Combustion and Flame

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What is Combustion?

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Combustion is a redox chemical reaction that takes place between fuel and an oxidant to give a mixture of gases and heat and light. Combustion does not always give rise to fire. But when it does, the flames are an indicator of the reaction. Incomplete combustion, the reactants get oxidized or burn in oxygen to give a limited number of byproducts. For Example- when 1 molecule of hydrocarbon burns in oxygen, it gives carbon dioxide and water. Thus, it can be deduced that end product from the burning of an element in oxygen gives the oxides of the same element. Which means, carbon yields carbon dioxide, sulfur yields sulfur dioxide, and so on.

When not, enough oxygen is available for the continuation of chemical reaction or burning of elements in oxygen, it produces more by products. For example- in incomplete combustion of Hydrocarbon, carbon, hydroxide and carbon monoxide are produced.


Combustible and Non-Combustible Substances

  • Substances that easily catch fire are combustible substances. Example- paper, coal, wood, etc.

  • Substances that do not catch fire easily are non-combustible substances. Example- water, glass, sand, etc.


Fuel

  • Fuel is a substance that produces a usable form of energy on combustion. Like fossil fuel, biogas, etc.

  • The physical state of the fuel may vary. That is, it may be solid, liquid or gas.

  • Fuels are of two types- Natural and Artificial depending upon the source.


Fire

When chemical combustion takes place between oxygen and fuel, it produces a visible heat and light source- fire. The fire keeps burning until there is enough oxygen and fuel to continue the combustion. The temperature at which a combustible substance catches fire when heated in the presence of oxygen is called ignition temperature. Thus, it’s a necessary parameter for generating fire. Some substances like diesel, petrol have very low ignition temperatures. Which means that they easily catch fire with a flame. These substances are known as inflammable substances.


A Fire Can Only Be Generated If

  • Fuel or combustible material is present.

  • A flame or heat source is present to bring the temperature to the ignition temperature of the fuel.

  • Oxygen is present to sustain combustion.

  • When any of these factors are removed or controlled, the fire is controlled.


Types of Combustion

Combustion is of Two Types

  1. Rapid Combustion - In rapid combustion, heat and light is released rapidly in a short span of time.

  2. Spontaneous Combustion - It is spontaneous and occurs without the application of heat. In such a type of combustion, the substance catches fire on its own. Forest fires are one of the examples of this type of combustion.


What is Flame?

The flame is the glowing gaseous part of the fire. It is visible light energy released by combustion of fuel. There are different zones of the flame which are as follows-

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  1. The Outermost Zone - It is the hottest part of the flame. Moreover, it is a result of complete combustion and is blue in colour. The outermost zone of the flame is non-luminous, which means it doesn’t produce light.

  2. The Middle Zone - This zone is moderately hot and yellow in colour. It is the brighter part of the flame and occurs as a result of partial combustion of fuel.

  3. The Innermost Zone - It is the least hot zone of the flame. The innermost zone is black due to the unburnt wax vapours.


Fire Control

Fire is quite hard to control because it continues to burn in the presence of oxygen. To control fire, at least, one or all of the factors of combustion must be brought under control.

To sum up- Oxygen, fuel and ignition temperature of the substance plays a key role in combustion. So, by controlling one of these factors, fire can be controlled.


Calorific Value of a Fuel

  • The heat energy generated by the complete combustion of net 1kg of fuel is termed as the calorific value of the fuel at constant pressure and under normal conditions. It is expressed in unit- Kilojoules per Kilogram- Kj/Kg

  • When the energy obtained by combustion of fuel is converted into useful work, it indicates the efficiency of fuel.

  • The calorific value of a fuel is directly proportional to its efficiency. Which means, if the calorific value is high for fuel, its efficiency is also high. And Vice versa.

FAQ (Frequently Asked Questions)

1. What is the Smoke?

When solid unburnt particles from the combustion of a substance get dispersed in air, they generate smoke. It is of black colour because of the presence of unburnt carbon particles. Smoke from the burning of fuels and other substances is a major pollutant in the atmosphere.

2. Explain the Working of a Matchstick.

  • The bulb of the matchstick is made up of red phosphorus. On heating, the red phosphorus turns into white phosphorus.

  • White phosphorus ignites spontaneously and increases the temperature of the wooden stick which is also an easily combustible substance.

  • When the temperature reaches the ignition point, the matchstick starts burning.

3. How Does a Fire Extinguisher Work?

A fire extinguisher is a device to control uncontrolled fire. It does so, by cutting off the supply of oxygen as it is the main component of combustion. or it lowers the temperature of the fuel. Or does both to control fire.

4. How Does the Burning of Fuel Give Rise to Pollution?

  • Burning of natural fuels like wood, coal, fossil fuels, release unburnt carbon particles in the air, called smoke. Smoke causes respiratory problems like asthma.

  • Incomplete burning of fuel releases carbon monoxide which is a very dangerous gas.

  • Burning of fuel also releases carbon dioxide which is a major greenhouse gas and raises the global temperature leading to a phenomenon called global warming.

  • Burning some fuels also release sulfur dioxide and nitrogen oxide. These harmful gases mix with water molecules in the atmosphere and turn into acid causing acid rains.