Combustion is the rapid chemical reaction between oxygen and a combustible material that releases heat and light. Usually the combustible material is a hydrocarbon and ambient air supplies the oxygen. Complete combustion occurs when there is sufficient oxygen to convert all of the carbon to carbon dioxide and all of the hydrogen to water. Incomplete combustion means that there is either unburned or partially reacted fuel, i.e., carbon monoxide, hydrogen, etc.
Methane is the main constituent of natural gas. It reacts with oxygen to form carbon dioxide and water.
CH4 + 2 O2 → CO2 + 2 H2O
This stoichiometry is typical of all hydrocarbons. One atom of carbon requires one molecule of oxygen and four atoms of hydrogen require one molecule of oxygen. The theoretical air is that needed for complete combustion of the carbon and hydrogen, i.e., two molecules of oxygen for one molecule of methane. Excess air is that supplied in addition to what is required. For example, 20% excess air means that the air supplied is 1.2 times the stoichiometric amount.
The following reaction represents the complete combustion in oxygen of an arbitrary carbon-based fuel compound. For combustion in air, add N2 to both sides of the reaction equation in an amount equal to the oxygen input multiplied by 3.774.
Fig. 8-14 shows how the composition of the flue gases depends on the amount of combustion air.
