How pulverised coal industrial boilers work
Industrial pulverised coal boilers, typically 50 to 250 TPH, serve paper mills, chemical plants and process cogeneration where steam demand is large and continuous. Coal is ground in bowl or ball-tube mills to a fine powder and carried to the burners by primary air from a PA fan; secondary air from the FD fan enters the burner windbox after the air heater. Around 80% of the coal ash leaves the furnace as fine fly ash, so with Indian coal at 30 to 45% ash the ESP inlet dust is heavy. The gas passes the superheater, economiser and a tubular or rotary air heater, then the ESP and the ID fan. Oil or gas support is used during start-up and at low load.
Where the FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Secondary air duct
- Burner windbox
- Furnace
The fan, often two in parallel, draws ambient air through an inlet silencer and pushes it through the air heater to the burner windbox. A separate PA fan supplies the air that dries and carries coal from the mills, either as cold air through its own air heater section or as hot air taken after the air heater. FD fan outlet pressure is typically 200 to 350 mmWC, set by the air heater, ducts and windbox.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 250 to 320°C, downstream of the fan |
| Pressure at the fan outlet | Typically 200 to 350 mmWC |
| Share of total air | Usually 75 to 85% as secondary air, rest as primary air |
| Dust | Clean air; coal dust near the bunker bay |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for pulverised coal industrial boilers
| Blade form | Backward curved aerofoil, for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 6 or 8 |
| Bearings and cooling | Pedestal bearings; oil lubricated on large fans |
| Flow control | Variable speed drive or inlet vanes on total air flow |
| Also specified | Inlet silencer, flow measuring device, non-return dampers for parallel fans |
What this FD fan has to cope with
Air flow measurement and control
Pulverised coal combustion needs accurate air flow, because both too little and too much air cause problems: unburnt carbon and CO on one side, high flue gas loss on the other. A reliable flow measuring section in the FD duct, and a fan with a stable curve, give the combustion control a good signal to work with.
Air heater leakage
Some of the FD fan's output leaks to the gas side across the air heater. As leakage grows, the fan runs faster for the same windbox air, and the ID fan works harder. Duty selection includes expected leakage, and oxygen readings before and after the air heater show when repair is due.
Response to fast load changes
Process plants can change steam demand quickly. The FD fan must lead or follow coal flow closely, with air leading on load increase and lagging on decrease, to avoid smoky or fuel-rich conditions. A variable speed drive with correctly tuned response rates handles this. Oxygen trim then corrects the steady state.
Energy and control
The FD fan on an industrial pulverised coal boiler runs continuously and follows process steam demand. Inlet vane control is acceptable near full load but wastes energy at the 60 to 80% loads common in practice. A variable speed drive, combined with keeping excess oxygen near 3 to 4% and reducing air heater leakage, gives the largest saving on both the FD and ID fans.
ID fan and FD fan in pulverised coal industrial boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP | Ahead of the air heater |
| What it handles | Flue gas with fine fly ash | Clean ambient air |
| Temperature at the fan | 140 to 165°C | Ambient, 25 to 45°C |
| Main design concern | Fly ash erosion behind the ESP | Accurate air flow and load response |
| Control | Furnace draft | Total air flow and excess oxygen |
See also the Induced Draft (ID) Fan for pulverised coal industrial boilers.
Questions buyers ask
What is the difference between the FD fan and the PA fan on a pulverised coal boiler?
The FD fan supplies secondary air to the burner windbox for combustion. The PA fan supplies primary air to the mills, which dries the coal and carries it to the burners. The PA fan works at higher pressure and smaller volume; the forced draft fan moves most of the combustion air.
Does the FD fan handle hot air on a pulverised coal boiler?
No. It takes ambient air and pushes it through the air heater, where the air is heated to 250 to 320°C before the windbox. The fan is selected at ambient density. Hot PA arrangements take some hot air after the air heater, but that is the PA fan's duty.
How do I know if my FD fan is oversized?
Signs include inlet vanes or dampers running mostly closed, high noise, and power far above what the air flow needs. Comparing the measured duty point against the fan curve shows how far it is from best efficiency. A new impeller or a speed change often corrects it.
Information needed to quote an FD fan for pulverised coal industrial boilers
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Air split. Secondary and primary air flows at full and minimum load, and the PA arrangement.
- Parallel running. Number of fans and whether one fan must carry partial load alone.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.