Natural gas is composed almost entirely of methane although it does contain small amounts of other gases--ethane, propane, butane, and pentane. Methane is composed of a molecule of one carbon atom and four hydrogen atoms. Natural gas is colourless, non-toxic, invisible, and odourless, although an odourant is added to natural gas. Known as mercaptan, this odourant is an important safety measure because it provides a distinct smell (much like the smell of rotten eggs) in the event of a gas leak.
Natural gas is lighter than air and rapidly dissipates into the air when it is released. When natural gas burns, a high-temperature blue flame is produced and complete combustion takes place producing only water vapour and carbon dioxide. Natural gas has a heating value of about 1000 BTUs per cubic foot. However, when it burns improperly, it can produce carbon monoxide – a deadly, poisonous gas.
Natural gas has a flammability range of approximately 5 to 15 percent. That means that any mixture containing less than 5 percent or greater than 15 percent natural gas to air would not support combustion. Natural gas, when mixed with air and exposed to an ignition source, is combustible.
Propane gas is similar to natural gas in many ways and is also used as a fuel. The most significant difference between propane and natural gas is that propane gas is heavier than air. If propane gas leaks, it does not tend to vent safely into the air. Rather, it settles in low areas, both inside a structure or outside. Liquid propane is stored in tanks and vapourizes when it is released from the tank because propane vaporizes at any temperature above -44°F. The flammability range of propane is approximately 2 – 10 percent propane in air.