How bagasse spreader stoker boilers work
In a bagasse spreader stoker boiler, bagasse is fed through chutes on the front wall and thrown across the furnace by mechanical rotors or by jets of air from distributor nozzles. Fines and pith burn in suspension while they fall, and only the heavier fibre lands and burns on the grate below, which may be dumping, pinhole or travelling. Suspension burning gives quick response to load changes but carries much more unburnt fibre and ash up with the gas than a bed-burning grate. The gas passes the superheater, bank, economiser and air heater, then a mechanical collector and ESP or scrubber. Spreader air, overfire air and undergrate air come from separate fans.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Grate
- Furnace
The FD fan pushes ambient air through the air heater into the plenum under the grate. Separate fans supply distributor air to the spreader nozzles and overfire air to secondary nozzles. Because so much bagasse burns in suspension, undergrate air is often only 30 to 50% of the total. The FD fan pressure depends on the grate type below the spreaders.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 200 to 250°C, downstream of the fan |
| Pressure at the fan outlet | About 120 to 250 mmWC depending on grate |
| Share of total air | About 30 to 50% as undergrate air |
| Dust | Bagasse pith around the intake |
| 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 bagasse spreader stoker boilers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on steam pressure with oxygen trim |
| Also specified | Inlet screen; coordinated control with distributor and overfire air fans |
What this FD fan has to cope with
Wrong air split
If the FD fan supplies too much undergrate air, the bed burns too hot and fines are lifted back into suspension; too little, and fibre piles up on the grate and burns slowly. Coordinated control of the FD, distributor and overfire air fans from one combustion controller keeps the split right as bagasse quality changes.
Grate type changes the duty
Spreader stokers sit over dumping, pinhole or travelling grates, and each has a different resistance, from low on open dumping grates to much higher on pinhole grates. The FD fan is sized for the actual grate below the spreaders, not for the spreader system, and is reselected if the grate is changed during a retrofit.
Air heater leakage
Air heater leaks pass FD air straight to the gas side and raise both FD and ID duty, and on bagasse boilers the leaks grow through the season as sand erodes the tube ends. The FD selection includes a realistic leakage allowance, so the grate is not short of air in the last weeks before the overhaul.
Energy and control
Because spreader stokers follow mill load quickly, the FD fan sees frequent changes. A variable speed drive with oxygen trim follows them efficiently and keeps excess air low, reducing the wet gas volume through the ID fan. Correct air split between undergrate and overfire air often saves more than the fan drive itself.
ID fan and FD fan in bagasse spreader stoker boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After ESP | Before the air heater |
| What it handles | Wet flue gas with fines and unburnt fibre | Ambient air |
| Temperature at the fan | 150 to 180°C, higher with afterburning | Ambient, 25 to 45°C |
| Main design concern | Carry-over and fire risk | Air split with spreader and overfire air |
| Control | Furnace draft | Steam pressure with oxygen trim |
See also the Induced Draft (ID) Fan for bagasse spreader stoker boilers.
Questions buyers ask
How much undergrate air does a bagasse spreader stoker boiler use?
Usually about 30 to 50% of the total combustion air goes under the grate from the FD fan. The rest is distributor and overfire air from separate fans, because a large share of the bagasse burns in suspension above the grate. The exact split depends on bagasse fineness and the grate below the spreaders.
Is the distributor air fan the same as the FD fan?
No. The distributor or spreader air fan supplies higher pressure air to throw bagasse across the furnace. The FD fan supplies lower pressure undergrate air through the air heater. On some boilers the distributor air is tapped from the FD duct and raised by a booster fan, but it is sized as a separate duty.
What FD fan pressure suits a spreader stoker bagasse boiler?
It depends on the grate below the spreaders and the air heater resistance. Typically the FD fan develops about 120 to 250 mmWC at its outlet. Pinhole grates need the upper end of that range, while dumping and travelling grates sit lower. We confirm it from measured plenum pressure on existing boilers.
Information needed to quote an FD fan for bagasse spreader stoker 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. Shares of undergrate, distributor and overfire air.
- Grate under the spreaders. Dumping, pinhole or travelling grate and its design pressure.
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.