Combustion refers to the rapid oxidation of fuel accompanied by heat or heat and light. Complete combustion requires an adequate oxygen supply. The objective of good combustion is to release all the heat from fuel by controlling temperature, turbulence for mixing fuel and oxygen, and reaction time. Stoichiometry calculates the theoretical air required for combustion and can determine excess air by measuring flue gas CO2 levels. A certain amount of excess air is needed for complete combustion but too much leads to heat losses.

1. Combustions

2. Principles of combustion
• Combustion refers to the rapid oxidation of fuel accompanied by the production of
heat, or heat and light.
• Complete combustion (complete oxidation) of a fuel is possible only in the
presence of an adequate supply of oxygen. (Air: 20.9% oxygen, 79% nitrogen and
other gases).
• Rapid fuel oxidation results in large amounts of heat.
• Solid or liquid fuels must be changed to a gas before they will burn.
• Fuel gases will burn in their normal state if enough air is present.
• Nitrogen is considered to be a temperature reducing diluter.
• Nitrogen reduces combustion efficiency by absorbing heat from the combustion of
fuels and diluting the flue gases.
• Nitrogen reduces the heat available for transfer through the heat exchanger
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 mixture.
• Nitrogen also can combine with oxygen (particularly at high flame temperatures) to
produce oxides of nitrogen (NOx), which are toxic pollutants.
• Carbon may also combine with oxygen to form carbon monoxide, which results in
the release of a smaller quantity of heat. Carbon burned to CO2 will produce more
heat per unit of fuel than when CO or smoke are produced.

3. • The objective of good combustion is to release all of the heat in the fuel
which is accomplished by controlling
o Temperature which should be high enough to ignite and maintain
ignition of the fuel
o Turbulence or intimate mixing of the fuel and oxygen
o Time should be sufficient for complete combustion.
• Commonly used fuels like natural gas and propane generally consist of
carbon and hydrogen so the flue gases mainly consist of CO2 and H2O.
• Water vapor formed removes heat from the flue gases, which would
otherwise be available for more heat transfer.

4. Stoichiometric calculation of air
required
• Stoichiometry is a branch of chemistry that deals with the relative
quantities of reactants and products in chemical reactions.
• For combustion air is needed. The amount of air needed can be
calculated by:
• The first step is to calculate the theoretical amount of air required for
combustion. For example, for each kg of fuel burnt, 14.12 kg of air is
required.
• The second step is to calculate the theoretical CO2 content in the flue
gases, for example 15.5%.
• The next step is to measure the actual CO2 percentage in the flue
gases, for example 10%.
• This can then be used to calculate the % of excess air, for example 55%.
• Once you know this you can calculate the weight of constituents in e.g.
100 kg of fuel with 55% excess air: CO2, H2O, SO2, O2, N2
• Finally you can convert the weight into volume of constituents. This
could result in for example 10% CO2 and 7.5% O2.

5. Concept of Excess Air
• For optimum combustion, the real amount of combustion air must be
greater than that required theoretically. This additional amount of air is
called “excess air”.
• A certain amount of excess air is needed for complete combustion of fuel.
• Too much excess air leads to heat losses and too little excess air leads to
incomplete combustion.
• You can tell if there is too little or too much excess air by measuring the
CO2 in the flue gases.
• The amount of excess air required depend on various factors like type of
fuel used, type of firing system, size of fuel in case of solid fuel etc.
Combustion analyzer

7. Residual oxygen (%)
Excess air (%)
This illustrates the relation between residual oxygen and excess air. The
higher the percentage of oxygen measured in the flue gas, the more
excess air there is. The typical range of O2 percentage that indicates
combustion is good, e.g. 6-8%, depends on type of fuel used and the firing
system.

8. Exercise
A sample of coal has the following
composition:
Carbon = 80%
Hydrogen = 20%
Calculate the amount of theoretical air
required for complete combustion.