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### Theory:

Any substance that ignites to produce energy in the form of heat is known as fuel. Combustible materials are also called fuels. These are inflammable substances.
Classification of fuels:

Fuels can be classified into different kinds based on their:
• State
• Exhaustibility
• Occurrence
State:
The fuels are classified based on their state as:

 State Example Solids Coal Liquids Petrol Gases Natural gas

Exhaustibility:

The fuels are classified based on their exhaustibility as:

 Renewable sources These are non-depletable resources, for example, sunlight, air. Non-renewable sources These are resources which can be depleted when overused, for example, coal, petroleum, etc.

Occurrence:

The fuels are classified based on their origin as

 Natural fuels These are fuels that occur in nature, for example, coal, natural gas and petroleum. Synthetic fuels These are fuels derived from natural gas, such as producer gas, diesel, etc.

Fuel Efficiency:

The measure of the conversion of chemical potential energy to kinetic energy or work is known as the efficiency of a fuel. In simpler terms, the measure of the conversion of one form of energy to another form is known as efficiency.

Calorific Value:

Calorific value is the measure of fuel efficiency. The quantity of heat produced during the process of complete combustion for one kg of a fuel is known as its calorific value. This is expressed in terms of kilojoule per kg (kJ/kg).

Higher the calorific value higher is the efficiency of the fuel, for example, coal and petrol. The calorific value of petrol is higher than coal, so its efficiency is higher than coal.

 Fuel Calorific Value Coal $$25000$$-$$33000$$ Petrol $$45000$$

Characteristics of ideal fuels:
• It should be easily available.
• Should possess high calorific value and low moisture content.
• Moderate ignition temperature.
• No harmful gases or substances should be released on reactions.