Air pre-heaters are used as heat recovery surfaces in boilers used in industry and power plants. Air pre-heaters in boiler applications are of three types.
- Recuperative or tubular air pre-heater
- Regenerative air pre-heaters
- Steam coil air pre-heaters
The steam coil air pre-heaters are used in the majority of boilers to keep the cold end temperature higher during cold start-ups. The recuperative and regenerative air pre-heaters are for heating the combustion air to the boiler. Designers select these air pre-heaters depending upon the economics, the heat to be recovered from the flue gas leaving the economizer in the boiler, the capacity of the plant, etc.
In all the three designs, plugging is seen to be a threat. The steam coil air pre-heaters can get plugged across the fins by anything that is carried along with the air being heated, depending on the location. As these air pre-heaters are used only at low loads and start-ups, if they get plugged there is a large amount of pressure drop across the length and the forced draft power consumption goes higher. The designers are now using a by-pass duct to house the steam coil air pre-heater. This saves the power consumed by the forced draft fan during higher loads. Placing this in the by-pass duct also enables any required on line maintenance to be done.
In the case of tubular air pre-heaters, the plugging gets initiated during cold start-up due to reasons like unburned oil deposit on the cold end side, no flow region because of duct connections, tube leakage in the economizer area, infective soot blowing, wet steam during air pre-heater soot blower, frequent start-ups, etc. It is normally seen that the plugging gets initiated from the cold end side except in the case of a no flow region due to a duct connection.
In the case of the regenerative air pre-heaters, the plugging is caused due to large number of start-ups, poor oil gun performance, ineffective soot blowing, ash particles settling, water washing of air pre-heaters with inadequate drying, low frequency of operation of air pre-heater soot blowers, high amount of air pre-heater leakage with prolonged low load operation, etc.
In both recuperative and regenerative air pre-heaters, the plugging rate goes up when ammonia is used for containing sulphur oxide emissions. Selective catalectic reduction and NOx control with higher than 10 ppm of ammonia slip have also been reported to have caused a higher rate of air pre-heater plugging.
In the case of both recuperative and regenerative air pre-heaters, getting plugged affect the boiler performance adversely.
- The higher the plugging, the lesser the heat transfer surface available, and hence the flue gas temperature leaving the air pre-heater increases. This reduces the boiler operating efficiency and increases fuel consumption
- Increases the velocity of flue gases thus leading to higher erosion rate of the heat transfer surface
- Increases the downtime of the boiler for maintenance of air pre-heaters
- Increases the auxiliary power consumption of the boiler
- In the case of regenerative air pre-heaters, preferential plugging can cause vibration and combustion oscillations
Air Pre-heater Fires in Large Capacity Boilers - 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
Cold End Corrosion in Boiler and Its Prevention - In steam generating units using fuels with sulphur has a potential hazard of sulphur corrosion at the cold end of the boiler. The severity depends on many factors like percentage of sulphur in fuel, excess air, moisture in flue gas etc. Many options are available to contain cold end corrosion
Both Regenerative Air Pre-Heater Motors Trip - Tips to Handle - In regenerative type air heater in boiler when the electrical motor trips always the air motor start on auto. Failure of this will lead to a major breakdown of air pre-heater and loss of availability of boiler.