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
- 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.