Combustion of Fuels - Definition, Types, Structure of Flame

We use different kinds of fuel for various purposes at home, in industry and for running automobiles. These fuels are cow dung, wood, coal, charcoal, petrol, diesel, compressed natural gas (CNG) and many more.

What is Combustion?

A chemical process in which a substance reacts with oxygen to emit heat is called combustion. The substance that undergoes combustion is called combustible or fuel which can be solid, liquid or gas. 

In some cases, light is also emitted during combustion as a flame. e.g. burning of wood.

Types of Combustion

1. Rapid Combustion: When gas burns rapidly and produces heat and light. Such combustion is called rapid combustion.
2. Spontaneous Combustion: Combustion in which a material suddenly bursts into flames, without application of any cause is called spontaneous combustion.
3. Complete combustion: Complete combustion is the mixture of fuel with oxygen without remaining fuel over time, friction and high enough temperature to ignite all the fuel components.
4. Incomplete combustion: It is caused by inadequate air-fuel mixing, insufficient residence time, insufficient temperature and low total excess air.
5. Explosion: When a sudden reaction takes place with the release of heat and light and the evolution of a large amount of gas. It is called an explosion. Eg, firecrackers.

Necessary Conditions for Combustion are-

1. Presence of a combustible substance, a fuel and a supporter of combustion.
2. Heating the combustible substance to its ignition temperature and maintaining it.
3. The presence of air or oxygen.

What are Combustible and Non-combustible Substances?

Substances like paper and wood which easily catch fire are combustible substances while the substances which do not catch fire readily are non-combustible substances, such as sand, water, glass.

Combustion Reaction

A combustion reaction occurs when a substance reacts quickly with oxygen (O₂). Combustion is commonly called burning, and the substance that burns is usually referred to as fuel. 

The products of a complete combustion reaction include carbon dioxide (CO₂) and water vapour (H₂O). The reaction typically gives off heat and light. The general equation for a complete combustion reaction is:

Fuel + O₂ → CO₂ + H₂O

The fuel that burns in a combustion reaction contains compounds called hydrocarbons which are compounds that contain only carbon and hydrogen. e.g. Methane and Ethane. Natural gas is a fuel that is commonly used in home furnaces and gas stoves. The main component of natural gas is methane (CH₄). The combustion of methane is represented by the equation:

CH₄ + 2O₂ → CO₂ + 2H₂O

Fuel

The sources of heat energy for domestic and industrial purposes are wood, charcoal, petrol, kerosene etc which are called fuels. They help us to fulfill day to day needs. A good fuel is one that is readily available. It is cheap and burns easily in the air at a moderate rate. It produces a large amount of heat. It does not leave behind any undesirable substances. There is probably no fuel that could be considered as an ideal fuel.

Types of Fuels

1. Solid Fuels: Examples are wood and coal. These are used to cook food in homes. Coal is also used in industries.
2. Liquid Fuels: Examples are kerosene and petrol. Kerosene is used in stoves to cook food and petrol is used as fuel.
3. Gaseous Fuels: Fuels like natural gas and petroleum gas are used in industries. CNG is used to run automobiles.

Fuel Efficiency

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 kilojoule per kg (kJ/kg). Efficiency is the proportion of energy released by a combustion process that is converted into useful work. It is directly proportional to its efficiency. The calorific value of some fuels is given in the table below.

Fuel	Calorific Value
Cow dung	8,000
Wood	22,000
Coal	33,000
Biogas	40,000
Diesel	45,000
Kerosene	45,000
Petrol	45,000
Methane	50,000

Ignition Temperature: The lowest temperature at which a combustible substance catches fire when heated in air is called its ignition temperature.

Characteristics of an Ideal Fuel are:

1. The ideal fuel should be cheap, easily available and readily combustible.
2. It should have a high calorific value.
3. It does not produce harmful gases or residues that pollute the environment.
4. It should be dry and should have less moisture content as dry fuel increases its calorific value.
5. The combustion speed of a good fuel should be moderate.
6. The combustion of a good fuel should not be explosive.

Flame

Fire is the result of a chemical combustion reaction between oxygen and some sort of fuel.​​​​​​ How long a fire lasts depends on how much fuel and oxygen are available. 

