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6. Combustion and Flame

This chapter talks about various phases of fire, types of combustion and ways to extinguish unwanted fire

A chemical process in which a substance reacts with oxygen to give off heat is called as combustion. The substance that undergoes combustion is said to be combustible or Fuel.

For example – when we burn magnesium ribbon, magnesium oxide is formed and heat & light are produced. In this, magnesium is the combustible substance.

For the combustion to take place, oxygen is required. In the absence of oxygen, the flame will go off.

6.1      Requirements for Combustion

The following are the requirements for Combustion to take place

  1. Presence of a combustible material (Fuel)
  2. Presence of Oxygen
  3. Minimum temperature at which the Fuel catches fire (Ignition Temperature)

The substances which have very low ignition temperature & easily catch fire with the flame are called as Inflammable substances. For example – Alcohol, LPG, Petrol

The lowest temperature at which a substance catches fire is called its ignition Temperature. Different substances catch fire at different temperature so the ignition temperature for different substance is different

6.1.1      Types of fuels

The following are the different types of fuels used

  1. Wood
  2. Coal
  3. Charcoal
  4. Petrol
  5. Diesel
  6. LPG
  7. CNG
  8. Cow dung cakes

6.2      How to control Fire?

For the fire to take place, the following 3 things are required – Fuel, Air (oxygen) and Ignition Temperature. To extinguish fire, any of these 3 requirements are removed from the environment and the fire is extinguished.

The following rules describe the behaviour of eliminating one of the above three materials

  1. Fuel – It cannot be eliminated, as the combustible material is ignited and already has a very high temperature
  2. Air – If we cut off the supply of air, it will extinguish the fire.
  3. Temperature – If we bring down the temperature, the fire will get extinguished.

6.2.1      How to fire extinguishers work

Fire extinguishers work on the principle of cutting off one of the 3 main requirements for combustion

The following are the different types of fire extinguishers and their uses

Fire Extinguisher Type How does it work Use On Don’t Use On
Water Reduces the temperature thus bringing it below Ignition Temperature Fires involving wood, paper etc Fire involving
  • Electrical appliances, because of risk of electric shock
  • Oil and Petrol
Liquefied/ Compressed CO2 It is heavier than oxygen and covers the fuel like an envelope, thus cutting off oxygen supply

Also, when liquefied CO2 is released, it cools the fuel

Fire involving
  • Electrical appliances, because of risk of electric shock
  • Oil and Petrol
 
Sodium/Potassium Bicarbonate powder These release CO2 near fire Fire involving
  • Electrical appliances, because of risk of electric shock
  • Oil and Petrol
 

To control fire, we need to do the following

  1. We need to put fire extinguisher (water, Co2) on the fire
  2. The combustible material cools down
  3. The temperature of the combustible material goes down its ignition temperature
  4. Also, water vapours surround the combustible material, cutting off the supply of air
  5. Thus, the fire is extinguished

6.3      Types of Combustion

The following are the types of Combustion

  1. Rapid Combustion – When the fuel or the combustible substances catch fire rapidly and produces heat & light, it is called as rapid combustion. For example – Cooking Gas
  2. Spontaneous Combustion – When a material suddenly bursts into flames, without the application of any apparent cause, it is called as spontaneous combustion. For example – Forest fire
  3. Explosion – A sudden reaction with the release of heat, light and sound in which large amount of Gas is released is called as Explosion. For example – Burning of crackers leads to explosion

6.4      The Structure of a Flame

The flame has multiple zones. The structure of the flame suggests that

  • Outermost zone is the zone of complete combustion, so it is the hottest zone
  • The middle zone is of partial combustion, so it is moderately hot
  • The innermost zone is the zone of still unburnt fuel, so it is the least hot zone

DIAGRAM

6.5      Fuel Efficiency

A good fuel is one which is readily available, is cheap, burns at a moderate rate, produces large amount of heat & is least polluting. For example – Wood, Charcoal, kerosene, petrol and diesel

The efficiency of the fuel depends on the factors like the amount of heat & light it produces & the cost of fuel.

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its Calorific value. It is expressed as kilojoule per kg (KJ/kg)

The following table gives the relative comparison of calorific value of various commonly used fuels

Fuel Calorific Value
Wood 17,000 – 22,000
Coal 25,000 – 33,000
Biogas 35,000-40,000
Petrol, Diesel, Kerosene 45,000
Methane, CNG, LPG 50,000-55,000
Hydrogen 1,50,000

Note that Hydrogen has the maximum calorific value. This is because; it has strong affinity to O2 and therefore requires less energy to combine

6.6      The Harmful Effects of Fuels

The increase in burning of fuels leads to harmful effects on the environment as well as human beings. For example

  1. Carbon fuels like wood, coal, petroleum etc release unburnt carbon particles which are dangerous pollutants. These pollutants cause diseases related to respiratory tract. For example – Asthma
  2. Incomplete combustion of carbon fuels give out carbon monoxide, which is a very poisonous gas. It can even kill people.
  3. The combustion of mist fuels releases CO2 is the environment. Rise in the concentration of CO2 in air leads to Global Warming.
  4. Burning of Coal & diesel releases Sulphur Dioxide gas (SO2). SO2 is very suffocating & corrosive. Petrol engines give out gaseous oxides of nitrogen. The oxides of sulphur & nitrogen dissolve in rain water & form acids leading to the formation of Acid Rain.

6.7      Important Questions

  1. If we cover a burning candle with an inverted glass, the flame of the candle goes off. This is because air is needed for the flame to burn & covering the candle with the glass will cut the air supply to the flame & the flame will go off.
  1. If we put water in paper cup and put it on flame, the paper cup does not burn. This is because the heat supplied to the paper cup is transferred to water by conduction & the in the presence of water, the ignition temperature of the paper is not reached.
  1. It is strictly advised not to use water o extinguish electrical fires. It is so because water may conduct electricity & harm those who are trying to extinguish the fire.
  1. Water can not be used to extinguish fire involving petrol. It is so because water is heavier than oil. Water will sink below the oil and oil keeps burning on the top.
  1. Co2 is the best extinguisher for fires involving electrical equipment & inflammable materials like petrol. It is so because CO2 is heavier than oxygen. It covers the fire like a blanket, so the contact between the fuel & the oxygen is cut off & the fire is doused.
  1. Kerosene oil produces flame whereas charcoal burns without a flame. This is because kerosene oil vaporises during burning giving out flames whereas charcoal does not vaporize and does not produce a flame.
  1. Goldsmith blows the outermost zone of the flame with a metallic blow –pipe for melting gold. This is because the outermost zone is the zone of complete combustion & is the hottest zone.
  1. We should not sleep in a room with burning or simmering coal fire in it. It is so because burning of coal produces carbon monoxide which is a very poisonous gas & it can kill persons sleeping in that room.