How biomass CFBC boilers work
A biomass CFBC boiler burns chipped wood, agro-residues or mixed biomass in a fast, dilute bed of sand and ash fluidised at 4 to 6 m/s. Solids leave the top of the furnace, are caught in hot cyclones and returned through loop seals, so combustion stays near 800 to 870°C across the whole furnace height. Fuels usually carry 25 to 50% moisture, 3 to 15% ash, and potassium and chlorine that foul superheaters and make the bed prone to agglomeration. After the cyclones, the gas passes superheaters, economiser and a tubular air heater, then an ESP or bag filter, the ID fan and the chimney. A primary air fan fluidises the bed through the nozzle grid, a secondary air fan stages combustion higher up, and small high pressure blowers drive the loop seals.
Where the ID fan sits
- Riser furnace
- Hot cyclones
- Superheaters and economiser
- Air heater
- ESP or bag filter
- ID fan
- Chimney
The fan sits after the ESP or bag filter, so it sees only the fines that escape the final collector: a mix of biomass ash, fine bed sand worn down in the loop and some unburnt carbon. Upstream resistance is high because the cyclones, the back-pass tube banks and the air heater all add loss, and the fan must also hold a small suction at the furnace top. On a 20 to 60 TPH unit the fan inlet suction is typically 300 to 450 mmWC, often split across two fans running in parallel.
Gas at the ID fan
| Gas temperature, normal | 140 to 165°C after the air heater |
| Maximum or upset | Up to 200°C with fouled back-pass surfaces or cyclone burning |
| Dust at the fan | Fine biomass ash with worn bed sand; 30 to 100 mg/Nm³ after an ESP or bag filter |
| Moisture and dew point | Water vapour of 15 to 25% by volume with wet fuel; water dew point 55 to 65°C |
| Corrosive species | HCl and potassium chloride from straw and agro-residues; corrosion risk if casing surfaces fall below about 120°C |
| 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 biomass CFBC boilers
| Blade form | Backward curved, with wear protection at the leading edges for the sand fraction |
| Material of construction | Mild steel casing; impeller in high strength steel with wear-resistant liners on the blade inlet |
| Drive and arrangement | Arrangement 8, or double inlet arrangement 6 for large volumes, direct coupled |
| Bearings and cooling | Pedestal bearings outside the gas path with a cooling disc; temperature and vibration sensors |
| Flow control | Variable speed drive on furnace pressure, inlet damper for isolation |
| Also specified | Casing insulation to keep metal above the acid and chloride dew point, drains, and margin for the wettest fuel |
What this ID fan has to cope with
Gas volume swing with fuel moisture
Moving from dry husk to fresh wood chips can raise the flue gas volume per tonne of steam by a quarter or more, because every kilogram of water in the fuel leaves as vapour. An ID fan sized on design fuel runs out of capacity in the monsoon. The duty is set on the wettest fuel the plant expects to fire, with speed control to absorb the range.
Chloride corrosion at cold spots
Agro-residues and some wood species release chlorine, and HCl condenses with water on surfaces below about 120°C. Uninsulated casings, inspection doors and the shaft seal area corrode first. Casing insulation, sealed doors and a short warm-up with hot gas before full flow prevent wet starts.
Sand fines in the gas
Silica bed sand attrites in the cyclone loop into fine particles that pass the collector. After a bag filter the quantity is small, but during bag failures or ESP field trips the fan sees much more. Leading edge liners give margin for these periods without resorting to heavy radial blades.
Energy and control
Biomass CFBC boilers in power plants run near full load for long periods, but fuel moisture shifts the gas volume by the week. A variable speed drive on the ID fan follows these shifts without throttling. Reducing air ingress at the air heater, ash hoppers and expansion joints lowers the gas volume the fan must handle, and on two-fan installations running both fans at matched speed avoids one fan working near stall.
ID fan and FD fan in biomass CFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter, before the chimney | Before the air heater, feeding the windbox |
| What it handles | Humid flue gas with biomass ash and sand fines | Clean ambient air |
| Temperature at the fan | 140 to 165°C, up to 200°C upset | Ambient |
| Main design concern | Gas volume range with fuel moisture | High pressure and stable minimum flow |
| Control | Furnace pressure | Fluidising air flow |
See also the Forced Draft (FD) Fan for biomass CFBC boilers.
Questions buyers ask
What size ID fan is needed for a biomass CFBC boiler?
The size is set by flue gas volume at the fan inlet, which depends on fuel moisture, excess air and air ingress, and by the suction needed to pull gas through the cyclones, back-pass, air heater and collector. We work from the boiler heat balance for the wettest fuel and confirm against measured data on existing plants.
Why does my CFBC ID fan run at maximum speed in the rainy season?
Wet fuel adds water vapour to the flue gas and raises its volume. The fan has to move more gas at the same furnace pressure, so it speeds up. If it reaches full speed before full load, the fan was sized on drier fuel and may need rerating with a new impeller or a higher speed.
Do biomass CFBC boilers use one or two ID fans?
Smaller units often use a single ID fan. Larger units commonly use two fans in parallel, each sized for 50 to 60% of the full gas flow, so that the boiler can keep running at reduced load if one fan trips. Parallel fans need matched curves and isolating dampers, and each must be stable when running alone at its higher share of the flow.
Information needed to quote an ID fan for biomass CFBC 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 range. The wettest and driest fuels expected, with moisture, ash and chlorine content if known.
- Fan arrangement. Whether one fan or two in parallel is required, and the capacity each must carry alone.
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.