Page 52 - Synthetic Fuels Handbook

40 CHAPTER TWO

Relative to air, methane is less dense (Table 2.4) but the other hydrocarbon constituents
of unrefined natural gas (i.e., ethane, propane, butane, etc.) are denser than air (Fig. 2.4).
Therefore, should a natural gas leak occur in field operations, especially where the natural
gas contains constituents other than methane, only methane dissipates readily into the air
and the higher molecular weight hydrocarbon constituents pose a considerable risk if the
ambient conditions are such that they accumulate at ground level.

TABLE 2.4 Boiling Point and Density of Methane Relative to
Air and Water

Boiling point (760 mm Hg) −161.5°C (−258.7°F)
Gas specific gravity 0.55–0.64 (air = 1.00)
Specific gravity of liquefied natural gas 0.42–0.46 (water = 1.00)
Gas density (varies slightly) 0.0438 lb/scf

2.6.2 Heat of Combustion (Energy Content)

All of the hydrocarbon constituents of natural gas are combustible, but nonflammable non-
hydrocarbon components (carbon dioxide, hydrogen sulfide, nitrogen, and helium) detract
slightly from the heating value of natural gas.
The heat of combustion (energy content) of natural gas is the amount of energy obtained
from the burning of a volume of natural gas and is measured in British thermal units (Btu).
The value of natural gas is calculated by its Btu content. One Btu is the quantity of heat
required to raise the temperature of one pound of water of 1°F at atmospheric pressure. A
cubic foot of natural gas has an energy content of approximately 1031 Btu, but the range of
values is between 500 and 1500 Btu depending upon the composition of the gas.
Thus, the energy content of natural gas is variable because natural gas has variations in the
amount and types of energy gases (methane, ethane, propane, butane) it contains: the more
noncombustible gases in the natural gas, the lower the energy (Btu). In addition, the volume
mass of energy gases which are present in a natural gas accumulation also influences the Btu
value of natural gas. The more carbon atoms in a hydrocarbon gas, the higher its Btu value.
It is necessary to conduct the Btu analysis of natural gas at each stage of the supply chain.
Gas chromatographic process analyzers are used in order to conduct fractional analysis of the
natural gas streams, separating natural gas into identifiable components. The components and
their concentrations are converted into a gross heating value in Btu-cubic foot.
In the United States, at retail, natural gas is often sold in units of therms (th) (1 therm =
100,000 Btu). Wholesale transactions are generally done in decatherms (Dth), or in thou-
sand decatherms (MDth), or in million decatherms (MMDth). A million decatherms is
roughly a billion cubic feet of natural gas.
The gross heats of combustion of crude oil and its products are given with fair accuracy
by the equation:
Q = 12,400 − 2100d 2
where d is the 60/60°F specific gravity. Deviation from the formula is generally less than
1 percent.

2.6.3 Volatility, Flammability, and Explosive Properties

The boiling point (boiling temperature) of a substance is the temperature at which the vapor
pressure of the substance is equal to atmospheric pressure.