Combustion and Flame
Combustion
A chemical process in which a substance reacts with oxygen to give off heat and light is called combustion.
Combustible Substances
Fuel
Substances which easily catch fire are combustible substances, such as paper, coal, wood.
Non - Combustible Substances
Substances which do not catch fire readily are non-combustible substances, such as sand, water, glass
Any substance, which upon combustion produces a usable amount of energy is known as fuel. For example, fossil fuels, biogas, nuclear energy, etc.
Fuels can be solid, liquid or gas depending on their state.
Ignition Temperature
The lowest temperature at which a combustible substance catches fire when heated in air is called its ignition temperature.
Inflammable Substances
The substances, which have very low ignition temperature and can easily catch fire with a flame are known as inflammable substances. Examples: diesel, LPG, acetone.
Fire is the result of a chemical combustion reaction between oxygen and some sort of fuel.
Types of Combustion
Rapid Combustion
Spontaneous Combustion
Explosion
Flame
Flame is the zone of combustion of a combustible substances.
Structure of Flame
The outermost zone is the hottest among all zones and is blue in colour and this is due to complete combustion. It is the non-luminous part of the flame.
The middle zone of the candle flame is moderately hot and is yellow in colour, and partial combustion of fuel takes place. It is the bright part of the flame.
The innermost zone of the flame is the least hot and is black in colour. This is due to the presence of unburnt wax vapours.
Substances which vaporize during burning gives flame.
Fuel Efficiency
Efficiency is that proportion of energy released by a fuel combustion process which is converted into useful work.
The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).
Burning of Fuels Leads to Harmful Products
Carbon fuels like wood, coal, petroleum release unburnt carbon. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.
Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas.
Increased concentration of carbon dioxide in the air is believed to cause global warming.
Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.
The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts. CNG is a cleaner fuel.