Page 35 - Handbook of Natural Gas Transmission and Processing Principles and Practices

1.6. Natural Gas Properties
1.6.1. Chemical and Physical Properties

Natural gas is colorless, odorless, tasteless, shapeless, and lighter than air (see Table 1.1). The
natural gas after appropriate treatment for acid gas reduction, and moisture and hydrocarbon dew
point adjustment, would then be sold within prescribed limits of pressure, heating value, and
possibly Wobbe Index (often referred to as the Wobbe Number).
One of the principal uses of natural gas is as a fuel, and consequently, pipeline gas is normally
bought and sold on the basis of its heating value that can be produced by burning the gas. The
heating value of natural gas is variable and depends on its accumulations which are influenced by
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the amount and types of gases they contain. The gas industry always uses the gross heating value
(frequently called higher heating value) in custody transfer. Obviously, the numerical difference
between the two net and gross heating values is the heat of condensation of water at the specified
conditions. Heating values for custody transfer are determined either by direct measurement, in
which calorimetry is used, or by computation of the value on the basis of gas analysis (Gas
Processors Association, 1996). The heating value of natural gas is measured in British thermal unit
(Btu). A British thermal unit is the energy required to raise the temperature of 1 pound of water by
1°F. For larger industrial customers, the abbreviations MBtu (1000 Btu) or MMBtu (million Btu) are
more commonly used. Since meters measure volume and not energy content, a conversion factor of
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1000 Btu/ft is commonly used by gas companies to convert the volume of gas used to its heat
equivalent, and thus calculate the actual energy use.
The Wobbe Index (defined as the gross heating value of the gas divided by the square root of the
specific gravity) gives a measure of the heat input to an appliance through a given aperture at a
given gas pressure. Using this as a vertical coordinate and the flame speed factor as the horizontal
coordinate, a combustion diagram can be constructed for an appliance, or a whole range of
appliances, with the aid of appropriate test gases. This diagram shows the area within which
variations in the Wobbe Index and flame speed factor of gases may occur for the given range of
appliances without resulting in either incomplete combustion, flame lift, or the lighting back of
preaerated flames. This method of prediction of combustion characteristics is not sufficiently
accurate to eliminate entirely the need for the practical testing of new gases.
Since natural gas as delivered to pipelines has practically no odor, the addition of an odorant is
required by most regulations in order that the presence of the gas can be detected readily in case of
accidents and leaks. This odorization is provided by the addition of trace amounts of some organic
sulfur compounds to the gas before it reaches the consumer. The sulfur compound, a chemical
odorant (a mercaptan also called a thiol with the general formula R–SH and the odor of rotten eggs)
is added to natural gas so that it can be smelled if there is a gas leak. The standard requirement is
that a user will be able to detect the presence of the gas by odor when the concentration reaches 1%
of gas in air. Since the lower limit of flammability of natural gas is approximately 5%, this
requirement is equivalent to one-fifth the lower limit of flammability. The combustion of these trace
amounts of odorant does not create any serious problems of sulfur content or toxicity.

Table 1.1

Properties of Natural Gas

Properties Value
Relative molar mass 17–20
Carbon content, weight% 73.3
Hydrogen content, weight% 23.9
Oxygen content, weight% 0.4
Hydrogen/carbon atomic ratio 3.0–4.0
Relative density, 15°C 0.72–0.81
Boiling point, °C −162
Autoignition temperature, °C 540–560
Octane number 120–130
Methane number 69–99
Stoichiometric air/fuel ratio, weight 17.2
Vapor flammability limits, volume % 5–15
Flammability limits 0.7–2.1

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