What is combustion?
Combustion is a chemical process in which a substance reacts with oxygen and releases heat. The substance that burns is called a combustible substance or fuel. Combustion may also produce light.
Wood, LPG, petrol and candle wax are combustible. Glass, sand and stone are non-combustible under ordinary conditions.
The fire triangle
Three things are needed for a fire to continue:
a fuel,
oxygen from air,
enough heat to keep the fuel at or above its ignition temperature.
The ignition temperature is the lowest temperature at which a substance catches fire. A match does not ignite merely by touching the box; friction raises its chemicals to their ignition temperature.
Materials with very low ignition temperatures that catch fire easily are inflammable substances. Petrol, LPG and alcohol are examples and must be stored carefully.
A fuel does not burn simply because oxygen is present. It must first reach its ignition temperature.
Controlling a fire
A fire can be stopped by removing any side of the fire triangle.
Remove heat: water cools wood or paper below its ignition temperature.
Cut off oxygen: sand, a fire blanket, foam or carbon dioxide can cover the burning material.
Remove fuel: closing the valve of a gas cylinder stops the fuel supply if this can be done safely.
Water must not be thrown on an electrical fire because ordinary water can conduct current. It is also unsafe for burning oil: the oil floats on water and may spread the flames. Carbon dioxide extinguishers are commonly used for electrical equipment and flammable liquids.
Types of combustion
Type | What happens | Example |
|---|---|---|
Rapid combustion | burns quickly after a flame or spark is applied | LPG in a gas stove |
Spontaneous combustion | catches fire without an external flame | some piles of coal or oily rags |
Explosion | very fast reaction producing heat, light, sound and gases | a firecracker |
How a candle burns
Heat from the flame melts wax near the wick. Liquid wax rises through the wick, changes into vapour, and the wax vapour burns. The solid wax itself does not burn directly.
A candle flame has three main zones:
Zone | Appearance | Combustion and temperature |
|---|---|---|
Inner zone | dark | unburnt wax vapour; least hot |
Middle zone | yellow and luminous | partial combustion; glowing soot particles |
Outer zone | faint blue or nearly invisible | complete combustion; hottest |
Goldsmiths blow air toward the outer zone because its complete combustion produces the highest temperature.
Choosing a good fuel
A good fuel should be readily available, affordable, easy to store and transport, and safe to use. It should have a high calorific value, burn steadily, and produce little smoke or harmful residue.
Calorific value is the heat produced by the complete combustion of 1 kg of a fuel. It is measured in kilojoules per kilogram (kJ/kg).
No fuel is ideal in every situation. LPG burns more cleanly than coal in a kitchen, while a vehicle needs a fuel that can be carried safely and release energy at a controlled rate.
Combustion and pollution
Incomplete combustion can produce poisonous carbon monoxide.
Burning coal and diesel releases sulphur compounds that contribute to acid rain.
Carbon dioxide from burning fuels adds to global warming.
Soot and fine particles harm the respiratory system.
Good ventilation and properly adjusted burners help fuels burn more completely. A blue gas flame usually shows more complete combustion than a yellow, sooty flame.
Summary
Fuel, oxygen and heat above the ignition temperature are all required for fire.
Cooling, smothering or removing fuel can stop combustion.
The outer zone of a candle flame is the hottest because combustion is complete there.
A useful fuel combines high calorific value with safety, availability and low pollution.