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Chapter 3

PRINCIPLES AND

PRODUCTS OF COMBUSTION

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Principles of combustion

Combustion is a chemical reaction that takes place at a

high temperature and results in the oxidation of gas.

For combustion to occur three ingredients are required.

1. Fuel (hydrocarbon)

2. Oxygen (normally taken from air, which consists of 

approx. 20% oxygen and 80% nitrogen)

3. Ignition (heat)

Note: Nitrogen is an inert gas and does not burn.

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Combustion process

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Products of complete combustion

The combustion process results in:

1. heat (what is required)

2. water vapour (H2O)

3. carbon dioxide (C02)

Note: Nitrogen passes through combustion unchanged.

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Causes of incomplete combustion

•Lack of air 

—insufficient air will not provide enough

oxygen to complete combustion.

• Over-gassing—if a burner is over-gassed there is no

guarantee sufficient air will be available for combustion.

• Blocked or inadequate flue—failure to remove

combustion products will affect the combustion

process.

• Impingement of the inner cone—a solid object in the

inner cone of the flame will cool the fuel below ignition

temperature. This will result in incomplete combustion.

Incomplete combustion produces carbon monoxide,

which is harmful!

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Important!

 Appliances and burners are designed and certified to

ensure that, when fitted and adjusted correctly,

complete combustion will take place.

Correct installation and commissioning of the appliance

will ensure the appliance performs safely and

efficiently.

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Types of burners

1. Post-aeratedGas is supplied at the burner head with all the air for 

combustion coming from around the burner head.

2. Aerated

 Approximately half the air used is entrained into the

burner and mixed with the gas prior to burning. The

remainder of the air comes at the point of combustion

at the burner head.

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Post-aerated burners

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 Aerated burners

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 Aeration adjustment

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Burner adjustment

 An aerated burner should be adjusted

when burning the

correct maximum

amount of gas.

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Burner adjustments

a. Correct flame

b. Over-aerated

c. Under-aerated

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Combustion—Summary

• For combustion to occur three ingredients are required.

1. Fuel (hydrocarbon)

2. Oxygen (normally taken from air)

3. Ignition (heat)

• The combustion process results in:

1. heat (what is required)

2. water vapour (H2O)

3. carbon dioxide (C02)

• Incomplete combustion results in:

 – carbon monoxide

• Burner types: post-aerated, aerated (bunsen), forced

and induced draught.

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