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 FD fan sits
- Inlet silencer and screen
- FD (primary air) fan
- Air heater
- Windbox
- Nozzle grid
- Furnace bed
The primary air fan sits at ground level near the air heater and delivers ambient air into the air heater, then to the windbox under the nozzle grid. It must overcome the air heater, the grid nozzles, which are deliberately given a high pressure drop for even distribution, and the weight of the bed itself. Total pressure is commonly 1,000 to 1,500 mmWC. A separate secondary air fan feeds staged air higher in the furnace, and loop seal blowers handle the cyclone return legs.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 180 to 250°C, downstream of the fan |
| Pressure at the fan outlet | Typically 1,000 to 1,500 mmWC |
| Dust | Clean air; screened against biomass dust from the fuel yard |
| Flow range | Minimum fluidising flow must be held even at low load |
| 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 CFBC boilers
| Blade form | Backward curved at high tip speed, with a steep, stable pressure curve |
| Material of construction | Mild steel casing; high strength steel impeller for the tip speed |
| Drive and arrangement | Direct coupled at two-pole or four-pole speed, arrangement 8 |
| Bearings and cooling | Standard pedestal bearings; no cooling needed |
| Flow control | Variable speed drive or inlet vanes, with a minimum flow limit for fluidisation |
| Also specified | Inlet silencer, non-return damper where two fans run in parallel, and a stall-free margin at minimum flow |
What this FD fan has to cope with
Loss of fluidisation at low load
Biomass CFBC boilers turn down by reducing secondary air first, because primary air cannot fall below the flow that keeps the bed fluidised. A fan that becomes unstable near that minimum flow lets the bed slump and agglomerate. The fan is selected so that minimum fluidising flow sits well to the right of its stall point on the curve.
Bed agglomeration raises grid pressure
Potassium in biomass ash forms sticky coatings on bed sand, and as particles grow and fuse, the bed pressure drop and fluidising behaviour change. The operator then has to drain coarse bed and add fresh sand while keeping the bed moving. The fan needs pressure margin over the clean bed condition so that fluidisation can be held during this recovery instead of tripping the boiler.
Noise at high tip speed
A primary air fan at 1,000 mmWC and above runs at high tip speed and produces strong blade passing tones that carry out through the inlet and the casing walls. In an open boiler house this can exceed workplace limits. Inlet silencers sized for the flow, acoustic lagging on the casing and first duct sections, and flexible connections that stop structure-borne noise are normally part of the supply.
Energy and control
The primary air fan on a CFBC boiler is one of the largest auxiliary loads, because it supplies a large air flow at high pressure. A variable speed drive saves more than inlet vanes at part load, but the saving is limited by the minimum fluidising flow. Reducing grid nozzle wear and air heater leakage keeps the required pressure down. The secondary air fan, at lower pressure, gives most of the turndown.
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 Induced Draft (ID) Fan for biomass CFBC boilers.
Questions buyers ask
What is the difference between the PA fan and FD fan on a CFBC boiler?
On many CFBC boilers the FD fan and the primary air fan are the same machine, supplying fluidising air under the grid. Where a separate secondary air fan is also fitted, it is sometimes called the SA or FD fan. Both take ambient air and push it through the air heater, and both are selected at ambient density.
Why is the primary air fan pressure so high on a biomass CFBC boiler?
The air has to pass the tubular air heater, the windbox and the grid nozzles, which are designed with a deliberately high pressure drop so that air spreads evenly across the grid. It then has to support the weight of the bed above. Together these typically need 1,000 to 1,500 mmWC on industrial units, well above a grate-fired boiler FD fan.
Can a CFBC primary air fan be run with a variable speed drive?
Yes, and it is now common on new and retrofitted units. The control system must hold a minimum speed or flow that keeps the bed fluidised at every load, and the fan curve must remain stable at that minimum. Turndown below that point is then taken by the secondary air fan, which can be reduced much further without upsetting the bed.
Information needed to quote an FD fan for biomass CFBC 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 and grid data. Design bed pressure drop, grid nozzle pressure drop and minimum fluidising air flow.
- Air split. Primary, secondary and loop seal air quantities and which fan supplies each.
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.