Combustion: You may have noticed that some materials such as paper burn easily, while materials such as iron do not. This is because the paper is a combustible substance whereas iron is a non-combustible substance. Combustion is a chemical reaction between two or more substances, usually involving oxygen, that produces heat and light in the form of a flame. To learn more about combustion, read the below article.

Define Combustion

The process by which substances burn in air with the release of heat or both heat and light is known as combustion.

A substance may be classified into two categories based on its combustibility. The substance that burns in air or oxygen to produce heat and light is called combustible substances.

 For example, petrol, wood, coal, paper, kerosene, etc, are combustible substances. The substance that does not burn in air or oxygen is called a non-combustible substance. For example, metals, mud, stone, glass, cement, etc, are non-combustible substances.

Combustion Reaction

Combustion is a chemical change. When a fuel undergoes combustion, the product obtained depends on the amount of oxygen available for combustion. If there is enough oxygen to support the combustion or burning process, the combustion is called complete combustion.

For example,

Methane on complete combustion in sufficient oxygen gives carbon dioxide, water, and energy.

During combustion, substances like coal, or charcoal react with oxygen in the air to form carbon dioxide and water.

A simpler example, seen in the combustion of hydrogen and oxygen, which is a commonly used reaction in rocket engines. The resultant product will be water vapour.

If there is insufficient oxygen to support the combustion or burning process, the combustion is called incomplete combustion.

For example, when methane is allowed to burn in insufficient oxygen, the products obtained are carbon monoxide, water, and energy.

Combustion Process

During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are referred to as exhaust. The majority of the exhaust is caused by chemical reactions between the fuel and oxygen.

When a hydrogen-carbon-based fuel burns, water, and carbon dioxide are released into the atmosphere. However, the exhaust can also contain chemical combinations formed by the oxidizer alone. If gasoline is burned in the air that contains \(21\% \) oxygen and \(78\% \) nitrogen, the exhaust may contain nitrous oxides (nitrogen + oxygen).

Because of the heat transferred to the exhaust during combustion, the temperature of the exhaust is high. Because of the high temperatures, the exhaust is usually a gas, but there can also be liquid or solid exhaust products. For example, soot is a type of solid exhaust produced by certain combustion processes.

Conditions Necessary for Combustion

Combustion does not occur all the time in nature. It takes place only if the following conditions are met.

Let us now discuss the conditions necessary for the process of combustion to take place.

1. Combustible Substance

A substance that can burn or a combustible substance is the main requirement for combustion.

2. Supporter of Combustion

Combustion occurs in the presence of oxygen, a supporter of combustion. For example, during the combustion of magnesium, charcoal, and coal, air or oxygen is a supporter of combustion. A sufficient supply of a combustion supporter is required for burning. When the supply of oxygen is cut off, combustion stops.

3. Attaining Ignition Temperature

The ignition temperature or kindling point of a substance is the lowest temperature at which a substance spontaneously ignites in a normal atmosphere. This is the third most important condition for combustion to occur. Each combustible substance has a different ignition temperature.

Some substances have a very low ignition temperature, while others have a high ignition temperature. To catch fire, substances with high ignition temperatures must be heated to their ignition temperature.

Other substances with low ignition temperatures easily catch fire at room temperature, such as white phosphorous, which catches fire when exposed to air and is thus stored in water. Petrol, kerosene, LPG, CNG, alcohol, and other such substances are examples of inflammable substances. Handling such substances requires extreme caution because even a small spark can start a major fire.

