Heat Loss Because of Unburned Fuel

Heat loss because of unburned fuel is depending on type of fuel. If the fuel used is gas or oil, the combustion process if it is not performed perfectly will produce CO; it can be called heat loss. This CO will be highly toxic (for gas fuel) and will be attended by smoke and fouling (for oil fuel). For gas and oil fuel, carbon can be burned nearly 100 %. But for coal fuel, carbon can be burned until 98% - 99% for Circulating Fluidized Bed Combustion Boiler and Pulverized Fuel Firing Boiler, 90% - 95% for Bubbling Fluidized Bed Combustion Boiler, and 80% - 90% for Stoker Boiler. Heat loss because of unburned fuel generally occurs in solid fuel firing.

The steam boiler with type stoker or grate will need long time to burn carbon. Some of carbons leave the furnace with unburned condition. Amount of unburned fuel depending on type of coal, thicker coal and grate stoker running too fast will increase excessive unburned fuel.

Unburned fuel can be seen in fly ash and dust contained in flue gas before discharge to stack. To minimize heat loss because of unburned fuel, operator of steam boiler must adjust balance draft through some of air system devices like force draft fan, secondary air fan, and induced draft fan to burn fuel completely and overcome the resistance of the flue gas passes the steam boiler. Unburned fuel or carbon will influence on combustion efficiency and show an efficiency loss. 

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