How biomass AFBC boilers work
A biomass AFBC boiler burns crushed or chipped biomass in a bubbling bed of sand and ash, about 600 to 900 mm deep, fluidised at 1.5 to 2.5 m/s and held at 750 to 850°C. Rice husk, groundnut shell, wood chips, coconut residues and mixed agro-waste are common, often in blends with coal. Fuel is fed overbed by spreader or underbed through pneumatic points. Fine ash and some sand elutriate from the bed and leave with the gas, while coarse material is drained from the bed. The gas passes the boiler bank, economiser and air heater, then a multiclone and an ESP or bag filter before the ID fan. The FD fan supplies fluidising air under the distributor plate, and an overfire air connection or small secondary fan completes combustion of volatiles.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Air plenum compartments
- Distributor nozzles
- Bubbling bed
The FD fan delivers ambient air to the air heater and then to the plenum below the distributor plate, which is usually divided into compartments so that sections of the bed can be slumped for turndown. Resistance includes the air heater, the plenum dampers, the nozzles and the bed itself, typically 700 to 1,000 mmWC in total. Overfire air for volatile burnout is taken from the same fan through a branch or from a small separate fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 120 to 200°C, downstream of the fan |
| Pressure at the fan outlet | Typically 700 to 1,000 mmWC |
| Dust | Clean air, but the fuel yard can be dusty |
| Turndown method | Compartment slumping plus 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 biomass AFBC boilers
| Blade form | Backward curved with a rising pressure curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, two-pole or four-pole speed |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive with compartment dampers |
| Also specified | Inlet screen and silencer; branch for overfire air where used |
What this FD fan has to cope with
Uneven fluidisation
If air pressure falls at part load, compartments far from the main duct fluidise poorly, and parts of the bed run hot and still. Biomass ash rich in potassium then fuses into clinker in those spots. A fan that holds plenum pressure through the load range, with balanced ducting and individual compartment dampers, keeps every active compartment fluidised evenly.
Clinker raises plate resistance
Biomass ash with high potassium and low calcium can sinter the bed into lumps. As clinker forms, the bed resists fluidisation and more plenum pressure is needed to keep it moving. The FD fan needs pressure margin over the clean bed condition so that the operator can keep the bed active while draining clinker and adding fresh sand.
Nozzle blockage after a trip
When the boiler trips, the bed slumps and fine bed material can fall back into the distributor nozzles, partly blocking them. At restart the plenum pressure needed to break the bed loose is higher than in normal running. A fan with pressure margin for partly blocked nozzles at restart avoids a cold, slumped bed and repeated failed light-ups.
Energy and control
The fluidising air fan cannot be turned down freely because the bed needs a minimum velocity, so AFBC boilers turn down by slumping compartments. A variable speed drive matched to the active compartments saves energy compared with an outlet damper. Keeping the air heater tight against leakage and holding oxygen at the economiser outlet near target avoids pushing air the bed does not need.
ID fan and FD fan in biomass AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the final collector, before the chimney | Before the air heater, feeding the plenum |
| What it handles | Flue gas with biomass ash and sand fines | Clean ambient air |
| Temperature at the fan | 145 to 170°C | Ambient |
| Main design concern | Sand erosion and blend changes | Bed pressure and even fluidisation |
| Control | Furnace draft | Bed air and active compartments |
See also the Induced Draft (ID) Fan for biomass AFBC boilers.
Questions buyers ask
How much pressure does an AFBC boiler FD fan need?
Usually 700 to 1,000 mmWC at the fan outlet, covering the air heater, ducts and compartment dampers, the distributor nozzles and the bed itself. Deeper beds, finer nozzles and heavier bed material need more. We confirm the figure from the boiler design or from plenum pressure measured at site, and add margin for clinker and nozzle blockage.
Why does the AFBC boiler FD fan current rise over time?
Clinkering of the bed, sand build-up from over-filling, blocked nozzles, or a leaking air heater all raise either resistance or air flow, and the fan draws more power. Comparing the plenum pressure with the bed height and with the figures recorded when the bed was fresh usually shows which cause is at work.
Can one FD fan supply both bed air and overfire air on a biomass AFBC boiler?
Yes. On many smaller units a branch from the FD fan supplies overfire air above the bed through a set of nozzles, controlled by its own damper. Where the overfire nozzles need more pressure or a separately controlled flow, a dedicated small fan is used instead, which also makes combustion tuning on different biomass blends easier.
Information needed to quote an FD fan for biomass 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, number of compartments and plenum pressure at full load.
- Overfire air. Whether overfire air comes from the FD fan or a separate fan, and its quantity.
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.