flammable and must therefore be stored away from sources of ignition and in a well-ventilated area, so that any leak can disperse safely. A special chemical, mercaptan, is added to give LPG its distinctive, unpleasant smell so that a leak can be detected. The concentration of the chemical is such that an LPG leak can be smelled when the concentration is well below the lower limit of flammability. Worldwide, LPG is used heavily for domestic purposes such as cooking and heating water.

LNG is not the same as compressed natural gas (CNG). CNG is natural gas that is pressurized and stored in welding bottle-like tanks at pressures up to 3,600 psig. Typically, CNG is the same composition as pipeline quality natural gas, i.e., the gas has been dehydrated (water removed) and all other elements reduced

to traces so that corrosion is prevented. CNG is often used as a vehicle transportation fuel and is delivered to an engine as low-pressure vapor (up to 300 psig). CNG is often misrepresented as the only form of natural gas that can be used as vehicle fuel. However, LPG and LNG are also common transport fuels.

LNG is also not synonymous with Gas-to-Liquids (GTL). GTL refers to the conversion of natural gas to products like methanol, dimethyl ether (DME), middle distillates (diesel and jet fuel), specialty chemicals and waxes. While the technology for producing each of these distinct products was developed years ago,

only methanol is currently in widespread commercial production. DME and specialty lubricants and waxes from natural gas are in limited commercial production. Middle distillate can be directly substituted for diesel fuel in existing compression ignition engines. The advantage of GTL diesel is that it contains almost no sulfur or aromatics and is well suited to meet current and proposed cleaner fuel requirements of developed economies.