What are the different methods of firing a Boiler ?

Firing refers to the application of heat to a boiler.  Firing is done by different methods. 

Fuel fired Boilers
The most common method is by burning fuel such as wood, oil, coal or gas.

Waste Heat Recovery Boilers
These boilers function by recovering the heat from the gases or effluents of other industrial processes.  For instance, the exhaust gas from a power plant can be used to heat a boiler. 

Electrically heated Boilers
These boilers are used for heat generation.  They are not used for power generation.  They are generally of small capacity.  They can be used in hospitals for sterilizing equipment.  They can be used for laundry and for domestic heating purposes. 

Nuclear Powered Boilers
These boilers are used in Nuclear Power Plants.  The nuclear fission reaction which occurs in the reactor produces enormous amounts of heat.  This heat can be used to heat water and produce steam.  This steam is used in power plants to generate electricity or to drive submarines