What is meant by the calorific value of a fuel?

What is meant by the calorific value of a fuel?

 

  • Fuels are substances that can normally burn readily in air or oxygen to produce large quantity of heat.
  • Examples of some common fuels are petroleum, natural gas and coal. These are called fossil fuels.
  • Different types of fuels have different heats of combustion. Heat of combustion is expressed in kilojoules per mole.
  • In our daily life, it is not very practical to measure the quantity of a fuel in mole. We normally measure the quantity of a fuel by its mass.
  • The calorific value or fuel value of a fuel is the amount of heat energy given out when one gram of the fuel is completely burnt in excess of oxygen.
  • The unit for fuel value is kJ g-1.
  • Fuel value is also known as heat value.
  • A fuel with a high fuel value releases a lot of heat per gram when it burns.
  • Different types of fuels have different fuel values. Some fuels have very high fuel values but have low density, thus occupy a bigger volume. Others have high density but have low fuel values.
  • Some fuels can cause environmental pollution. There are fuels with high fuel values but are not very economical, not easily available or non-renewable.
  • The learning of thermochemistry helps us in the selection of a suitable fuel for a specific purpose.
  • The table below shows the fuel values of some food and common fuels.


    Substance Fuel value (kJ g-1)
    Apple 2
    Egg 6
    Bread 11
    Wood 20
    Coal 30
    Butter 34
    Gasoline 34
    Kerosene 37
    Natural gas 50
    Hydrogen 143

The selection of a fuel to be used in industries depends on the following factors:

  • Fuel value of the fuel
  • Cost of the fuel
  • Availability and sources of the fuel
  • Effect of the fuel on the environment
  • Technology used to harness the energy from the fuel
  • Storage space needed to store the fuel

Fuels with the following qualities are normally chosen:

  • High fuel value
  • Cheap
  • Easily available
  • Do not pollute the environment
  • Easily harnessed
  • Require less storage space

Calorific value example: The heat of combustion of ethanol is -1376 kj mol-1. Calculate the fuel value of ethanol.

[Relative atomic mass: H, 1; C, 12; O, 16]

Solution:



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