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 FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Plenum compartments
- Distributor nozzles
- Ash bed with in-bed tubes
The fan delivers ambient air through the air heater to a compartmented plenum under the distributor plate. On high ash coal boilers with in-bed tubes, the bed is relatively deep and dense because it is made of the coal's own ash, and total pressure at the fan outlet is typically 900 to 1,200 mmWC. A separate PA fan supplies conveying air where coal is fed pneumatically underbed. Bed drain rate, more than load, sets the pressure the fan sees from hour to hour.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 150 to 220°C, downstream of the fan |
| Pressure at the fan outlet | Typically 900 to 1,200 mmWC |
| Dust | Coal dust in the boiler house |
| Turndown | Compartment slumping with speed or vane control |
| 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 high ash Indian coal AFBC boilers
| Blade form | Backward curved with a steep, stable curve |
| Material of construction | Mild steel; high strength impeller for tip speed |
| Drive and arrangement | Direct coupled, arrangement 8, two-pole or four-pole |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive with compartment dampers |
| Also specified | Inlet silencer and pressure margin for bed height variation |
What this FD fan has to cope with
Bed grows with high-ash coal
Coal with 40% ash adds bed material quickly. If draining falls behind, the bed deepens and the plenum pressure needed rises. A fan without margin then cannot keep the bed fluidised and compartments slump. Pressure margin and bed pressure monitoring help the operator manage drain rates.
In-bed tube erosion and air distribution
In-bed tubes on high ash coal boilers erode fastest where the air velocity is uneven, near worn or blocked nozzles and at the bed edges. Changes in plenum pressure shift the distribution set during commissioning. The fan must deliver steady plenum pressure through the campaign, so that tube wear stays even and nozzle condition, not fan behaviour, is the variable to manage.
Coarse stones and tramp material
Indian coal often contains stones, shale and tramp iron that do not burn and sink to the distributor plate, where they hinder fluidisation in patches. Bed drains remove them, but the fan must keep the rest of the bed moving meanwhile. That needs pressure in hand rather than a fan already running at the top of its curve.
Energy and control
The FD fan on a high ash coal AFBC boiler is a large, high pressure machine running continuously. Variable speed drives give the best savings when combined with compartment control. Keeping bed height in the design range avoids paying for extra pressure, and maintaining distributor nozzles prevents gradual pressure rise.
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 Induced Draft (ID) Fan for high ash Indian coal AFBC boilers.
Questions buyers ask
Why does the FD fan pressure rise on an Indian coal AFBC boiler?
Usually because the bed is getting deeper as ash from the coal accumulates faster than it is drained, or because distributor nozzles are blocked with fines or clinker. Checking bed pressure against the design value and adjusting the drain rate normally restores the plenum pressure. A steady rise over months points to nozzle condition instead.
Is a separate fan needed for coal feeding on an AFBC boiler?
With underbed pneumatic feeding, yes. A PA fan supplies conveying air at high pressure to carry the crushed coal through pipes to feed points in the bed. With overbed feeding through spreaders, the FD fan supplies all the combustion air, and the spreaders may take a small high pressure air supply of their own.
What FD fan pressure is typical for a high ash coal AFBC boiler?
Usually 900 to 1,200 mmWC at the fan outlet. That is higher than on biomass AFBC boilers, because the high ash coal bed is denser and deeper and in-bed tubes add to the bed resistance. We confirm the figure from the boiler data or from measured plenum pressure at site, and add margin for bed height variation.
Information needed to quote an FD fan for high ash Indian coal AFBC 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.
- Bed design. Bed depth, compartments, in-bed tubes, and design plenum pressure.
- Coal feed. Overbed or underbed feed, and the PA fan duty if separate.
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.