Page 49 - Synthetic Fuels Handbook

NATURAL GAS 37

2.6 PROPERTIES

The properties of unrefined natural gas are variable because the composition of natural gas
is never constant. Therefore, the properties and behavior of unrefined and refined natural
gas (Table 2.2), as determined by a series of standard test methods (Table 2.3), are best
understood by investigating the properties and behavior of the constituents.

TABLE 2.2 General Properties of Unrefined (Left Hand Data)
and Refined (Right Hand Data) Natural Gas

Relative molar mass 20–16
Carbon content (weight %) 73–75
Hydrogen content (weight %) 27–25
Oxygen content (weight %) 0.4–0
Hydrogen-to-hydrogen atomic ratio 3.5–4.0
Density relative to air @15°C 1.5–0.6
Boiling temperature (°C/1 atm) –162
Autoignition temperature (°C) 540–560
Octane number 120–130
Methane number 69–99
Vapor flammability limits (volume %) 5–15
Flammability limits 0.7–2.1
Lower heating/calorific value (Btu) 900
Methane concentration (volume %) 100–80
Ethane concentration (volume %) 5–0
Nitrogen concentration (volume %) 15–0
Carbon dioxide concentration (volume %) 5–0
Sulfur concentration (ppm, mass) 5–0
http://www.visionengineer.com/env/alt_ng_prop.php.

Thus, assuming that the natural gas has been cleaned (i.e., any constituents such as
carbon dioxide and hydrogen sulfide have been removed and the only constituents remain-
ing are hydrocarbons), the properties and behavior of natural gas becomes a study of the
properties and behavior of the relevant hydrocarbons (see for example, Speight, 2005a).
The composition of natural gas varies depending on the field, the formation, or the
reservoir from which it is extracted and that are an artifact of its formation. The different
hydrocarbons that form natural gas can be separated using their different physical proper-
ties as weight, boiling point, or vapor pressure. Depending on its content of higher molecu-
lar weight hydrocarbon components, natural gas can be considered as rich (5 or 6 gal or
more of recoverable hydrocarbon components per cubic feet) or lean (less than 1 gal of
recoverable hydrocarbon components per cubic feet).

2.6.1 Density
Density is a physical property of matter, is a measure of the relative heaviness of hydrocar-
bons and other chemicals at a constant volume, and each constituents of natural gas has a
unique density associated with it. Another term, specific gravity is commonly used is rela-
tion to the properties of hydrocarbons. The specific gravity of a substance is a comparison
of its density to that of water. For most chemical compounds (i.e., those that are solid or
liquid), the density is measured relative to water (1.00). For gases, the density is more
likely to be compared to the density of air (also given the number 1.00 but this is arbitrary