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Let’s begin by defining the Combustion first!
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What is Combustion?
Combustible elements of fuels chemically combine with oxygen to give heat and light. This process is known as combustion.
It is defined as:
”Combustion is a rapid chemical reaction in which a combustible element rapidly combine with oxygen to produce a large amount of heat and light at high temperature”.
In all types of fuels, the main combustible elements are carbon, hydrogen and a small quantity of sulphur which chemically combine with oxygen to produce heat and light.
These reactions can be represented in quality and quantity by chemical equations. Oxygen required for the combustion process is obtained from the air or other means.
In some situations, reactions can take place with oxygen such as magnesium burn with nitrogen and antimony with chlorine.
3Mg+N2= Mg3N₂+heat + light
3Sb+3Cl2= 25bCl3 + heat + light
We shall consider those reactions which only take place in the presence of oxygen.
Types of Combustion-
On the basis of the time taken in the process, the combustion can be classified as:
- Rapid Combustion.
- Self Combustion.
They are briefly described below:
1. Rapid Combustion-
Combustion of fuel which takes place rapidly in a short duration of time is called rapid combustion.
Combustible and gaseous fuels burn rapidly whereas solid fuels take a longer time to burn.
Example of Rapid Combustion-
- Methane etc.
These can burn instantly in contact with a spark.
Combustion that takes place without an external source of heat is called self-combustion. In such a combustion process there is no need to supply heat from outside.
Self-combustion is also called ”Spontaneous combustion”.
Example of Self-Combustion-
- Friction produces a spark which causes combustion
- Burning of yellow phosphorus
In this type of combustion, chemical substances combine with oxygen sharply. During an explosion heat, light, and a high volume of gases are produced instantly with noise.
- Must Read: Liquid Fuels: A Detailed Guide All you Need
Example of Explosion-
- Ammonia etc.
These substances burn with explosions.
Classification of Cumbustion-
Combustion can also be classified on the basis of combustion control and they are-
- Controlled Combustion
- Uncontrolled Combustion
1. Controlled Combustion-
A combustion process which can be controlled is called controlled combustion. This process can be stopped, reduced or increased as per the requirement.
Example of Controlled Combustion-
- Nuclear combustion in a nuclear reactor
- Engine Ignition combustion
2. Un-Controlled Combustion-
A combustion process which cannot be controlled and further this combustion can not be stopped, is called uncontrolled combustion.
Example of Un-Controlled Combustion-
- Bombs explosions
- Nuclear Bomb etc.
Necessary Conditions for Combustion-
The following conditions are required for the combustion process to be completed:
- Combustible elements.
- Ignition temperature.
They are described below in brief:
1. Combustible Elements-
In any fuel, there are combustible and non-combustible elements. Combustible elements are those which actually take part in the combustion process.
Where combustible elements are carbon, hydrogen and sulphur and Non-combustible elements are nitrogen, oxygen, ash and water-vapour etc.
Combustible elements of a fuel combine with oxygen to produce heat and light. Oxygen is obtained from air.
If combustible elements of fuel can be proportioned by weight or volume, then the chemical equations can be used to find the required quantity of oxygen for the complete combustion of fuel.
By the known quantity of required oxygen, minimum required air can be calculated. For this, the knowledge of combustible elements, their molecular weights, atomic weights and chemical reactions is essential.
3. Ignition Temperature-
The temperature at which the ignition of different combustible elements of a fuel start, is called the ignition temperature. This temperature is different for each element.
It is important to know these temperatures of ignition of different elements.
Steps of combustion process-
The combustion process takes place in the following sequential steps:
- Evaporation of moisture or water contents.
- Evaporation of volatile elements and their burning
- Burning of Combustible elements (carbon, sulphur etc.)
- Remains of Combustible elements after burning, like ash etc.
Examples of combustion-
Here are five examples from many of the combustion examples-
- Burning Wood
- Coal Burning
- Burning of Liquid Fuels such as petrol and diesel
- Burning of gaseous fuels such as LPG and CNG
- Thermal Plants combustion
Combustion of fuel-
The combustion of fuel is the chemical reaction between oxygen and hydrocarbons in the presence of atmospheric air.
It is expressed by chemical equations in terms of quality and quantity. Chemical equations represent the nature of reactions which take place during the combustion of fuel.
To construct a chemical equation we must know the chemical reactions that take place between elements, compounds their atomic weights and molecular weights etc.
They are described in down below:
Elements are those substances which cannot be further divided into other substances. There are 110 such elements found so far. All other substances are derived from these elements.
Prominent elements are, hydrogen, oxygen, nitrogen, iron, carbon, copper, aluminium, tin etc. All the elements are symbolised at the international level.
The compounds are formed by the combination of different elements. In this way, an infinite number of compounds can be made.
The physical properties of compounds are entirely different from their elements. Prominent examples of compounds are Water (H2O), Carbon dioxide (CO2) and salt (Na2CI) etc.
Atoms and Molecules-
The elements are made up of minute and chemically indivisible particles known as atoms.
The smallest quantity of a substance which can exist by itself in a chemically recognizable form is known as a molecule. A molecule may consist of one atom, two atoms, three atoms or even more.
Hydrogen is the lightest known substance. By taking the weight of a hydrogen atom as unity, it is possible to obtain the relative weights of other atoms and molecules.
It may be noted that the actual weights of these atoms are extremely small. The atomic weight of an element is the number of times, the atom of that element is heavier than the hydrogen atom.
For example, the atomic weight of oxygen is 16. It means that the oxygen atom is 16 times heavier than the hydrogen atom.
Types of Combustion are done at this point. I hope you found the information you were looking for and further sparked your interest even more.
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Thermodynamics: Definitions, Laws, Types, Importance and Applications
Abhishek Tiwary is a blogger by passion and a Quality Engineer by profession. He completed his B.Tech degree in the year 2017. Now working in a reputed firm. He loves to share his knowledge with others.