Fuel is a substance that has carbon content and produces energy on combustion. This energy is utilised in various industries and households.

Fuels are combustible and produce energy on combustion. Combustion is a reaction in which the substance reacts with oxygen to give product and heat. The product produced has lower energy as compared to the fuel. All reactions that produce heat as a by-product are known as exothermic reactions. Nuclear reaction, Combustion are examples of this. Fuels have carbon as the main content. Examples of fuels are Charcoal, wood, coal, petrol, diesel etc.

Fuels which are present in the earth crust are known as fossil fuels. Examples of common fossil fuels are coal and petroleum oils.

Definition :-

Fuel in simple terms is a material burnt to produce heat and energy. It can be an artificially or naturally occurring substance that burns in the presence of oxygen to produce energy.

Origin :-

The first use of fuel was around 2 million years ago by Homo erectus. They mainly burnt wood. In early times the source of fuels was of plant or animal origin.

Charcoal was used in around 6000 Going further distillation of crude oil to form kerosene was done by Persian chemists. Later in 9th century the oil field were found and exploited in place such as Baku and Azerbaijan

 Steam engines were discovered in the UK in 1769 with this coal being started in the locomotive industry. In the 19th century gas came into picture and by the 20 th and 21st century the production of electricity from coal started. Further during the industrial revolution due to rising global warming fossil fuels came and now we are trying to shift to more renewable options like biofuel.

Classification of Fuels

Fuels can classify into two main categories –

1) Occurrence and

2)  State of aggregation

Occurrence -: On the basis of occurrence fuels can be classified into primary and secondary. Primary fuels are the ones which are extracted from nature as such. They are natural fuels. Examples of primary fuels are – coal, petroleum, natural gas, wood and so on. Secondary fuels are the ones which are extracted from the primary fuels. The other name of primary fuels is artificial fuels. Examples of secondary fuels are kerosene, petrol, diesel, charcoal etc.

  • Aggregation-: On the state of aggregation fuels are classified into solid, Liquid and gas.
  • Solid fuel –All fuels that exist in solid state. Example – coal, wood, charcoal etc
  • Liquid Fuel – Fuels that exist in liquid state. Example – Petrol, diesel, kerosene etc
  • Gaseous Fuels –All the fuels that occur in gaseous state. Examples – natural gas, Biogas.

Comparison between solid, liquid and gaseous Fuels

Every fuel in its state has certain merits and demerits these are discussed below –

Solid Fuels -:

Merits –

1) High calorific value

2) Easy to Handle and transport

3) Low cost of production

4) Moderate ignition temperature

Demerits –

1) Low thermal efficiency

2) High ash content

3) Large amount of heat goes unutilized.

4) Combustion is difficult to control.

Liquid Fuels -:


1) less moisture content.

2) Easy to store for a long period.

3) Combustion can be easily controlled

4)less amount of air is required for complete combustion

Demerits –

1) The storage is costlier in comparison to solid fuel

2)They produce bad odour

3) Inflammable

4)Risk of fire hazards are high

Gaseous Fuels-:

Merits –

1) Burn easily without soot 

2) Economical

3) Combustion can be easily controlled

4) They don’t need special burners.

Demerits –

 1) Highly Inflammable

2) High risk of fire Hazards

3) Require more storage space and hence the cost gets higher.

Characteristics of a good fuel 

A fuel which has the following properties is considered good -:

1) High calorific value.

2) Generate large amounts of energy.

3)  Not evolve harmful gases- like SO2, Cox etc 

4) Easy to handle and transport

5) Moderate ignition temperature 

6) Readily available

7) Economical Rate

8) No liberate any unpleasant odor during combustion

9)High burning efficiency

10)Uniform shape in case of solid fuel

Calorific value

It is the amount of heat generated to burn a unit mass of fuel completely. It has the same unit as heat or energy. 

Units of calorific value

Calorific value is expressed in the following units – Calorie, Kilocalorie, British thermal units, centigrade heat unit.

Determination of Calorific value 

The calorific value of fuels can be determined with the help of Calorimeters. Some important Calorimeters that are used are as follows-:

1) Bomb Calorimeter

2) Boy’s gas Calorimeter

Properties of Some Fuels 

Wood -It is the oldest type of fuel discovered. Wood is vegetable tissue and contains substances such as lignin, fat, tar and sugar. The main constituents of any wood variety are – water, lignin, sugar, Fat -tar and cellular tissue. Wood ignites easily and has an average ignition temperature.

Coal -In India we find coal in Madhya Pradesh, Bengal, and Bihar. The Indian coal is low grade and finds its use in railways (30%) and other activities like iron work.

Petroleum- It is a natural fuel with around 79.5 to 87.1% of carbon content as its chief constituent. It also has other elements like oxygen, hydrogen, sulphur etc. It is composed of hydrocarbons like naphthalene and paraffins.

Natural gas – It is in close relation with petroleum products. It has a calorific value of 12,000 to 14,000 kcal/ m3. Its availability in America is immense. Inters of its chemical properties it is poisonous and odourless gas. It has a specific gravity of 0.57 to 0.7 .


Fuels are an asset to any economy without which survival is not possible. From our cars to our gas stove everything needs fuel to work. Thus, we should use these resources in a sustainable manner.