How coal and biomass co-fired CFBC boilers work
CFBC boilers are well suited to burning coal with a large share of biomass, from wood chips and bagasse to husk and agricultural residue, because the circulating hot solids even out the differences in fuel reactivity. Fuels are fed into the lower furnace and burn at 820 to 880°C, with solids carried up, separated in a hot cyclone and returned through a loop seal. Biomass shares of 30 to 70% by heat are common in captive and independent power plants. Alkali and chlorine from biomass raise the risk of agglomeration in the loop seal and of corrosive deposits on the superheaters. After the back pass and air heater the gas passes an ESP or bag filter and the ID fan.
Where the FD fan sits
- Inlet silencer
- FD (primary air) fan
- Air heater
- Air box
- Distribution grid
- Fluidised bed
The primary air fan sits at ground level and pushes ambient air through the air heater to the air box. A separate secondary air fan supplies air to ports higher in the furnace, where biomass volatiles burn. Loop seal blowers fluidise the cyclone return. PA fan outlet pressure is typically 1,000 to 1,500 mmWC. Some designs also use primary air to carry biomass into the furnace through feed chutes.
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 |
| Share of total air | About 40 to 60% as primary air, falling as biomass share rises |
| Dust | Clean air; biomass dust near the fuel yard |
| 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 coal and biomass co-fired CFBC boilers
| Blade form | Backward curved, high pressure, with a stable characteristic over a wide flow range |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings, oil lubricated on larger fans |
| Flow control | Variable speed drive with protected minimum fluidising flow |
| Also specified | Inlet silencer and screen; non-return dampers for parallel fans |
What this FD fan has to cope with
Agglomeration in the bed and loop seal
Biomass alkalis form sticky compounds with bed sand, and agglomerates grow where fluidisation is weak, often in the loop seal or lower bed corners. The PA fan must hold fluidising air above the minimum at all loads and blends. Bed material refresh and bed temperature control are the other parts of the defence.
Changing PA share with fuel blend
On coal the boiler may run at 55 to 60% primary air; on high biomass, 40 to 45%. A PA fan selected only at the coal point may operate inefficiently or near its stall region on biomass. The selection is checked at both ends of the blend range.
Biomass fibre and dust at the intake
Biomass yards create fine dust and fibre. If the PA fan intake is near the yard, the material builds on the impeller and fouls the air heater. Taking air from a clean area and fitting a screen or filter with a regular cleaning schedule prevent this.
Energy and control
The primary air fan is one of the largest auxiliary loads in a co-fired CFBC plant. Its flow must stay above the fluidising minimum, so most turndown is taken by the secondary air fan. A variable speed drive on the PA fan, and bed inventory kept at design pressure, minimise the high pressure air it must supply. Checking fan power on each blend helps set the best operating split.
ID fan and FD fan in coal and biomass co-fired CFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Ahead of the air heater and grid |
| What it handles | Flue gas whose volume changes with blend | Clean ambient air |
| Temperature at the fan | 145 to 170°C | Ambient, 25 to 45°C |
| Main design concern | Wide gas volume range across blends | Stable fluidisation as PA share changes |
| Control | Furnace draft | Bed fluidisation, with SA taking turndown |
See also the Induced Draft (ID) Fan for coal and biomass co-fired CFBC boilers.
Questions buyers ask
Why does the primary air share change when we fire more biomass?
Biomass releases most of its volatiles quickly and burns higher in the furnace than coal. More of the air is therefore supplied as secondary air above the dense bed, and less as primary air through the grid. The forced draft (PA) fan's duty point moves accordingly.
What is the FD fan on a co-fired CFBC boiler?
On most CFBC boilers the primary air fan is the FD fan, supplying fluidising air through the grid at high pressure. A separate secondary air fan supplies the upper furnace. Both are needed, and on a co-fired unit their split changes with the fuel blend.
Can biomass be fed with primary air on a CFBC?
Some designs use a share of primary air, or a dedicated sealing air fan, to carry biomass through feed chutes and prevent back-flow of hot gas. This takes air from the PA system and must be included in the fan duty. Check the boiler maker's feed system design.
Information needed to quote an FD fan for coal and biomass co-fired 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.
- Air split. Primary and secondary air flows for coal alone and for the highest biomass share.
- Bed. Design bed pressure and bed material refresh arrangement.
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.