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.

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.

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