Combustion Technology (Pty) Ltd
|Combustion||Perfect Combustion||Incomplete Combustion|
|Efficiency Types||Testing||O2 versus CO2|
|Three T's of Combustion||NOx||Draft|
|Flue Gas Test Procedures||Acceptable Test Results||Accurate Testing|
|Savings Potential||Training Centre|
Combustion refers to the rapid oxidation of fuel accompanied by
the production of heat, or heat and light. Complete
combustion of a fuel is possible only in the presence of an adequate supply of oxygen.
1,600 to 2,000 cubic feet of air is required to burn one gallon of #2 fuel oil at 80%
efficiency (at sea level). About 15 cubic
feet of air is required to burn one cubic foot of natural gas at 75% efficiency (at sea
Oxygen (O2) is one of the
most common elements on earth making up 20.9% of our air. Rapid fuel oxidation results in large amounts of heat. Solid or liquid fuels must be changed to a gas
before they will burn. Usually heat is
required to change liquids or solids into gases. Fuel
gases will burn in their normal state if enough air is present.
Most of the 79% of air (that is not
oxygen) is nitrogen, with traces of other elements. Nitrogen
is considered to be a temperature reducing dilutant that must be present to obtain the
oxygen required for combustion.
Nitrogen reduces combustion efficiency
by absorbing heat from the combustion of fuels and diluting the flue gases. This reduces the heat available for transfer
through the heat exchange surfaces. It also
increases the volume of combustion by-products, which then have to travel through the heat
exchanger and up the stack faster to allow the introduction of additional fuel air
This nitrogen also can combine with
oxygen (particularly at high flame temperatures) to produce oxides of nitrogen (NOx),which
are toxic pollutants - more on this later.
Air for combustion is divided into four types depending upon its role and the design of the particular burner. Air will be referenced in this manual and seminar as primary, secondary, excess and dilution air.
Primary air provides a percentage of the combustion
air, but more importantly, controls the amount of fuel that can be burned.
Excess air is
supplied to the combustion process to ensure each fuel molecule is completely surrounded
by sufficient combustion air. As a burner
tune-up improves the rate at which mixing occurs, the amount of excess air required can be
Dilution air does
not participate directly in the combustion process and is primarily required to attempt to
control stack draft and reduce the likelihood that moisture in the flue gases will
condense in the vent system --- which directly influences combustion air intake, safety