How high ash Indian coal AFBC boilers work
AFBC boilers were widely adopted in India because a bubbling bed can burn high-ash domestic coal, washery rejects and middlings that grate boilers handle poorly. Coal with 35 to 50% ash and 2,800 to 4,000 kcal/kg is crushed to below about 6 mm and fed into a bed held at 820 to 900°C, where in-bed evaporator tubes absorb a large share of the heat. The coal's own ash forms the bed, so bed material is drained continuously to hold bed height. Much of the remaining ash elutriates as fine, quartz-rich fly ash, often 40 to 80 g/Nm³ before the collectors. The gas passes the boiler bank, economiser and air heater, then a multiclone and ESP, before the ID fan and chimney.
Where the ID fan sits
- Bubbling bed with in-bed tubes
- Freeboard
- Boiler bank and economiser
- Air heater
- Multiclone
- ESP
- ID fan
- Chimney
The fan follows the ESP, which on these boilers is often three or four fields deep because the inlet dust load is so high. A multiclone ahead of the ESP takes the coarse fraction. Suction at the fan inlet is typically 250 to 350 mmWC. On older plants, ESPs designed for lower-ash coal can be overloaded when the plant switches to rejects, and the dust at the fan rises accordingly.
Gas at the ID fan
| Gas temperature, normal | 140 to 170°C after the air heater |
| Maximum or upset | Up to 210°C with eroded or fouled air heater tubes |
| Dust at the fan | Quartz-rich fly ash; 100 to 300 mg/Nm³ after an ESP, far more when fields trip |
| Moisture and dew point | Water dew point 40 to 50°C; washed coal can be wetter |
| Corrosive species | Low sulphur, 0.3 to 0.8%; acid corrosion is rarely limiting |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 high ash Indian coal AFBC boilers
| Blade form | Radial tipped or backward curved with heavy liners, depending on measured dust at the fan |
| Material of construction | Mild steel casing with replaceable scroll liners; impeller in wear-resistant plate |
| Drive and arrangement | Direct coupled, arrangement 8, at four-pole or six-pole speed to keep tip speed low |
| Bearings and cooling | Pedestal bearings with cooling disc and vibration monitoring |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Bolted liners at the cut-off and side plates, inspection doors, and a spare rotor policy |
What this ID fan has to cope with
Heavy quartz fly ash
Indian coal ash contains a large fraction of free quartz, harder than most wear plates. Even after a good ESP, the fan handles enough fine quartz over a year of running to thin blades noticeably. Low tip speed, wear-resistant plate and bolted liners that can be replaced at site keep the fan running between planned shutdowns.
ESP field trips
An ESP that loses a field for a few hours can pass ten times its normal outlet dust. On high-ash coal, a single bad week can remove as much metal from the blades as a year of normal running. Monitoring ESP performance and fan vibration together gives early warning of rapid wear.
Changes to washery rejects
Plants that switch from washed coal to rejects to cut fuel cost raise both ash content and gas volume per tonne of steam. The ESP and fan were often sized for the better coal. The fan then runs at full speed and the higher dust load accelerates wear. Checking the duty before the fuel change avoids surprises.
Energy and control
High ash coal AFBC boilers in captive and process plants run continuously at 60 to 100% load. A variable speed ID fan saves considerably against damper control, and replacing a worn, re-welded radial wheel with a new lined wheel recovers efficiency. Sealing the bed drain and air heater against air ingress lowers both gas volume and dust carried to the fan.
ID fan and FD fan in high ash Indian coal AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, before the chimney | Before the air heater, feeding the bed plenum |
| What it handles | Flue gas with heavy quartz fly ash | Clean ambient air |
| Temperature at the fan | 140 to 170°C | Ambient |
| Main design concern | Abrasive ash and ESP reliability | Fluidising a deep, dense ash bed |
| Control | Furnace draft | Bed air and compartments |
See also the Forced Draft (FD) Fan for high ash Indian coal AFBC boilers.
Questions buyers ask
Which blade type is best for an ID fan on a high ash coal AFBC boiler?
Radial tipped blades tolerate heavy dust best, but are less efficient. Backward curved blades with heavy liners are more efficient and work well after a reliable ESP. The choice depends on the measured dust at the fan, the ESP's trip history and how the plant values efficiency against maintenance.
How long should an ID fan impeller last on an Indian coal AFBC boiler?
With a well-maintained ESP, a lined impeller in wear-resistant plate typically runs several years, with liner changes at planned shutdowns. With frequent ESP field trips, or only a multiclone ahead of the fan, life can fall to months. Keeping tip speed low is the single most effective design choice, because erosion rises steeply with velocity.
Can a worn ID fan rotor from an AFBC boiler be rebuilt?
Yes. A worn rotor can often be rebuilt with new blades, liners and hard facing, then dynamically balanced. Where wear is severe, a new rotor in wear-resistant plate, designed for the actual duty, is usually better value. We do both, for any make of fan.
Information needed to quote an ID fan for high ash Indian coal AFBC 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.
- Coal and rejects. Ash content of coal and rejects fired, and the blend ratio.
- ESP data. Number of fields, outlet dust and how often fields trip.
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.