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 Introduction to LNG

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Composition of Natural Gas and LNG

Natural gas is composed primarily of methane, 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 natural gas. The Figure 5 provides a typical natural gas composition.18 The liquefaction process requires the removal of some of the non-methane components such as water and carbon dioxide from the produced natural gas to prevent them from forming solids when the gas is cooled to about LNG temperature (-256o F). As a result, LNG is typically made up mostly of methane as shown in Figure 5 .

Figure 5 . Typical Natural Gas and LNG Composition

Examples of LNG composition are shown in Table 1.

Table 1 . LNG Composition with Geographic Variation

LNG COMPOSITION (Mole Percent)
Source
Methane
Ethane
Propane
Butane
Nitrogen
Alaska
99.72
0.06
0.0005
0.0005
0.20
Algeria
86.98
9.35
2.33
0.63
0.71
Baltimore Gas & Electric
93.32
4.65
0.84
0.18
1.01
New York City
98.00
1.40
0.40
0.10
0.10
Sand Diego Gas & Electric
92.00
6.00
1.00
-
1.00
Source: Liquid Methane Fuel Characterization and Safety Assessment Report. Cryogenic Fuels Inc. Report No. CFI-1600, Dec. 1991

LNG is odorless, colorless, non-corrosive, and non-toxic. However, as with any gaseous material besides air and oxygen, the natural gas vaporized from LNG can cause asphyxiation in an unventilated confinement.

Appendix 1 explains the differences between LNG and other products used in the industry such as Natural Gas Liquids (NGLs) , Compressed Natural Gas (CNG) , Liquefied Petroleum Gas (LPG) and Gas -to-Liquids (GTL).

18 Danesh, Ali: PVT and Phase Behavior of Petroleum Reservoir Fluids, Elsevier, 1998.