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Sample Problem

In an experiment, 4.5kg of a fuel was completely burnt. The heat produced was measured
to be 1,80,000KJ. Calculate the calorific value of the fuel.

Answer. The calorific value of a fuel is the heat produced by burning 1Kg of fuel. Now, the
heat produced by burning 4.5kg fuel = 1,80,000 kJ

180000
So, the heat produced by burning 1 kg fuel = = 40,000kJ/kg.
4.5
Thus, the calorific value of the given fuel is 40,000kJ/kg.

Characteristics of an ideal fuel

1. It has a high calorific value, or it produces a large amount of heat.
2. It burns easily in the air at a moderate rate.
3. It has a proper ignition temperature.
4. It does not produce any harmful gases or leaves any residue after burning.
5. It is cheap, readily available, and easy to transport.

Flame

A flame is a region where the combustion of gaseous substances takes place. Only those
solid and liquid fuels which vaporise on being heated, burn with a flame.
Some of the substances which burn by producing flames are LPG, biogas, wax, etc.

Some fuels burn without flame, for example, charcoal, coal, etc.

When fuels burn, the type of fame produced depends on the proportion of oxygen which is
available for burning the fuel.

1. When the oxygen supply is insufficient, then the fuels burn incompletely producing
mainly a yellow flame. The yellow flame is caused by the glow of hot unburnt carbon
particles produced due to incomplete combustion fuel. This yellow flame produces light,
so it is said to be a luminous flame.
2. When the oxygen supply is sufficient, then the fuels burn completely producing mainly a
blue flame. This blue flame does not produce much light, so it is said to be a non -
luminous flame

Structure of a flame
A flame consists of three zones. These are the innermost zone, middle zone, and the
outermost zone.

I. The innermost zone of a flame is dark. It consists of hot, unburnt vapours of the
combustible material. It is the least hot zone of the flame.
II. The middle zone of a flame is yellow. It is bright and luminous. The fuel burns partially
in the middle zone because there is not enough air for burning in this zone. The partial
burning of fuel in this zone produces carbon particles. These particles become hot
and emit light.
III. The outermost zone of a flame is blue. It is a non-luminous zone. In this zone,
complete combustion of the fuel takes place because there is plenty of air around it.

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