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Air Pre-heater Fires in Large Capacity Boilers

written by: Dr V T Sathyanathan • edited by: Lamar Stonecypher • updated: 7/9/2010

The damage due to air pre-heater fire is very severe depending upon the intensity of the fire. In many cases it has made the regenerative air pre-heater elements a molten mass. The boiler availability loss will be high if a fire takes place in the air heater.

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    Air pre-heaters are heat recovery systems used in boilers to improve boiler operating efficiency. There are two major types of air pre-heaters: the recuperative and the regenerative. For higher sizes boilers, say 350 t/hr and above, designers adopt the regenerative type. This is mainly due to the compactness of regenerative air pre-heaters for higher heat duty. In both cases, air heater fires are not uncommon in boilers, but more common in the regenerative type. When a boiler is operated with pulverized coal with a higher load, then the probability of air heater fire is low. An air pre-heater fire can happen during startup or shutdown when there is a possibility of a large volume of unburnt particles reaching the air pre-heater. This unburnt gets deposited in the air pre-heaters elements and gets ignited when the temperature rises and particles that are still burning reach this area.

    The plant operator will have to quickly respond to symptoms that show a tendency for air pre-heater fire to start. The possibility of detecting a fire in the air pre-heater in a boiler is very difficult from the operating parameters of boiler. However, indications of an air pre-heater fire can be verified from a certain parameter changing its trend suddenly. When both air and flue gas temperature leaving the air pre-heater increase suddenly with a steep gradient, then the reason for it is to be verified. The specific causes for an air pre-heater fire, the plant response, and the immediate operator response required during this are discussed.

    The specific causes

    • Heavy Unburnt Oil / soot deposits in air pre-heater due to
      • Poor atomization of oil being fired
      • Worn-out oil burner tips leading to coarser particles of oil
      • Low air flow through the burner leading to unburnt particles
      • Improper air distribution between primary, secondary etc
      • Wrong tip position with respect to diffuser or spinner
    • Inadequate frequency of operation of APH Soot blowers
    • Plugged Nozzles of APH Soot blowers
    • Low blowing steam pressure in APH Soot blowers

    Plant Response

    • Sharp rise in air temp at APH outlet will be noticed
    • Increase in flue gas temperature at air pre-heater outlet

    Immediate Operator Action

    • Trip unit on emergency
    • Stop all fans immediately
    • Close all dampers on the air and flue gas side
    • Flood APH with water through air heater water washing system or otherwise
    • Rotate APH to avoid buckling of the air pre-heater rotor

    Adopting some of the good operating practices of boiler can prevent air pre-heater fire totally during the life of the boiler.

    • Reduce number of startups of boiler to a bare minimum.
    • Avoid operating boiler in low loads requiring oil support for a prolonged duration.
    • It is a good practice to check oil viscosity characteristics on a periodic basis and whenever there is a change in supplier.
    • Clean oil gun tips and lap the internals to satisfy the design requirements.
    • Never restart the oil burners without purging during startup.
    • Boiler startup operation must be avoided when air pre-heater soot blowers are not available.
    • Always make it a practice to soot blow all the air pre-heaters, the non working air pre-heaters also must be soot blown by keeping the isolation damper marginally open to allow the soot to be blown out.
    • During startups, low load operation, and shutting down, make it a practice to watch the trend of air and gas temperature leaving the air pre-heaters.
    • Operate air pre-heater soot blowers regularly.
    • Any fire detecting device provided by supplier has to be kept active and tested as and when possible.

    To repair the air pre-heater damaged due to fire takes large amount of time and needs spares for replacements. Some of the spares of large air heaters will not be available off-the-shelf which can further delay the process of repair. The boiler availability loss will be high if a fire takes place in an air heater. Responding quickly to suspected air pre-heater fire immediately and taking corrective action can save a lot of money, downtime, and generation loss.