How coal and biomass co-fired AFBC boilers work
Many AFBC boilers in India fire coal together with biomass such as rice husk, wood chips, groundnut shell, mustard stalk or pellets, at shares from about 10% to well over 50% of heat input. The fuels are fed overbed or underbed into a bubbling bed at 800 to 880°C. Biomass is lighter, more volatile and often wetter than coal, so more of it burns in the freeboard and it brings potassium, sodium and chlorine into the furnace. These alkalis can cause bed agglomeration and sticky deposits on superheaters. The gas passes the back pass, economiser and air heater, then a multiclone and ESP or bag filter before the ID fan. Fuel mix often changes with season and price.
Where the FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Compartment air boxes
- Nozzles
- Bubbling bed
The fan sits at ground level and pushes ambient air through the air heater into the compartment air boxes. Overbed or secondary air, needed to burn the biomass volatiles released in the freeboard, comes either from a branch with its own control damper or from a separate fan. Fan outlet pressure is typically 800 to 1,100 mmWC, set by nozzle and bed resistance. Underbed coal feeding, where used, has its own transport air fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 150 to 200°C, downstream of the fan |
| Pressure at the fan outlet | Typically 800 to 1,100 mmWC |
| Air split | Overbed air share rises with biomass share |
| Dust | Clean air; husk and wood 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 AFBC boilers
| Blade form | Backward curved, high pressure, stable at low compartment flows |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings; no cooling needed |
| Flow control | Variable speed drive on steam pressure, with separate control of overbed air |
| Also specified | Inlet silencer and screen against biomass fibre; compartment dampers |
What this FD fan has to cope with
Bed agglomeration from biomass alkalis
Potassium in biomass ash forms low melting silicates with bed sand, and agglomerates grow where fluidisation is weak. Holding fluidising air above the minimum at all loads, and not shifting too much air from bed to overbed, keeps the bed moving. Fan selection with stable low flow operation is one part of the defence.
Wrong air split with a changing blend
Biomass releases most of its energy above the bed, coal mostly within it. A fixed split set for coal leaves biomass volatiles unburnt; one set for biomass starves the coal. Independent control of fluidising and overbed air, each with flow measurement, lets operators set the split for each blend.
Biomass fibre at the intake
Husk, wood dust and stalk fibre in the boiler house can be drawn into the FD fan and build up on the impeller, causing unbalance. An inlet screen and an intake taken from a clean area avoid this, and the screen is cleaned on a schedule.
Energy and control
Co-fired boilers run on different blends through the year, and the air needed per tonne of steam changes with each. A variable speed FD fan, trimmed on flue gas oxygen, keeps air close to what each blend needs. Shifting turndown to overbed air where the bed allows, and keeping bed height at design, reduce the high pressure air the FD fan must supply.
ID fan and FD fan in coal and biomass co-fired AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Ahead of the air heater and air boxes |
| What it handles | Flue gas with mixed ash and alkali salts | Clean ambient air |
| Temperature at the fan | 150 to 175°C | Ambient, 25 to 45°C |
| Main design concern | Gas volume of the wettest blend | Air split and stable fluidisation |
| Control | Furnace draft | Steam pressure, with separate overbed air |
See also the Induced Draft (ID) Fan for coal and biomass co-fired AFBC boilers.
Questions buyers ask
Why does a co-fired AFBC need more overbed air?
Biomass is highly volatile and much of it burns above the bed. Without enough overbed air, volatiles leave the furnace unburnt, raising CO and losing efficiency. As the biomass share rises, more air goes above the bed and less through it, which changes the FD fan duty.
Can biomass co-firing cause defluidisation in an AFBC boiler?
Yes. Alkalis in biomass ash form sticky compounds with bed sand, and if fluidising air is weak these grow into agglomerates. Keeping air above the minimum fluidising flow, controlling bed temperature and refreshing bed material reduce the risk. The forced draft fan must be stable at the lowest air flow used.
Does the FD fan need to change when we add biomass?
Not always. The total air for a given steam output changes little, but the split between bed and overbed air shifts. Check that the FD fan can supply the fluidising air at the bed pressure required and that the overbed air supply can meet the higher share.
Information needed to quote an FD fan for coal and biomass co-fired AFBC 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. Fluidising and overbed air flows for coal alone and for the highest biomass share.
- Biomass. Types of biomass and their ash and alkali content.
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.