Each steam boiler has different thermal efficiency. There is no steam boiler which has thermal efficiency 100% because there are some heat losses which reduce the efficiency. The sources of heat loss are dependent on type of boiler. They can be different based on type of fuel, combustion / firing process, and so on. Solid fuel usually influence higher heat loss than liquid or gaseous fuel.
The heat loss can derived from:
4. Heat loss that can not be countable.
Thermal efficiency (%) = 100 % - All of heat losses (%)
All of heat losses above influence calculation of combustion and thermal efficiency. Design new steam boiler should calculate performance of boiler include combustion and thermal efficiency calculation to determine good chemistry between sorbent, flue gas, residue and air in the combustion process to reach highest efficiency. To achieve maximum thermal efficiency and minimize unburned fuel operator of steam boiler should perform control and maintenance of burner, minimize quantity of air to reduce heat loss in stack, and make sure the feed water before be supplied to steam boiler is clean to enhance absorption of heat in the pressure parts.