Combustion is the chemical reaction (burning) of fuel with the oxygen in air. Fuel is primarily carbon (C) and hydrogen (H) atoms bound together (as gas, oil, coal, etc), often with some sulfur (S), nitrogen (N), and other elements included. Air is primarily oxygen molecules (O2) (20.9%), and nitrogen molecules (N2) (78%), with a small amount of argon (Ar) (1%), carbon dioxide (CO2), etc. The nitrogen is generally inert (non-reactive) except in small amounts at high temperatures.(FUEL)C + H & (AIR) O2 (and N2,Ar,..) forms CO2 (carbon dioxide), and H2O (water vapor)and heat is liberated in the process.
In addition, some of impurities in the fuel (S, N,...) will react with the O2 in air to form SO2 (sulfur dioxide), and NO (nitric oxide), and if insufficent air is present, CO (carbon monoxide) and/or smoke may be formed.
A. Combustion Efficiency:
Generally a continuous measurement of O2 is used to insure that there is sufficient excess air to burn all of the fuel. [If not, unburned fuel can lead to an explosive situation]. Sometimes CO2 is used for control. In addition, a smoke meter may be used as a secondary indicator of excess air. When smoke appears the air must be increased. AIM offers both extractive and in-situ O2 analyzers, as well as across-stream Opacity / Dust monitors.
B: Pollution Emissions:
Both SO2 and NO are "pollutants", and today we want to minimize the amount of pollutants that are put into the atmosphere. The AIM equipment can measure these, to insure that the plant is operating within approved limits, and for feedback control of the combustion process.
However combustion is never perfect. If there is more air than fuel, then some of the SO2 will form SO3; and SO3 plus H2O produces sulfuric acid (H2SO4)...a strong corrosive acid, which may corrode the duct and stack lining, as well as foul the atmosphere.
Also some of the NO will react with the O2 to form NO2. The sum of NO and NO2 is called NOx. Furthermore NO2 will react with water to form nitric acid (HNO3), another strong acid. (This suggests that one control for H2SO4 and HNO3 is to minimize the excess air, and in fact that works).
And when the volume of air is less than the fuel, there will be incomplete combustion, with some fuel left over, and other molecules produced. For example: C + O2 with insufficient air, will generate CO (carbon monoxide), as well as soot (carbon particles) which appears as opacity or smoke. The S will form into hydrogen sulfide (H2S) which smells like "rotten eggs".
In general optimum combustion occurs when there is a perfect mixture of air and fuel with little or no excess of either. AIM offers the analytical tools to maintain this relationship
