Ash accumulation in boiler penthouse can be caused both due to operational deviations and due to boiler performance deterioration or maintenance. The main factor which results in ash accumulation in penthouse is the positive pressure created at the top of the boiler furnace. The rate of ash accumulation depends on the amount of pressurization and the furnace roof sealing effectiveness. The upper furnace can get pressurized if the furnace vacuum maintained is not adequate. Normally it is a practice to maintain minus 10 millimeter of water column one meter below the furnace roof. This ensures that the pressure just below the furnace roof is negative which means no ash can penetrate the roof sealing and find its way to the penthouse.
The pressure in the boiler furnace just below the roof can become positive when the induced draft fan get loaded fully and more of the combustion air is send to the furnace. The combustion air requirement depends on the amount of fuel being burnt to carry the load in the boiler. One should not mistake this for the weight of fuel being sent to the furnace, but it is the million calories being supplied by the fuel. This can happen when the boiler is being consciously over loaded due to any reason.
The induced draft fan can get fully loaded when in such boilers the air ingress to the boiler increases above a limit. A large amount of air ingress in boilers happens at many locations, starting from furnace to induced draft fan inlet. In furnace one of the major areas is the bottom ash hopper design and the sealing system provided and maintained. The peep holes provided in the furnace at various locations to view the flame condition inside the furnace is also a source of air ingress if they are not properly closed.
In the other areas of second pass, economizer outlet flue gas duct, air pre-heater, air pre-heater flue gas outlet duct to electrostatic precipitator, the electrostatic precipitator and the flue gas duct from electrostatic precipitator outlet to induced draft fan are locations prone for air ingress to the boiler and over load the induced draft fan. The air ingress into the boiler at any location can be evaluated by doing oxygen measurement in the inlet and outlet of the sections. The percentage air ingress can be worked out from the equation (O2 outlet – O2 inlet) divided by (21 – O2 outlet). It is normally seen that the expansion bellows given at various locations are a cause of concern for air ingress into the system.
Ash accumulation in the penthouse of the boiler can also be caused from improper combustion leading to pressurization of the furnace near the roof. It is seen that when the primary air flow in pulverized coal fired boiler is just near the settling velocity, then it can lead to coal particles settling in the coal pipe very near the burner and getting puffed into the furnace. This leads to furnace pressure oscillation enabling the ash particle to get pushed to the penthouse of the boiler.
Inspection and maintenance of the furnace roof sealing is another important factor which can avoid ash ingress into the boiler penthouse.