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 ID fan sits
- Furnace and burners
- Superheater
- Economiser
- Air heater
- ESP
- ID fan
- Chimney
The fan, often two in parallel on larger units, sits after the ESP. Suction at the inlet is typically 250 to 400 mmWC, covering the furnace, back pass, air heater and ESP. Because pulverised firing sends most of the ash to the ESP, inlet dust is commonly 40 to 80 g/Nm³ on Indian coal, and the fan is protected only as well as the ESP performs. Air heater leakage adds to the gas volume.
Gas at the ID fan
| Gas temperature, normal | 140 to 165°C after the air heater |
| Maximum or upset | Up to 200°C with fouled air heater or a mill out |
| Dust at the fan | Fine fly ash with silica and alumina; 50 to 100 mg/Nm³ after the ESP, much more with a field out |
| Moisture and dew point | Water dew point about 45 to 55°C |
| Corrosive species | Coal sulphur typically 0.4 to 0.8% for Indian coal, higher for some imported coals; acid dew point 110 to 135°C |
| Density at the design temperature | About 0.83 kg/m³ at 155°C, 69% 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. |
ID Fan selection for pulverised coal industrial boilers
| Blade form | Backward curved aerofoil or plate, with wear protection at blade inlets |
| Material of construction | Mild steel casing; impeller with wear-resistant plate or hard-faced leading edges |
| Drive and arrangement | Direct coupled, double inlet arrangement 6 or 8 on larger units |
| Bearings and cooling | Pedestal bearings with forced or oil-bath lubrication and cooling disc |
| Flow control | Variable speed drive or hydraulic coupling; inlet vanes on older units |
| Also specified | Vibration monitoring, inspection doors, isolating dampers for parallel fans |
What this ID fan has to cope with
Fly ash erosion when ESP fields trip
With 40 to 80 g/Nm³ at the ESP inlet, losing even one field puts a large dust load on the fan. Pulverised coal fly ash is fine, so it erodes evenly across the blades rather than in deep grooves, but the wear is fast. Wear protection at blade inlets and a load reduction rule when a field trips limit damage.
Air heater leakage raising fan duty
Rotary and tubular air heaters both leak as they age. Leaking combustion air adds volume at the ID fan and cools the gas. Over time the fan approaches its limit at full load. Measured oxygen rise across the air heater shows how much leakage to allow for.
Load swings from process demand
Paper and chemical plants change steam demand quickly when machines or reactors start and stop. Furnace pressure must be held while mills and burners respond. A drive with a fast draft loop and enough speed range keeps the furnace stable without tripping on high or low pressure.
Energy and control
Industrial pulverised coal boilers often run between 60 and 100% load following process demand, and the ID fan is the largest auxiliary drive. Inlet vane or damper control wastes much of its power below full load. A variable speed drive on furnace draft gives a substantial saving, and keeping air heater leakage and ESP pressure drop low reduces both gas volume and fan pressure.
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 Forced Draft (FD) Fan for pulverised coal industrial boilers.
Questions buyers ask
Why does the ESP matter so much for a pulverised coal boiler ID fan?
Pulverised firing sends around 80% of the coal ash out of the furnace as fly ash, so the ESP handles a very heavy load. The ID fan sees only what the ESP lets through. If a field trips, dust at the fan rises sharply and the impeller wears fast.
Should an industrial PC boiler have one or two ID fans?
Larger units often use two fans in parallel so the boiler can keep running at reduced load if one fan trips. Smaller units use one. With two induced draft fans, a stable curve and matched controls are essential to avoid hunting at part load. Each fan is usually sized for about 60% of full load.
How much does air heater leakage affect the ID fan?
Leakage of 8 to 15% is common on older rotary air heaters, and all of it must be handled by the ID fan. It raises volume and fan power. Repairing seals can recover a meaningful part of the fan's capacity and reduce power at the same time.
Information needed to quote an ID fan for pulverised coal industrial boilers
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- ESP. Number of fields, inlet and outlet dust, and how often fields trip.
- Air heater. Type of air heater and measured leakage from oxygen readings.
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.