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 ID fan sits
- Bubbling bed and freeboard
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan follows the final collector. On many 10 to 30 TPH units the collector is a multiclone followed by an ESP or bag filter, but older plants may have only a multiclone and a wet scrubber after the fan. The fan works against the freeboard draft, the boiler and economiser banks, the air heater and the collectors, typically 200 to 300 mmWC at the inlet. The furnace is held at slight suction above the bed so that hot bed material does not puff out of the feed points.
Gas at the ID fan
| Gas temperature, normal | 145 to 170°C after the air heater |
| Maximum or upset | Up to 210°C when the air heater is fouled or bypassed |
| Dust at the fan | Fine ash with bed sand fines; 50 to 150 mg/Nm³ after an ESP or bag filter, several g/Nm³ after a multiclone only |
| Moisture and dew point | Varies with blend; water dew point 45 to 60°C |
| Corrosive species | Chlorides and alkali from some agro-residues; sulphur is low unless coal is co-fired |
| Density at the design temperature | About 0.81 kg/m³ at 165°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 biomass AFBC boilers
| Blade form | Backward curved with liners after an ESP or bag filter; radial tipped where only a multiclone precedes the fan |
| Material of construction | Mild steel casing; wear-resistant plate or lined impeller |
| Drive and arrangement | Direct coupled, arrangement 8, moderate tip speed |
| Bearings and cooling | Pedestal bearings outside the gas stream with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Scroll liners at the cut-off, casing drain, access door, and margin for blend changes |
What this ID fan has to cope with
Erosion from elutriated bed sand
Silica sand is added to start the bed and wears into fines that leave with the gas. Sand is harder than most biomass ash and wears blade tips and the scroll cut-off quickly if the collector lets it through. Liners at these points and a fan speed kept below about 1,000 rpm extend impeller life between shutdowns.
Unburnt fines from light fuels
Light fuels such as husk, sawdust and shell fines can lift out of the bed before they burn and finish in the freeboard, leaving unburnt carbon in the fly ash. That carbon settles in the fan casing and inlet box and can smoulder after a trip. A casing drain, an access door and a short purge run before restart let the operator clear deposits safely.
Fuel blend changes the duty
A plant that moves from dry groundnut shell to fresh wood chips or wet coconut residues raises both the gas volume and its moisture content within days. A fan sized on a single design fuel then runs at full speed with the damper wide open. Sizing on the actual range of blends fired through the year, with speed control, keeps headroom available for the wet months.
Energy and control
Biomass AFBC boilers in process industries often follow steam demand from shift to shift, and fuel blends change with availability. An ID fan on a variable speed drive tied to furnace draft typically saves 25 to 40% of fan power compared with damper control at 60 to 80% load. Sealing the bed ash drain, feeders and hoppers against air ingress reduces the gas volume further.
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 Forced Draft (FD) Fan for biomass AFBC boilers.
Questions buyers ask
What causes vibration in the ID fan of an AFBC boiler?
Usually uneven erosion of the blades or patchy ash deposits on them. Erosion removes metal unevenly while deposits add it unevenly, and both shift the balance of the impeller. On AFBC boilers, sand fines escaping the collector are a common cause. Regular vibration readings, balancing at each shutdown, and liners replaced as a full set rather than piecemeal keep the fan steady.
Is an ID fan for a biomass AFBC boiler different from one for a coal AFBC boiler?
The arrangement is similar, but biomass gives more flue gas per tonne of steam, wetter gas and lighter, finer ash, while coal gives heavier and more abrasive ash with some sulphur. The biomass AFBC boiler ID fan is often larger in volume and less heavily lined, but it needs more margin for the moisture swing between fuels.
Can a wet scrubber be placed before the ID fan?
It can, but the fan then handles saturated gas at 55 to 65°C carrying droplets, and needs stainless or lined construction, drains and a mist eliminator upstream. Placing the fan before the scrubber keeps it dry and in mild steel, but it then has to tolerate the dust that the multiclone lets through. The choice depends on the collector and the emission limit.
Information needed to quote an ID fan for biomass 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.
- Fuel blends. The fuels and blend ratios fired through the year, with moisture and ash.
- Bed material. Bed sand type and make-up rate, and how much ash and sand is drained from the bed.
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.