What are the combustion losses in a boiler ?

When the fuel is burnt in the furnace of a boiler, all the energy in the fuel is not available to heat the boiler.  Some of the energy is lost in the form of losses.
The Losses in combustion of a boiler are categorized in to the following types.

Loss in the Flue gases
The heat generated by burning the fuel is present in the flue gases.  When these gases escape into the atmosphere, some amount of heat also escapes with the gases.

Hydrogen Losses
This refers to the heat used in evaporating moisture or water present in the fuel.  This is particularly significant in coal-fired boilers.

Losses due to improper combustion.
The improper combustion of fuel due to poor quality or inadequate air also results in loss of potential energy

Losses due to Convection and Radiation
The furnace of the boiler is insulated.  Despite this, some heat escapes from the furnace to the atmosphere.