Combustion and Flame - 8th science

On the basis of rapidness, combustion can be divided into three categories. These are Rapid Combustion, Spontaneous Combustion and Explosion.

(a) Rapid Combustion

When a combustible burns rapidly and produces heat and light, it is known as Rapid Combustion.

Example: LPG, CNG, Petrol, etc. burns rapidly. Thus combustions of LPG, CNG, petrol, etc. are some examples of rapid combustion.

Inflammable substances goes under rapid combustion.

(b) Spontaneous Combustion

Combustion in which a material suddenly catches fire without the application of any apparent cause is called Spontaneous Combustion.

Reference: By Ramiro Barreiro - Own work, CC BY-SA 3.0, Link

Example

(i) Catching of fire of phosphorous. Phosphorous has very low ignition temperature. Phosphorous catches fire at room temperature.

(ii) Catching of fire of coal dust. Coal dust in the coal mining areas catches fire suddenly because of heat of sun.

(iii) Catching fire in forest. Sometimes fire is broken out in forest due to heat of sun or due to lightning strike, etc.

(c) Explosion

Catching of fire in crackers due to chemical reaction. When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound. In such reaction a large amount of gas is formed in the reaction which burst out with heat, light and a boom sound. Such reaction is called Explosion.

Explosion can take place if pressure is applied also on the crackers.

Flame and Its Structure

Visible gaseous part of fire coming out from combustible is called Flame.

Some substance is burnt with flame while some is burnt without flame. Substances which produce gas i.e. vapourise while burning give flame and which do not vapourize while burning do not give flame.

Example: kerosene oil, molten wax, natural gas, LPG, etc. burns with flame. On the other hand coke and charcoal burn without flame.

Kerosene and molten wax rise through the wick and vapourized during burning and form flames. Charcoal does not vapourise and so does not produce a flame.

Structure of Flame : Different Zones of Flame

Substances which burns after vapourising produce flame and which burn without vapourising give no flame.

Flame has different zones. For example flame of candle has these different zones. These zones are called Innermost zone, middle zone and outer zone.

(i) Innermost zone

The innermost zone of a candle flame is the zone of unburnt wax. This innermost zone is least hot zone. And has unburnt wax vapours of black colour.

(ii) Middle Zone

The middle zone is the zone of partial combustion. This middle zone is of yellow colour. This middle zone is moderately hot zone.

(iii) Outer zone

The strong outer zone is the zone of complete combustion. This middle zone is of blue colour. The outer zone is the hottest part of flame.

Unburnt Carbon Particles in the Flame

Flame of candle produces smoke also. The smoke comes out from flame of a candle due to un-burnt particles of carbon. When a watch glass is placed over the flame, un-burnt particles of carbon coming out of flame get deposited over it in the black colour.

How a Goldsmith uses Flame to Heat the Gold or Silver

A goldsmith blows the outermost zone of a flame, which is the hottest part of flame, over gold or silver using a blow pipe. This melts the silver or gold to facilitate the jewelry making.

What is a Fuel?

Combustible substances are known as fuel. Fuel may be solid, liquid or gas. But all combustible substances are not considered as fuel rather substances which are used as the source of heat energy for domestic and industrial purposes are called fuels. For example wood, charcoal, petrol, kerosene, etc.

A good Fuel and An Ideal Fuel

A good fuel is that substance which is readily available, cheap, burns easily in air at moderate rate and produces a large amount of heat.

Thus a good fuel should have following characteristics

(i) Readily available

(ii) Cheaper in cost

(iii) Burns with moderate rate easily in air

(iv) Produce large amount of heat

(v) Does not leave any undesirable substances behind it.

In the light of the above criteria no fuel is considered as an Ideal Fuel. But fuels which fulfils most of the criteria mentioned above is considered as a good fuel.

Fuel Efficiency

Fuel that produces more heat in the less amount of fuel is called more efficient fuel. Fuel efficiency is measured in the term of Calorific Value.

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 kilojue per kg (kJ/kg).

This means fuel which gives more calorific value is the more efficient fuel.

Burning of Fuels Leads to Harmful Products

With increase in population the demands of fuel getting increased continuously. Increase fuel consumptions leads to many problems to environment as well as health.

Some of the harmful effects of the burning of fuel

(a) Carbon fuels like wood, coal and petroleum release un-burnt carbon particles which go in atmosphere and keep suspended in air for long time. These fine particles are dangerous pollutants and leads to respiratory diseases, such as asthma.

(b) Incomplete combustion of fuels gives out carbon monoxide gas. Carbon monoxide gas is very harmful and poisonous. Even a small amount of carbon monoxide gas can leads to death a person or other animal if inhaled.

(c) Burning of most of the fuels produces carbon dioxide gas which goes to environment. Increasing of concentration of carbon dioxide gas in the atmosphere is believed to cause of global warming. This global warming became the one of the most concerned threat for world.

(d) Combustion of coal and diesel releases sulphur dioxide gas along with carbon dioxide gas. Burning of petrol releases oxides of nitrogen also. These gases go in air and leads to acid rain. Acid rain has many harmful effects on crops, building, soil and water, etc.

These harmful gases are of corrosive nature and leads to many other problems also.

(e) On the other hand CNG and LPG are more efficient fuels than other fossil fuels. Burning of CNG and PNG create very less amount of pollution. Since CNG and LPG are cleaner fuels, use of these fuels can be resulted in better environment.

In India engines of automobiles are being modified to use CNG and PNG as fuels at the place of petrol and diesel.