How bamboo chip fired boilers work
Bamboo chips, rejects and bamboo dust are burnt in pulp and paper mills that use bamboo as a raw material, and in bamboo processing units in the north-east, Karnataka and Andhra Pradesh. Fresh chips hold 30 to 45% moisture, while air-dried rejects and dust fall to 15 to 20%. Bamboo has 2 to 5% ash, unusually rich in silica for a wood-like fuel, and a calorific value of about 3,000 to 3,600 kcal/kg as fired. Chips are long and splintery and burn on travelling or reciprocating grates, or in AFBC boilers alongside coal. Gas passes the boiler, economiser, air heater, multiclone and an ESP or bag filter to the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Grate zones
The fan sits near the air heater and pushes ambient air through it into zoned compartments under the grate. Fresh chips need hot air to dry and ignite, so the air heater outlet is typically 160 to 220°C. Bamboo slivers interlock and form mats that change the bed resistance along the grate. The fan works against the air heater, dampers and that variable bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 15 to 40°C |
| Air after the air heater | 160 to 220°C, downstream of the fan |
| Pressure at the fan outlet | Typically 120 to 200 mmWC |
| Bed condition | Splintery chips that mat and interlock |
| Dust | Clean air; bamboo dust near the chipper |
| 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 bamboo chip fired boilers
| Blade form | Backward curved, with pressure reserve for a matted bed |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on steam pressure, with zone dampers |
| Also specified | Inlet screen and an intake away from the chipping area |
What this FD fan has to cope with
Matted chip bed
Long bamboo slivers interlock on the grate and form mats with few gaps. Air finds the edges of the mats and leaves the centres burning slowly. An FD fan with pressure reserve, zone dampers and grate movement to break the mats keep combustion even. Uniform chip length also helps.
Fresh chips need preheat
Chips cut from green bamboo carry 40% or more moisture. Without hot air they dry slowly, ignition moves down the grate and output falls. A properly sized air heater, with an FD fan selected for its resistance, keeps the drying zone effective. Air leaks at the air heater are checked too.
Co-firing changes the air demand
When coal is fired with bamboo, the bed needs more undergrate air pressure and less overfire air than on bamboo alone. A fan with reserve pressure and separate damper control on undergrate and overfire branches lets the operator adjust the split for each fuel mix.
Energy and control
Mill boilers carry changing loads and changing fuel mixes. Speed control on the FD fan, tied to steam pressure with oxygen trim, avoids throttling losses and keeps excess air down across these changes. Drying chips before firing cuts the air heater duty and allows lower air flow for the same steam, reducing power on both FD and ID fans.
ID fan and FD fan in bamboo chip fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Ahead of the air heater |
| What it handles | Flue gas with silica-rich ash | Ambient air, preheated downstream |
| Temperature at the fan | 160 to 180°C | Ambient, 15 to 40°C |
| Main design concern | Silica erosion and moisture range | Pressure through a matted chip bed |
| Control | Furnace draft | Steam pressure |
See also the Induced Draft (ID) Fan for bamboo chip fired boilers.
Questions buyers ask
Why does a bamboo chip bed burn unevenly?
Long bamboo slivers interlock and mat, forming areas with few gaps where air cannot pass easily. Air takes the edges and leaves the centres slow-burning. More undergrate pressure, zone dampers and regular grate movement help break up the mats. Uniform chip sizing at the chipper reduces matting at the source.
How hot should the combustion air be for bamboo chips?
For fresh chips at 30 to 45% moisture, typically 160 to 220°C at the grate. Drier rejects and dust need less. The FD fan must be sized for the resistance of the air heater that provides this preheat. Where the mill also fires dried rejects, the zone dampers let the operator use less hot air on those sections.
Does co-firing coal with bamboo change the FD fan duty?
Yes. Coal needs higher undergrate pressure through a denser bed and less air per unit of heat than wet bamboo. The FD fan should have pressure reserve and speed control so it can serve both bamboo alone and the co-firing mix. Separate dampers set the split.
Information needed to quote an FD fan for bamboo chip fired 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.
- Fuel mix. Share of fresh chips, dried rejects, dust and coal at full load.
- Air heater. Hot air temperature and air side pressure drop at full load.
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.