Combustion Basics

Chemistry

Combustion is the rapid combination of oxygen with a fuel, such as natural gas, resulting in the release of heat. Most fuels contain carbon and hydrogen, and the oxygen usually comes from air. Combustion generally consists of the following overall reactions:

Stoichiometric or perfect combustion is obtained by mixing and burning exactly the correct proportions of fuel and oxygen so that no oxygen remains at the end of the reaction. If too much oxygen is supplied, the mixture is lean and the reaction is oxidizing. This results in a flame that is relatively shorter.

If too much fuel is supplied, the mixture is rich and the reaction is reducing. This typically results in a flame that is relatively longer and sometimes smoky. Most industrial burners are supplied with some excess air to mitigate the formation of unburned hydrocarbons, carbon monoxide and particulate matter.


Air Requirements for Perfect Combustion

The following table lists the proper proportions for perfect combustion of some gaseous fuels:

The products of full combustion for natural gas primarily consist of carbon dioxide, water vapor, nitrogen and excess oxygen. The oxygen supply for combustion usually comes from air. Because air contains a large amount of nitrogen, the required volume of air is much larger than the required volume of oxygen. The nitrogen absorbs some of the heat released in the reaction resulting in a much lower flame temperature compared to a reaction in pure oxygen.

Primary air is the air that is mixed with the fuel at or in the burner. Secondary air is usually the air that is brought in around the burner. Tertiary air is usually the air that is introduced downstream of the secondary air or through other openings in the combustion furnace.

Good combustion requires:

  • the proper proportioning of fuel and air
  • thorough mixing of the fuel and air
  • initial and sustained ignition of the mixture

 

Flame Temperature

The following graph illustrates how the theoretical flame temperature varies with the air/gas ratio:


Click on image for a larger view
click on image for a larger view

 

Heat Available from Natural Gas

When a perfect mixture of a fuel and air, originally at a specified reference temperature (usually 60°F) is ignited and then cooled to the reference temperature, the total heat released is termed the higher or gross heating value. The lower or net heating value is the gross heating value minus the heat released by the condensation of the water vapor in the combustion products.

For natural gas, which is primarily composed of methane, the higher heating value is typically in the range of 1000 to 1050 Btu/ standard cubic foot (scf). The chemical composition of natural gas varies slightly depending upon the location in North America. The exact value for your fuel can be obtained from your local natural gas distribution utility.

Perfect combustion of natural gas requires approximately 9.5 scf of air per scf of natural gas and results in approximately 10.5 scf of products of combustion. The values vary somewhat depending upon the natural gas composition.

 

For Further Information

  • “North American Combustion Handbook – Volume I ”; North American Mfg. Co., 3rd edition; 1986.
  • “Combustion Technology Manual”; Industrial Heating Equipment Association; 5th edition; 1994.

 

Source; Sunil Kumar, Last Updated 9-2002