How rice husk fired AFBC boilers work
In an atmospheric fluidised bed (AFBC) boiler, rice husk is fed into a bed of hot sand, typically 300 to 400 mm deep at rest, kept fluidised by air blown up through nozzles in a distributor plate. The bed runs at about 800 to 900°C, low enough to stay below the softening point of husk ash and to limit NOx. Bed and in-bed tubes give high heat transfer, and the boiler handles wetter or mixed husk better than a grate. Husk burns largely in the freeboard, and almost all its silica ash leaves with the gas along with fine sand worn from the bed. The gas passes the boiler bank, economiser and air heater, then a multiclone and ESP or bag filter before the ID fan.
Where the FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Windbox
- Distributor nozzles
- Fluidised bed
The fan sits beside the boiler and takes ambient air through an inlet silencer. It discharges through the air heater into the windbox under the distributor plate. The pressure needed is the sum of air heater and duct losses, the distributor nozzle drop and the weight of the fluidised bed, typically 800 to 1,200 mmWC at the fan outlet. Overbed or secondary air, and pneumatic husk feeding air where used, come from separate fans or a tapping from this one.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Typically 800 to 1,200 mmWC |
| Air after the air heater | 150 to 200°C, downstream of the fan |
| Minimum flow | Must stay above minimum fluidising velocity at all loads |
| Noise | High at this pressure; inlet silencer usually required |
| 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 rice husk fired AFBC boilers
| Blade form | High pressure backward curved, narrow width, for a steep stable curve |
| Material of construction | Mild steel; impeller designed for high tip speed with stress checks |
| Drive and arrangement | Direct coupled at 2,900 or 1,450 rpm depending on size, pedestal bearings |
| Bearings and cooling | Anti-friction bearings, no cooling on ambient air; vibration monitoring recommended |
| Flow control | Variable speed drive or inlet guide vanes, with a minimum flow limit in the control logic |
| Also specified | Inlet silencer, acoustic lagging where needed, non-return damper if a standby fan is fitted |
What this FD fan has to cope with
Bed slumping on fan trip
Loss of fluidising air drops the bed onto the distributor and can let hot sand back into the nozzles, and the boiler must be restarted from a slumped bed. Reliable bearings, vibration monitoring, a well-supported motor and, on larger units, a standby fan or two 60% fans reduce trips and the lost production that follows.
Operating near the surge region
At low load or with a partly blocked distributor, a fan selected too far to the left of its curve can become unstable and pulse the bed. We select the fan so that minimum fluidising flow at maximum bed pressure still falls on the stable part of the curve, with a safe margin from the peak.
Noise at high pressure
An 800 to 1,200 mmWC fan running at 2,900 rpm is loud at the inlet and radiates noise from the casing and ducts. An inlet silencer and, where needed, acoustic lagging on the casing and discharge duct keep the boiler house within acceptable levels, which matters where operators work beside the feeders all shift.
Energy and control
The fluidising air fan is usually the largest motor on an AFBC husk boiler. Bed height control matters as much as fan control: every extra 100 mm of bed adds roughly 100 mmWC of pressure. A variable speed drive with a minimum flow limit, a properly maintained distributor and controlled bed height together keep fan power close to what the boiler needs.
ID fan and FD fan in rice husk fired AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After ESP or bag filter | Before air heater and windbox |
| What it handles | Flue gas with husk ash and sand fines | Ambient fluidising air |
| Temperature at the fan | 140 to 170°C | Ambient, 25 to 45°C |
| Pressure | Suction 200 to 300 mmWC | Discharge 800 to 1,200 mmWC |
| Main design concern | Erosion from ash and sand | Stable high pressure, no trips |
| Control | Furnace draft | Load, with minimum fluidising flow |
See also the Induced Draft (ID) Fan for rice husk fired AFBC boilers.
Questions buyers ask
What is the FD fan pressure for an AFBC boiler burning rice husk?
Typically about 800 to 1,200 mmWC at the fan outlet, depending on bed depth, distributor design, air heater resistance and duct layout. The boiler designer's air system calculation gives the exact figure. On existing plants we confirm it by measuring windbox pressure and fan discharge pressure at normal bed height before selecting a replacement.
Why is a high pressure FD fan used for fluidised bed boilers?
The air must lift and fluidise a bed of sand several hundred millimetres deep, and pass through the distributor nozzles with enough pressure drop to spread evenly across the whole bed area. That needs several times more pressure than a grate boiler, where air only passes through a thin fuel layer on the grate bars.
Can we fit a VFD on the fluidising air fan?
Yes, with a minimum speed or flow limit in the control logic so that the bed never drops below minimum fluidising velocity, even at the lowest boiler load. The drive saves power at part load and gives smoother control than inlet vanes. It also allows soft starting of what is usually the largest motor on the boiler.
Information needed to quote an FD fan for rice husk fired 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 and distributor. Static bed height, distributor area and design windbox pressure.
- Redundancy. Whether one 100% fan or two 60% fans are required, and any standby arrangement.
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.