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 ID fan sits
- Fluidised bed and freeboard
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan is placed after the ESP or bag filter and pulls the furnace to a slight negative pressure above the bed, usually 3 to 5 mmWC. Upstream are the convection bank, economiser, tubular air heater, multiclone and final collector, so the suction at the fan is typically 200 to 300 mmWC. The multiclone takes most of the coarse sand and ash before the ESP. Any loss of ESP field or bag damage puts the full fines load onto the fan, so the fan is built to survive that period.
Gas at the ID fan
| Gas temperature, normal | 140 to 170°C after the air heater |
| Maximum or upset | Up to 220°C during bed slumping and restart or with fouled air heater |
| Dust at the fan | Silica ash and fine bed sand; typically 50 to 150 mg/Nm³ after ESP, much higher if a field trips |
| Moisture and dew point | Moderate; water dew point well below gas temperature on dry husk |
| Dust inlet loading upstream of collectors | High, often 15 to 30 g/Nm³, as nearly all ash is elutriated |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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. |
ID Fan selection for rice husk fired AFBC boilers
| Blade form | Backward curved with wear liners, or radial tipped where the collector is a multiclone only |
| Material of construction | Mild steel casing with liners at the scroll; impeller blades in wear-resistant plate |
| Drive and arrangement | Direct coupled, arrangement 8, moderate tip speed |
| Bearings and cooling | Pedestal bearings with cooling disc; vibration sensors on both bearings |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Replaceable liners, access door, casing drain and a pressure margin for fouled air heater |
What this ID fan has to cope with
Sand fines in the gas
Fluidised bed sand breaks down into fine, sharp particles that are carried over with the husk ash. Together they erode harder than husk ash alone, and the wear concentrates at the blade inlet edge and the scroll cut-off. A good multiclone ahead of the ESP, and bolted liners on the impeller and scroll, are the design answer.
Draft upset during bed slumping
When the FD fan trips or the bed is slumped for maintenance, the furnace pressure collapses and restart brings a surge of gas and dust. The ID fan control must hold draft through these events, so its drive is tuned for fast response and it carries a temperature margin for the hot restart.
Air heater fouling and leakage
Husk ash with fine sand deposits in tubular air heaters and erodes tube ends, raising resistance and adding leakage air. Over a season the fan duty creeps up. We size the fan with pressure margin for the fouled condition and leakage measured on similar plants.
Agglomeration and high excess air
If the bed agglomerates and operators raise air to keep it moving, gas volume rises sharply and the furnace draft becomes hard to hold. The fan needs capacity margin for high excess air operation, but not so much that it runs heavily throttled for the rest of the year. A variable speed drive resolves this conflict.
Energy and control
AFBC boilers in parboiling plants and captive power units often run at 60 to 90% load with long continuous runs. A variable speed drive on the ID fan cuts power compared with damper control, and also gives smoother furnace draft, which helps bed stability. Keeping the air heater clean is the second lever: each extra 25 mmWC of resistance costs fan power every hour the boiler runs.
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 Forced Draft (FD) Fan for rice husk fired AFBC boilers.
Questions buyers ask
Why does the ID fan wear faster on a rice husk AFBC boiler than on a grate boiler?
Nearly all the husk ash is carried out of the bed, along with fine sand worn from the bed material. The dust load to the collectors is higher than on a grate, and when the collector underperforms the fan sees hard, sharp particles. Liners and a good multiclone before the ESP control the wear.
What suction does the ID fan on an AFBC husk boiler work against?
Usually about 200 to 300 mmWC at the fan inlet, covering the furnace draft, convection bank, economiser, air heater, multiclone and ESP or bag filter. The figure rises as the air heater fouls and as bags age, so a margin is built in. Measuring static pressure at each point on site gives the most reliable figure for a replacement fan.
Should an AFBC boiler ID fan have a VFD?
Yes in most cases. A variable speed drive saves energy at part load and holds furnace draft steadily during bed disturbances, which helps fluidisation and reduces puffing at the feed points. An inlet damper is still fitted for isolation and start-up, and the drive is interlocked with the fluidising air fan so the two move together.
Information needed to quote an ID fan for rice husk fired AFBC boilers
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Bed material. Type of bed material, make-up rate and typical sand fines in fly ash if known.
- Collector arrangement. Whether a multiclone precedes the ESP or bag filter, and the ESP outlet dust.
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.