How coal washery rejects AFBC boilers work
Some captive plants burn coal washery rejects, with 45 to 60% ash and 1,800 to 2,800 kcal/kg, in bubbling bed AFBC boilers, usually blended with middlings or better coal to keep the bed hot. The crushed rejects are fed over or under the bed into a bubbling bed at 800 to 900°C, where in-bed tubes absorb much of the heat. A large share of the ash stays in the bed and is drained continuously, while fines are elutriated and leave with the gas through the superheater, convection bank, economiser and air heater. A multiclone and an ESP clean the gas before the ID fan. Unburnt carbon in the fly ash is higher than on a CFBC, and fly ash is sometimes returned to the bed.
Where the FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Compartment air boxes
- Nozzles
- Bubbling bed
The fan stands at ground level and pushes ambient air through the air heater into a distribution duct serving each bed compartment. Fan outlet pressure is typically 850 to 1,200 mmWC. Where rejects are fed under the bed, a separate transport air fan carries the crushed fuel. Overbed air may come from a branch of the FD fan or a separate fan. Bed ash drains in each compartment hold the bed at its design height.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 150 to 200°C, downstream of the fan |
| Pressure at the fan outlet | Typically 850 to 1,200 mmWC |
| Bed pressure range | Varies widely between ash drains |
| Dust | Clean air; reject dust from crushing near the boiler |
| 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 coal washery rejects AFBC boilers
| Blade form | Backward curved, high pressure, with a steeply rising curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings; no cooling needed |
| Flow control | Variable speed drive on steam pressure, with compartment dampers |
| Also specified | Inlet silencer and filter; bed pressure indication per compartment |
What this FD fan has to cope with
Fast-growing bed
With 45 to 60% ash in the fuel, the bed builds up within hours if the drain falls behind. Bed pressure and fan pressure rise together. A fan selected with pressure margin over the highest bed allowed between drains, and a bed pressure-based drain control, prevents fluidisation loss and the clinkering that follows.
Nozzle wear and air maldistribution
Hard bed material wears nozzles and their holes enlarge unevenly. Air then takes the easiest path, and dead zones form where the bed sinters. The FD fan cannot correct this on its own; checking nozzle condition and grid pressure drop at outages keeps the air distribution right.
Coarse ash settling at low air flow
Rejects ash includes coarse, dense particles that need more air to stay fluidised. If air flow is cut too far at low load, these settle and form a dead layer on the grid. A minimum air flow per compartment set above the coarse fraction's fluidising velocity prevents it.
Energy and control
On a rejects AFBC the FD fan carries high pressure continuously. The largest saving usually comes from keeping the bed at design height by regular drain, not from fan control alone. A variable speed drive then follows load and compartment changes efficiently, and trimming air on flue gas oxygen avoids heating excess air with a fuel that has little heat to spare.
ID fan and FD fan in coal washery rejects AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multiclone and ESP | Ahead of the air heater and air boxes |
| What it handles | Flue gas with abrasive ash and char | Clean ambient air |
| Temperature at the fan | 145 to 170°C | Ambient, 25 to 45°C |
| Main design concern | Erosion behind the ESP | Pressure margin for a fast-growing bed |
| Control | Furnace draft | Steam pressure and bed fluidisation |
See also the Induced Draft (ID) Fan for coal washery rejects AFBC boilers.
Questions buyers ask
Why does FD fan pressure keep rising on a washery rejects AFBC?
The fuel's high ash builds the bed up quickly. As bed height rises, more pressure is needed to fluidise it. If the bed drain is not keeping pace, the forced draft fan pressure climbs until it reaches its limit. Draining the bed to a set bed pressure solves this.
Does a rejects AFBC need a separate transport air fan?
If the rejects are fed under the bed through pneumatic pipes, yes. A high pressure transport air fan carries the crushed fuel into the bed. Overbed feeding by spreaders or screws avoids this fan. The choice depends on rejects size, moisture and the boiler design.
How much margin should an AFBC FD fan have?
Margin on pressure matters more than margin on flow. For washery rejects, the fan should handle the highest bed pressure allowed between drains plus the air heater and duct losses, with a further allowance for nozzle and air heater fouling. The boiler maker's bed design is the starting point.
Information needed to quote an FD fan for coal washery rejects 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 height and pressure, and the bed drain arrangement per compartment.
- Feeding. Overbed or underbed feeding, and transport air flow if underbed.
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.