What is the difference between Net Calorific Value and Gross Calorific Value in Boilers ?

Calorific value is the total energy contained in fuel.  It is indicated in kilojoules per litre or cubic metre.
All fuels, such as coal, oil or wood contain a small amount of water in the form of moisture.  When fuel is consumed in the furnace, some of the energy is used to evaporate the water contained in the fuel.  This water escapes as steam in the flue gases.
Gross Calorific Value
This includes the total energy in the fuel which  includes the energy used in heating the water.  The Gross calorific value is also known as the higher heating value.
Net Calorific Value
Net Calorific Energy is calculated after subtracting the energy used to evaporate the moisture in the fuel.  Net Calorific Energy is used for boiler efficiency calculation.  The Net calorific value is also known as the lower heating value.
The Net Calorific Value is generally 10 % less than the Gross calorific value.