The rapid oxidation of a fuel resulting in the release of usable heat and the
production of a visible flame.
+ 2O2 + 8N2 =
+ 2H2O + 8N2 + 1000 Btu Heat
equation illustrates that an air/fuel ratio consisting of 10 cf air and 1 cf natural
gas results in perfect combustion, and you obtain 1,000 Btus of heat.
carbon and hydrogen (that’s the origin of the term hydrocarbon), are common
to the fuels used in industrial processes . 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.
combustion conditions exist (no excess air and no excess fuel) the term stoichiometric
combustion is used. Also note that to attain perfect combustion (with air &
natural gas) fuel comprises 9.1% of the total input volume.
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, landfill, etc.).
condition produces the hottest flame and the minimum volume of exhaust.
fuels will burn continuously in self-sustained combustion as long as the percentage
of fuel in the air/fuel mixture falls within flammability limits.
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 of flammability.
The % of Natural
Gas by Volume is:
Limit (Lean) - 4.3 %
Combustion (Stoichiometric) - 9.1%
Limit (Rich) - 15.0 %
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 & upper explosive limits.
Perfect combustion for natural gas, with an input ratio containing approximately
9% fuel by volume, is well within flammability limits.
occurs at about 1,200°F. The initial heat that starts the chemical reaction
known as combustion can be provided by a match, burner pilot flame, or a spark
from an ignition transformer.
is attained, fuel burning systems need to:
- Mix and direct
the air/fuel supply,
- Provide for
stable combustion within flammability limits and
- Suitably remove
the products of combustion from the process involved.
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.
applications exist where either excess fuel is required (e.g., to provide a protective
atmosphere) or excess air is needed (e.g., to avoid discoloration when firing
sensitive brick or refractory or to promote temperature uniformity at low operating
temperatures). For many applications, burners set to achieve 10 to 15% excess
air (2 or 3% excess O2) are as close to perfect as is practical. In either case,
the air/fuel ratio should be maintained as close to perfect combustion as possible.
If neither excess fuel nor excess air is a process requirement, then a setting
as close to the stoichiometric ratio as possible should be the goal.
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.