This topic gives an overview of; 

  • Fuel
  • Types of Fuels
  • Fuel Efficiency
  • Burning of Fuels

What is a Fuel?

The sources of heat energy for domestic and industrial purposes are mainly wood, charcoal, petrol, kerosene, etc. These substances are called fuels. A good fuel is one which is readily available. It is cheap. It burns easily in air at a moderate rate. It produces a large amount of heat. It does not leave behind any undersirable substances.

There is probably no fuel that could be considered as an ideal fuel. We should look for a fuel which fulfils most of the requirements for a particular use. Fuels differ in their cost. Some fuels are cheaper than others.

Types of Fuels

Fuels can be divided into three main categories

  1. Solid fuels
  2. Liquid fuels
  3. Gaseous fuels

Solid fuels: Solid fuels left some ash or residue after combustion. The solid fuels are given below.

S. No. Solid Fuels
1 Coal
2 Cow dung cake
3 Wood

Liquid fuels: Most of the liquid fuels in use are the hydrocarbons which exist in the liquid phase at room temperature. The liquid fuels are given below.

S. No. Liquid Fuels
1 Petrol
2 Kerosene
3 Diesel

Gaseous fuels: Some of the gaseous fuels exist naturally at room temperature and some are made by the treatment of coal. The gaseous fuels are given below.

S. No. Gaseous Fuels
1  Biogas
3 Hydrogen

Fuel Efficiency

Suppose you were asked to boil a given quantity of water using cow dung, coal and LPG as fuel. 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). Calorific values of some fuels are given in Table.

Calorific Values of Different Fuels

Fuel Calorific Value
Cow dung cake 6000-8000
Wood 17000-22000
Coal 25000-33000
Petrol 45000
Kerosene 45000
Diesel 45000
Methane 50000
CNG 50000
LPG 55000
Biogas 35000-40000
Hydrogen 150000

Burning of Fuels 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. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.
  2. Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that room.
  3. Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming.
  4. Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil. 
  5. The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts. CNG is a cleaner fuel.

For centuries, wood was used as domestic and industrial fuel. But now it has been replaced by coal and other fuels like LPG. In many rural parts of our country, people still use wood as a fuel because of its easy availability and low cost. However, burning of wood gives a lot of smoke which is very harmful for human beings. It causes respiratory problem. Also, trees provide us with useful substances which are lost when wood is used as fuel. Moreover cutting of trees leads to deforestation which is quite harmful to the environment.

Global warming is the rise in temperature of the atmosphere of the earth. This results, among other things, in the melting of polar glaciers, which leads to a rise in the sea level, causing floods in the coastal areas. Low lying coastal areas may even be permanently submerged under water.


  • An ideal fuel is cheap, readily available, readily combustible and easy to transport. It has high calorific value. It does not produce gases or residues that pollute the environment.
  • Fuels differ in their efficiency and cost.
  • Fuel efficiency is expressed in terms of its calorific value which is expressed in units of kilojoule per kg.
  • Unburnt carbon particles in air are dangerous pollutants causing respiratory problems.
  • Incomplete combustion of a fuel gives poisonous carbon monoxide gas.
  • Increased percentage of carbon dioxide in air has been linked to global warming.
  • Oxides of sulphur and nitrogen produced by the burning of coal, diesel and petrol cause acid rain which is harmful for crops, buildings and soil.


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