How groundnut shell AFBC boilers work
In an atmospheric fluidised bed combustion boiler, crushed groundnut shell is fed into a bed of sand held in suspension by air blown up through a distributor plate. The bed runs at about 800 to 850°C, and the shell burns within it and in the freeboard above. Groundnut shell ash is rich in potassium, which can react with silica sand and form sticky agglomerates if the bed runs hot, so operators keep bed temperature down and drain and replenish bed material. The flue gas carries fine bed sand from attrition as well as shell ash through the boiler bank, economiser, air heater, multiclone and ESP or bag filter to the ID fan. Shell-fired AFBC units are typically 8 to 30 TPH, and many were converted from coal.
Where the FD fan sits
- Inlet filter and silencer
- FD fan
- Air heater, where fitted
- Air box compartments
- Distributor nozzles
- Fluidised bed
The fan delivers ambient air into the air box under the distributor plate, on some units through an air heater. The air passes through bubble cap nozzles, whose pressure drop spreads it evenly, then lifts and suspends a static bed of typically 300 to 450 mm of sand. The fan must overcome the distributor, the weight of the bed and the ducting, typically 800 to 1,200 mmWC in all. The air box is divided into compartments with their own dampers so sections can be slumped at low load.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | Up to about 150°C where fitted; many units take ambient air straight to the bed |
| Pressure at the fan outlet | Typically 800 to 1,200 mmWC |
| Dust | Clean air; an inlet filter protects the nozzles |
| Minimum flow | Set by the minimum fluidising velocity of the bed in each active compartment |
| 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 groundnut shell AFBC boilers
| Blade form | Backward curved high-pressure wheel, narrow and large in diameter; radial for the highest pressures |
| Material of construction | Mild steel, with an impeller strengthened for high tip speed |
| Drive and arrangement | Direct coupled at 2,900 or 1,450 rpm depending on pressure |
| Bearings and cooling | Pedestal bearings rated for the higher speed |
| Flow control | Variable speed drive on steam demand, with compartment dampers for slumping |
| Also specified | Inlet filter and silencer, and non-return dampers so bed material cannot fall back to the fan on a trip |
What this FD fan has to cope with
Defluidisation at low load
If air flow falls below the minimum fluidising velocity, the bed settles, hot spots form and the potassium-rich shell ash agglomerates the sand. Turndown therefore comes from slumping whole compartments rather than reducing air to all of them. The FD fan must hold a stable pressure while compartments switch in and out.
Noise from a high-pressure fan
A fluidising air fan at high tip speed is loud, and the noise travels out through the intake and back through the air box. An inlet silencer, acoustic lagging on the casing and a flexible connection to the air box bring it down, which matters where the boiler house is near offices or housing.
Sand backflow on a trip
When the fan trips, the bed collapses and sand can sift back through the nozzles into the air box and duct, and in the worst case reach the fan. Nozzles with back-sifting protection, non-return dampers and an interlock that closes the dampers on fan stop protect both the fan and the distributor.
Energy and control
The fluidising air fan is the largest power consumer on an AFBC boiler, and because it works at high pressure, a few points of efficiency show directly in the power bill. Running fewer compartments at full fluidisation rather than all of them at reduced air, and a variable speed drive matched to bed depth, both cut power. Holding bed height at the design level by draining surplus sand also lowers the pressure the fan must develop.
ID fan and FD fan in groundnut shell AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Feeding the air box under the bed |
| What it handles | Flue gas with shell ash and bed sand | Ambient fluidising air |
| Temperature at the fan | 150 to 175°C | Ambient, 25 to 45°C |
| Pressure | Suction of 200 to 300 mmWC | Discharge of 800 to 1,200 mmWC |
| Main design concern | Sand erosion | Stable pressure and fluidisation |
| Control | Freeboard draft | Steam demand and compartment slumping |
See also the Induced Draft (ID) Fan for groundnut shell AFBC boilers.
Questions buyers ask
Why does an AFBC boiler need such a high-pressure FD fan?
The air has to pass the distributor nozzles and lift the sand bed, which together create a pressure drop of about 800 to 1,200 mmWC. A grate boiler FD fan works at 100 to 200 mmWC. The AFBC fan therefore uses a narrow, high-speed impeller and draws considerably more power for the same air flow.
How is turndown achieved on a groundnut shell AFBC boiler?
By slumping compartments of the bed rather than reducing air to all of them. Each active compartment needs at least its minimum fluidising air, so the FD fan supplies full fluidisation to those in service and the dampers on idle ones close. This avoids bed settling and the agglomeration that potassium-rich shell ash encourages.
Can I reduce forced draft fan power on an AFBC boiler?
Often, yes. Keeping bed height at the design level, maintaining the distributor nozzles, replacing damper throttling with a variable speed drive and changing an inefficient impeller can all reduce power. A site test of pressure and flow shows how far the present fan is from its best efficiency point.
Information needed to quote an FD fan for groundnut shell 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. Static bed height, number of compartments and distributor pressure drop.
- Minimum fluidisation. Minimum air flow per compartment specified by the boiler maker.
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.