Calorific value:

Introduction:

The quality of a fuel is determined by the amount of energy released per unit mass or volume referred to as calorific value.

Definitions of Calorific value:

• Calorific value of a fuel is the amount of heat liberated when a unit mass or a unit volume of the fuel is burnt completely in air or oxygen.
• Fuels generally contain hydrogen in addition to carbon. During combustion, the hydrogen is converted to steam.
• In the determination of calorific value of the fuel if the products of combustion are cooled to ambient temperature (room temperature), the latent heat of steam is also included. This is referred to as gross calorific value (GCV) or higher calorific value.
• In practice, the products of combustion are allowed to escape and the amount of heat realized is lesser than the GCV (since the latent heat of vaporization is not released). This is net calorific value (NCV) or lower calorific value.

GCV       =       NCV        latent heat of steam

 

                                                                 

• Gross Calorific value is the amount of heat liberated when a unit mass or a unit volume of the fuel is burnt completely in air or oxygen and the products of combustion are cooled to ambient temperature.
• Net Calorific value is the amount of heat liberated when a unit mass or a unit volume of the fuel is burnt completely in air or oxygen and the products of combustion are allowed to escape.

Determination of Calorific Value of a Solid Fuel – Bomb Calorimeter:

Construction:

• The bomb calorimeter (shown in the fig.) consists of an outer cylindrical steel vessel (bomb) with an airtight screw and an inlet for oxygen.
• The bomb has a platinum crucible with a loop of wire. The ends of the wire project out and can be connected to a source of electric current.
• The bomb is immersed in a rectangular vessel (calorimeter) containing water, which is continuously stirred.
• A Beckmann thermometer is introduced into the calorimeter.

Working :

• A known mass of the fuel is made into a pellet and taken in the crucible.
• Oxygen is passed through the bomb.
• A known mass of water is taken in the calorimeter and is closed with the lid.
• The initial temperature of water is noted.
• The ends of the wire are connected to an electric source so as to ignite the fuel.
• The heat released is absorbed by water. The temperature of water rises.

The final temperature is noted.

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