Types of Combustion

Combustion of substances takes place under different conditions. Depending upon the rate at which combustion takes place, it has been classified into the following types:

a. Rapid Combustion

Rapid combustion is a type of combustion that produces a large amount of heat and light energy in a short period. It often results in fire. This form of combustion is used in machinery such as internal combustion engines. The burning of paper, hydrocarbon fuels such as kerosene, petrol, LPG,  etc., are examples of rapid combustion.

b. Spontaneous Combustion

The combustion process in which a substance burns without the presence of an external source of heat is referred to as spontaneous combustion. Substances with low ignition temperatures exhibit this type of combustion. For example, white phosphorous, which has an ignition temperature of \(35\) degrees Celsius, catches fire when exposed to air if the temperature outside rises above \(35\) degrees Celsius.

c. Slow Combustion

A slow combustion is a form of combustion which takes place very slowly and at low temperatures. Cellular respiration and rusting of iron are an example of slow combustion.

d. Explosion

In addition to the production of heat and light, combustion can result in the release of a large amount of gas. Excessive pressure is created by the sudden evolution of large amounts of gas, resulting in a loud noise. Such combustion is known as an explosion. The bursting of crackers is the best example of an explosion. The same type of combustion is used for exploding rocks and mountains for making roads, and the explosives used are dynamite.

How to Control Fire?

When a building is on fire, firefighters use a water jet to extinguish the flames. Water and its vapour help to cut off oxygen supply while also lowering the temperature of combustible materials. The principle of extinguishing fire involves removing the conditions that allow combustion to occur.

These are:

• 1. Removal of all combustible substances.
• 2. Reducing the temperature to below the ignition temperature.
• 3. Cutting off the supply of air (oxygen).

1. Removal of Combustible Substances

This may not always be possible, especially if the combustible material is already on fire. However, other combustible materials nearby can be removed to prevent the fire from spreading.

2. Reducing the Temperature to below the Ignition Temperature

This can be accomplished by sprinkling or pouring water on the fire. As it evaporates, water absorbs heat from the combustible substance, lowering its temperature. Pouring water, on the other hand, is not advised in certain types of fires. For example, if a person pours water on an electric fire, he or she may receive an electric shock. If water is poured on a fire involving gasoline, diesel, or kerosene, the fire may spread. Because these fuels are lighter than water, they would float and spread.

3. Cutting of Air Supply

This can be accomplished by sprinkling sand or spraying water on the fire. The combustible substance is covered in sand, which cuts off the air supply. Water is frequently sprayed on fire to produce a fine mist of droplets. The water evaporates to form water vapour, cutting off the supply of air to the combustible substance. The air supply can also be cut off by spraying the fire with carbon dioxide or foam from a fire extinguisher. This method is used to put out electrical and gasoline fires.

Summary

The process by which substances burn in air with the release of heat or both heat and light is known as combustion. The substance that burns in air or oxygen to produce heat and light is called combustible substances. The substance that does not burn in air or oxygen is called a non-combustible substance. Combustible substance, a supporter of combustion, and attaining ignition temperature are the condition necessary for combustion. Rapid combustion, spontaneous combustion, slow combustion, and explosion are types of combustion.

FAQs on Combustion

Q.1. What are the types of combustion?
Ans: There are four types of combustion. They are;
1. Rapid combustion
2. Spontaneous combustion
3. Slow combustion
4. Explosion

Q.2. What is the process of combustion?
Ans: A substance reacts with oxygen from the air in a combustion reaction. Combustion reactions occur at high temperatures and transfer energy to the surrounding environment in the form of light and heat. This is why when things burn, you see flames.

Q.3. What are 5 examples of combustion?
Ans: Five examples of combustion are
1. Burning of wood at home.
2. Fireworks
3. Combustion of natural gas
4. Burning off diesel and petrol in the car.
5. For the production of energy in thermal powerplant.

Q.4. What is the cause of combustion?
Ans: The cause of combustion are as follows:
1. To burn, there must be fuel.
2. To supply oxygen, there must be air.
3. An ignition temperature is required to initiate and sustain the combustion process.

Q.5. Is complete combustion possible?
Ans: No, Complete combustion is nearly impossible to achieve because the chemical equilibrium is not always reached or may contain unburned products such as carbon monoxide, hydrogen, and even carbon.