Theory:

Calorific value
Calorific value is the quantity of heat energy present in the food and fuels determined by their complete combustion at the standard condition and constant pressure. Its unit is written by expressing energy in kilojoule per specified amount in kilogram.
The calorific values of some fuels are listed below:
 
Fuel
Calorific value (\(kJ/kg\))
Cow dung
\(6000 - 8000\)
Wood
\(17000 - 22000\)
LPG
\(55000\)
Petrol
\(45000\)
Kerosene
\(45000\)
Diesel
\(450000\)

Properties of a good fuel:
  • Must have high calorific value.
  • Should not produce any poisonous products during combustion.
  • Must be transportable and easily storable.
  • Must have acceptable ignition temperature.
  • Should not cause pollution.
  • Should be cheap.
  • Must be readily available.
  • Should be controllable, i.e., combustion should be easy to begin and stop whenever required.
Formula for calculating calorific value:
 
\(\text{Calorific value}\) \(=\) \(\frac{\text{Heat produced}}{\text{Amount of fuel used}}\)
Example:
If \(5.5\) \(kg\) of a fuel is burnt and the quantity of heat produced is \(170,000\) \(kJ\), what is the calorific value of the fuel?
 
\(\text{Calorific value}\) \(=\) 1700005.5 \(=\) 30909 \(kJ/kg\)
Note: We can also calculate the mass of fuel used and the amount of heat produced by rearranging the formula.
 
Formula for calculating the mass of fuel:
 
\(\text{Amount of fuel used}\) \(=\) \(\frac{\text{Heat produced}}{\text{Calorific value}}\)
 
Formula for calculating amount of heat produced:
 
\(\text{Heat produced}\) \(=\)\(\ (\text {Calorific value} )\ \cdot\ (\text {Amount of fuel used} )\)
Ignition temperature
The least temperature at which a substance catches fire and burns is called ignition temperature.
If a substance has less temperature than its ignition temperature, it will not catch fire and burn.
Every substance has a different ignition temperature.

Inflammable substances:
 
A substance with the least ignition temperature and can catch fire quickly is known as inflammable.
 
Example:
  • Petrol
  • Alcohol
  • LPG (Liquefied Petroleum Gas)
  • CNG (Compressed Natural Gas)
Reference:
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