Flame is the visible and gaseous part of the fire. The substances which vaporize during burning, give flames. For example, kerosene oil and molten wax rise through the wick and are vaporized during burning and form flames. Charcoal does not vaporize and so does not produce a flame. 

Structure of flame

1. Outer Zone: The outermost zone is the hottest of all zones and is blue in colour. This is due to complete combustion. It is the non-luminous part of the flame.
2. Middle Zone: The middle zone of the candle flame is moderately hot and is yellow in colour, and partial combustion of fuel takes place. It is the bright part of the flame.
3. Innermost Zone: The innermost zone of the flame is the least hot and is black in colour. This is due to the presence of unburnt wax vapours.

Smoke: Smoke is an example of unburnt particles dispersed in the air. The black colour of smoke is due to the presence of unburnt carbon particles in the smoke.

Burning of Fuel leads to Harmful Products

The increasing fuel consumption has harmful effects on the environment-

1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles which are fine and dangerous pollutants that can cause respiratory diseases, such as asthma.
2. Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas that can kill a person sleeping in a room if the gas is released in that room.
3. Combustion of most fuels releases carbon dioxide into the environment. Increased concentration of carbon dioxide in the air causes global warming.
4. The burning of coal and diesel releases sulphur dioxide. It is an extremely suffocating and corrosive gas.
5. Petrol engines release oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rainwater and form acids. Such rain is called acid rain.

The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas) because CNG produces harmful products in very small amounts which makes it a cleaner fuel.

Unburnt Carbon Particles: Carbon fuels like wood, coal, candle,  petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.

CO Emission: Carbon monoxide is a poisonous gas, which is produced by the incomplete combustion of fuels. It is dangerous to burn coal in a closed room as the carbon monoxide produced can kill people sleeping in that room.

Sample Questions

Question 1: What is combustion?

Answer:

A chemical process in which a substance reacts with oxygen to emit heat is called combustion. The substance that undergoes combustion is called combustible or fuel which can be solid, liquid or gas. In some cases, light is also emitted during combustion as a flame. E.g. burning of wood.

Question 2: Write the general equation of combustion. Also, do the same for the combustion of methane.

Answer:

The general equation for a complete combustion reaction is:-

Fuel + O₂ → CO₂ + H₂O

The equation for combustion of methane is-

CH₄ + 2O₂ → CO₂ + 2H₂O

Question 3: Define fuel and classify them.

Answer:

The sources of heat energy for domestic and industrial purposes are wood, charcoal, petrol, kerosene etc which are called fuels. Types of Fuels are-

1. Solid Fuels: Examples are wood and coal. These are used to cook food in homes. Coal is also used in industries.
2. Liquid Fuels: Examples are kerosene and petrol. Kerosene is used in stoves to cook food and petrol is used as a fuel.
3. Gaseous Fuels: Fuels like natural gas and petroleum gas which are used in industries. CNG is used to run automobiles.

Question 4: What are calorific value and fuel efficiency? What is the relation between them?

Answer:

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 kilojoule per kg (kJ/kg). Efficiency is that proportion of energy released by a combustion process which is converted into useful work. It is directly proportional to its efficiency.

Question 5: What are the characteristics of an ideal fuel?

Answer:

The characteristics are as follows-

1. The ideal fuel should be cheap, easily available and readily combustible.
2. It should have high calorific value.
3. It does not produce harmful gases or residues that pollute the environment.
4. It should be dry and should have less moisture content as dry fuel increases its calorific value.
5. The combustion speed of a good fuel should be moderate.
6. The combustion of a good fuel should not be explosive.

Question 6: Define fire and flame.

Answer:

Fire is the result of a chemical combustion reaction between oxygen and some sort of fuel.​​​​​​ Flame is the visible and gaseous part of the fire. The substances which vaporize during burning, give flames.

Question 7: What are the effects of burning fuel on the environment?

Answer:

1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles which can cause respiratory diseases,.
2. Incomplete combustion of fuels gives carbon monoxide gas which can kill a person sleeping in a room.
3. Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air causes global warming.
4. Burning of coal and diesel releases sulphur dioxide.
5. Petrol engines releases oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain.