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 ID fan sits
- Bubbling bed and freeboard
- Superheater and bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan sits after the final collector and before the chimney. Suction at the inlet is typically 200 to 320 mmWC, and it rises as alkali-rich deposits build on the convective surfaces. With a bag filter, spark arrestors upstream protect the bags from glowing biomass char. Because gas volume per tonne of steam changes with the fuel mix, the fan's operating point moves along its curve from season to season.
Gas at the ID fan
| Gas temperature, normal | 150 to 175°C after the air heater |
| Maximum or upset | Up to 220°C with fouled back pass |
| Dust at the fan | Coal ash, silica-rich husk ash and alkali salts; 30 to 100 mg/Nm³ after the final collector |
| Moisture and dew point | Varies with blend; water dew point from about 45°C on coal to 60°C on wet biomass |
| Corrosive species | Chlorides and SO₂; HCl from biomass chlorine at the cold end |
| 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 coal and biomass co-fired AFBC boilers
| Blade form | Backward curved, with wear liners where husk ash is part of the blend |
| Material of construction | Mild steel casing and impeller; wear-resistant plate at the blade inlet for silica ash |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive on furnace draft, with headroom for the wettest blend |
| Also specified | Inspection doors, casing drain and access for washing deposits |
What this ID fan has to cope with
Fan undersized for the wet biomass blend
A fan sized on coal, or on a dry blend, runs out of capacity when the plant switches to more wet biomass. Wood chips at 40% moisture make far more gas per tonne of steam than coal. The duty is calculated for every expected blend and the fan is sized on the one with the largest gas volume.
Alkali and chloride deposits
Potassium and sodium chlorides from biomass condense on cooler surfaces and on fine ash, making sticky deposits in the back pass and, at times, on the fan blades. Deposits raise system resistance and cause unbalance. Margin on pressure, soot blowing and blade inspection at each outage keep these under control.
Low temperature corrosion from HCl
Biomass chlorine forms HCl in the flue gas. At low load the gas at the fan and the cold end of the air heater can approach the acid dew point, and chloride corrosion follows. Holding the air heater outlet temperature above a minimum and insulating the fan casing protect against it.
Char and fire risk in the collector
Light biomass char can reach a bag filter while still glowing. A fire in the filter also endangers the fan through high gas temperature. Spark arrestors, temperature alarms at the filter outlet and an ID fan rated for a short excursion above normal temperature are the usual defences.
Energy and control
Co-fired AFBC boilers change fuel mix often, which shifts both gas volume and furnace resistance. A fan sized for the wettest blend runs well below its rating on coal, so a variable speed drive is the most effective control. Recording ID fan speed and power for each fuel mix also helps the plant see the real auxiliary cost of each blend when deciding what to buy.
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 Forced Draft (FD) Fan for coal and biomass co-fired AFBC boilers.
Questions buyers ask
Does co-firing biomass with coal need a bigger ID fan?
Usually yes, if the biomass is wet or the share is large. Biomass with high moisture produces more flue gas per tonne of steam than coal. The induced draft fan should be checked against the wettest blend at full load before increasing the biomass share.
Why do deposits form on the ID fan when we burn more biomass?
Biomass brings potassium, sodium and chlorine. These form low melting salts that condense on fine ash and cooler surfaces, making deposits sticky. Some reach the fan, especially if the collector is upset. Regular inspection and cleaning at outages prevent unbalance. Smooth blade surfaces and keeping the gas above about 150°C at the fan also reduce sticking.
Can the existing coal AFBC boiler ID fan handle biomass co-firing?
Often it can at low biomass shares. At higher shares, check gas volume, system resistance and inlet temperature for the new blend against the existing fan curve. A higher speed, a new impeller or a variable speed drive may give the extra capacity within the existing casing.
Information needed to quote an ID fan for coal and biomass co-fired 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. Analysis of each fuel and the range of biomass shares expected through the year.
- Collector. ESP or bag filter, spark arrestors and the measured dust at the fan inlet.
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.