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Fundamentals of Combustion

What is combustion?

It 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.

Combustion Formula

CH4 + 2O2 + 8N2

CO2 + 2H2O + 8N2 + 1000 Btu Heat

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 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, 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 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, 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.

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.

Perfect, On-Ratio, Stoichiometric Combustion

  • All Fuel Combusted
  • Blue Near Burner Tile
  • Yellow Conical Flame Shape
  • Highest Flame Temperature
  • Minimum Exhaust Volume

Lean Combustion

  • Flue Products Oxidizing (Free O2)
  • Pale Blue Color
  • Shape – More Conical Flame
  • All Fuel Combusted
  • Flame Temperature Drops (Heating Excess Air)

Incomplete Combustion

  • Air Starved or Fuel Rich
  • CO and H2 Formed
  • Reducing Atmosphere
  • Predominantly Yellow Color
  • Shape Less Defined
  • Flame Temperature Drops

Results of Good Combustion Practice

  • Higher flame temperature
  • Greater heat transfer
  • Minimum exhaust volume
  • More available heat
  • Reduced fuel use