Characteristics of a good fuel

A fuel is considered good or valuable if It has a low ignition temperature, and It produces a large quantity of heat, that is, its calorific value it high.

Ideal fuels should be easy to store, leave behind little ash after being burnt, and burn easily. Their burning should produce no unwanted by-products. Coal, petroleum and natural gas meet nearly all these conditions. They are cheap, easily available, produce no harmful odour while burning, can be transported easily and their combustion can be controlled.

Types of Fuels

There are a number of ways of classifying fuels. Here we shall discuss the following ways.

1. On the basis of their physical state


2. On the basis of their occurrence

1. On the basis of their physical state

We know that matter can exist in three states-solid, liquid and gas. On this basis, fuels can be of the following types.

Solid fuels. Some solid fuels are wood, coal, cattle-dung cakes, bagasse (sugarcane from which juice has been extracted), etc.

Liquid fuels. Some liquid fuels are kerosene, liquefied petroleum gas (LPG), fuel oil, petrol, diesel, etc.


Gaseous fuels. Some gaseous fuels are natural gas, coal gas, water gas, producer gas, biogas, acetylene, etc.

2. On the basis of their occurrence

Some fuels occur naturally while other are derived by means of physical or chemical processes. On the basis of their occurrence we classify fuels as follows:

Natural fuels:

These are fuels that are used in the form in which they occur in nature. For example, wood, coal, natural gas, petroleum, cattle dung, agricultural waste obtained from crops, bagasse, etc. all natural fuels can be used directly to produce heat. Natural fuels are also called primary fuels. They occur in all the three states of matte.


Processed fuels:

These fuels are derived from natural fuels by various physical or chemical processes. Therefore, they are also called secondary fuels. For example, charcoal, coke, petrol, diesel, kerosene, water gas, coal gas, etc, are processed or manufactured secondary fuels. Processed fuels are also co three types- solid, liquid and gaseous.


Petroleum is a natural liquid fuel. It is also called crude oil. It is a slippery mixture with a strong odour. It is made up of thousands of compounds, mainly hydrocarbons, and compounds of oxygen, nitrogen and sulphur.

Formation of petroleum:


Petroleum is derived from marine life forms, which were buried in the sediments of the seabed millions of years ago. Heat and pressure under the layers of soil, rocks and water helped to bring about chemical changes. Bacterial action also helped to change the marine life forms into oil and gas.

Oil shale is a rock that contains a solid, waxy mixture of hydrocarbons called kerogen. It is crushed and strongly heated to vaporize kerogen which is condensed into dark brown shale oil. Tar sands are deposits of a mixture of fine clay, sand, water and variable amounts of bitumen, black, tar-like heavy oil. Tar sands are removed by surface mining, heated with steam at high pressure, to make the bitumen fluid float. The bitumen is then removed, purified and upgraded to synthetic crude oil before being refined.

Petroleum is detected in large underground reservoirs beneath impermeable rocks. It often floats over a layer of water and is held in this position under pressure beneath a layer of natural gas. In India, oil is found in Assam and Gujarat, at Bombay High and in the deltas of Godavari, Krishan and Kaveri.

Refining of petroleum:


Fractional distillation is a method of separating components of a mixture of liquids having different boiling points. The initial refining of petroleum into its various products is done by this process. When the petroleum mixture is heated, liquids with lower boiling points boil first, ad are collected one after the other.

Petroleum obtained from oil wells, is heated upto 3700C to 4300 C under pressure. Pressure in tubes keeps the crude oil from becoming a vapour. The hot oil flows to the bottom of the tall fractionating tower. Here the pressure is reduced and the liquid becomes a vapour. The vapour rises up in the tower, which may be more than 30m high. Hydrocarbons with a low boiling point range, such as those in gasoline, continue to move up the tower as a vapour. The condenser cools the vapours and liquefies them the shelves placed at different levels collect the condensed liquids within the tower. These shelves hold the liquids until they are drained off into separate storage tanks.

Refining of petroleum yields useful liquid fuels of commercial values. Some of them are gasoline, petrol, kerosene, diesel and lubricating oil.


1. Substances which burn to produce heat at a reasonable cost are called fuels.


2. A substance produced from the remains of plants and animals buried in soil and rocks is known as a fossil fuel.

3. Calorific value of a fuel is the amount of heat produced when one gram of fuel is burnt completely. It is expressed in J/g of kJ/g.

4. A good fuel has low ignition temperature and high calorific value.

5. Fuels can be liquid, solid or gaseous.

6. Wood, coal and petroleum are natural fuels. These are also called primary fuels.

7. Processed fuels are derived from natural fuels. These are also called secondary fuels.

8. Coal is a natural fuel. It is of four types-peat, lignite (brown coal), bituminous (soft) coal, and anthracite (hard coal).

9. Petroleum is a natural liquid fuel, which is a mixture of hydrocarbons.

10. Natural gas is chiefly made up of methane, though butane and propane are also present in small proportions.

11. The process of heating coal in the absence of air is called destructive distillation of coal.

12. Combustion is a process in which a substance burns in oxygen to produce heat.

13. Processed fuels are of three types-

a. Solid fuels such as coke and wood charcoal;

b. Liquid fuels such as gasoline, kerosene, diesel and petrol;

c. Gaseous fuels such as water gas, coal gas and producer gas.

14. Producer gas is a mixture of carbon monoxide and nitrogen.

15. In a soda-acid fire extinguisher, the property of acid to liberate CO2 by reacting with carbonates and bicarbonates of metals is made use of.