Combustion and Flame

What is Combustion?

Combustion is defined as a rapid exothermic reaction that liberates substantial energy as heat and flames as combustion reactions with the ability to propagate through a suitable medium.

Actually, combustion is a result of dynamic or time dependent events that occur on a molecular level among atoms, molecules, radicals and solid boundaries. Combustion generates air pollution, pollutant formation and oxidation kinetics as well as their reduction techniques are explained in this chapter.

Types of Combustion

1. Rapid combustion – When combustion occurs rapidly, it is called rapid combustion.
2. Spontaneous combustion – The type of combustion in which material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion. Eg, Burning of phosphorus.
3. Complete combustion – Complete combustion is the mixture of fuel with oxygen without remaining fuel over time, friction and high enough temperature to ignite all the fuel components.
4. Incomplete combustion – Incomplete combustion is typically caused by inadequate air-fuel mixing, insufficient residence time, insufficient temperature and low total excess air.
5. Explosion – When a sudden reaction takes place with the release of heat and light and evolution of a large amount of gas takes place it is called an explosion. Eg, firecrackers.

Necessary Conditions for Combustion

There are three necessary conditions for combustion. These are

• Presence of a combustible substance.
• Presence of a supporter of combustion.
• Heating the combustible substance to its ignition temperature.
• The presence of fuel plays an important role.
• The presence of air or oxygen.
• Ignition temperature is maintained (it is defined as the substance that catches fire at its lowest temperature).

Flames and Fuels

Substances that produce heat on combustion are called combustion fuels. In many rural parts of our country people still use wood as a fuel because of its easy availability and low cost. Global warming is the result of an increase of CO2 concentration in the atmosphere mostly due to the combustion of fuels.

• Solid Fuels – Examples are wood and coal. These are used to cook food in homes. Coal is also used in industries.
• Liquid Fuels – Examples are kerosene and petrol. Kerosene is used in stoves and in lamps to cook food and petrol is used as a fuel in automobiles.
• Gaseous Fuels – Examples are natural gas and petroleum gas. These are used in industries. CNG is used to run automobiles.

Pollution Caused by Combustion of Fuels

• Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids.
• Carbon monoxide (CO) is a toxic air pollutant produced largely from vehicle emissions.
• Fuels like wood, coal, petroleum etc, which are carbon fuels produce unburnt carbon particles, which cause respiratory diseases and skin diseases.
• Generally CO2 is released during burning of fuels. It causes the rise in the temperature of the atmosphere of earth, which is called global warming.
• Incomplete combustion of fuel releases many poisonous gases such as carbon monoxide. It can kill people sleeping in a room by causing suffocation.
• Combustion of coal and diesel generates sulphur dioxide. It can kill people sleeping in a room by causing suffocation.
• Burning of fuel releases oxides of nitrogen and sulphur. They dissolve in rain water and form acid. Such rain is called acid rain.