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Combustion Basics

What is Combustion:
The rapid oxidation of a fuel resulting in the release of usable heat and the production of a visible flame.
Combustion Equation:

CH4 + 2O2 + 8N2 =

CO2 + 2H2O + 8N2 + 1000 Btu Heat

The combustion 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.

Two elements, 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.

When perfect 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.

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, landfill, 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 flammability limits.

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 of flammability.

The % 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 & 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. 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.

Once ignition 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.

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.

Certain process 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.

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.