is the rapid oxidation of fuel resulting in the release of usable heat and the
production of a visible flame. Fuel, oxygen (air) and heat (temperature), as represented
in the combustion triangle, all must be present. Otherwise, burning will not start
or will not sustain itself after it starts. Take away any one of the three and
burning will stop.
Heat energy produced when burning a fuel gas is commonly expressed
in British thermal units (Btus). One Btu of heat will raise the temperature of
one pound of fresh water one degree Fahrenheit. Burning an ordinary wooden kitchen
match produces about 1 Btu of heat.
The heating value of a gas is the amount of heat released
when one cubic foot of the gas is completely burned. This heating value is expressed
in Btu per cubic foot of gas at standard pressure and temperature. The combustion
equation illustrates that an air/fuel ratio consisting of 10 cf of air and 1 cf
natural gas results in perfect combustion, and you obtain 1,000 Btus of heat.
CH4 + 2O2 + 8N2
CO2 + 2H2O + 8N2 + 1000 Btu
Two elements, carbon and hydrogen, are common to the fuels
used to produce heat. (That's the origin of the term hydrocarbon.) These elements,
when combined proportionally with oxygen and combusted, provide the usable heat
desired. The required oxygen is provided either in the form of room air or as
pure oxygen. Room air contains approximately 21% O2. The balance is N2 with small
amounts of water vapor, carbon dioxide, argon, hydrogen, and other elements.
Air is the usual source of oxygen for combustion and is a
critically important factor of a combustion system. All combustion systems are
designed for their air handling capabilities. When the constituents of a fuel
are known, the fuel's Btu capacity and the resulting volume of air to complete
combustion can be determined quickly. For general purposes, it's reasonably accurate
to assume that air is composed of 20% O2 and 80% N2.
When perfect combustion conditions exist (no excess air and
no excess fuel) the term stoichiometric combustion is used. To attain perfect
combustion (with air and natural gas), the fuel comprises 9.1% of the total input
Another thing to realize about the combustion equation is
that for each cubic foot of air input, 100 Btu of heat is liberated. This is valid
regardless of the fuel used (propane, oil, coal, etc.).
Also, this condition produces the hottest flame and the minimum
volume of exhaust.
Hydrocarbon fuels will burn continuously in self-sustained
combustion as long as the percentage of fuel in the air/fuel mixture falls within
The importance of flammability limits is illustrated when
an automobile engine floods. In this case, an excess of fuel produces an air/fuel
input mixture too rich to burn because the air/fuel ratio exceeds the upper limit
The percent of natural gas by volume
is: Lower Limit (Lean) - 4.3%
Perfect Combustion (Stoichiometric) - 9.1%
Upper Limit (Rich) - 15.0%
For natural gas which contains 95% methane, these limits are
approximately 4% for the lower, lean value and 15% for the upper, rich value.
These values are also known as the lower and upper explosive limits.
Perfect combustion for natural gas, with an input ratio containing
approximately 9% fuel by volume, is well within flammability limits.
Combustion occurs at about 1,200°F. A match, burner pilot
flame, or spark from an igniter can provide the initial heat that starts the chemical
reaction known as combustion.
Once ignition is attained, fuel-burning systems need to:
Mix and direct the air/fuel supply,
Provide for stable combustion within flammability limits,
Suitably remove the products of combustion from the process
Perfect combustion results from the input ratio that produces
the hottest flame and the minimum exhaust volume, or stoichiometric combustion
occurs when the air volume provided represents exactly 100% of the air (or oxygen)
required for combustion. When this condition exists, all fuel is consumed and
no trace of either combustible fuel or residual oxygen can be detected in the
exhaust flue gas.
Deviating from a perfect combustion input ratio impacts flame
color, flame geometry (or shape), flame temperature, exhaust or flue gas analysis,
and, therefore, efficient, economical and productive operation.