Why does Boiler back fire?

Why  does boiler back fire?

Following potential situations may cause back fire in biomass or bagasse fired boilers

Protections & Interlocks in power plants

1-Sudden trip of ID fans or closing of fans suction or discharge dampers:

If there is no interlock on ID fans trip to FD fans, a very huge back fire occurs in boiler which has potential to cause damages to the Boiler & operators.

Sudden closure of ID fans suction or discharge dampers will cause same back fire as above.

2-Malfunctioning of pneumatic dampers positioner may cause closure of ID fans dampers resulting into huge back fire in Boiler

3-APH tubes jam : Area required  for the flue gas flow is equals to Boiler capacity/9. Due to accumulation of ash in tubes, the flue gas flow area gets reduce. This situation causes backfire

This situation occurs if Boiler is run at lower load for long time without bypassing APH for FD/SA air, as at lower load flue gas temperature will be less, which get condense in APH tubes.

4-Higher moisture in the fuel: If the fuel has higher moisture that required, then there will be more requirement of combustion air, while maintaining this air fuel mixture back fire may occur.

5-Frequent variation in fuel moisture : This is experienced in sugar plants due to variation in bagasse moisture at mill outlet due to variation in imbibition water added to extract juice from cane.

Upon continuous variation of fuel moisture it become very difficult for operator to maintain balanced draught in furnace.

6-Improper spreading of fuel : Due to this furnace may experience bed heap combustion of fuel, due to this there will not be uniform distribution of bottom combustion air causing boiler back fire.

IBR acts, regulations & forms used

7-Sudden drop of boiler load: Due to this boiler air fuel ratio & draught system gets disturbed causing boiler back fire.

8-Fouling in Super heater coils : Higher moisture in fuel cause accumulation of bagasse & unburnt on super heater coils, which blocks the flue gas path causing boiler back fire

9-Accumulation & deposition of ash in ESP fields: This situation causes more resistance to flow of flue gas.

10-Improper air fuel mixture during start up & shutdowns may cause boiler back fire

11-Air leakage into & out of the system : Due to the air ingress & flue gas leakages, it becomes very  difficult for operator to maintain balanced draught.

12.Leakage or puncture of water wall tubes: Leakage or puncture of water wall tubes will result into high furnace pressure

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