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 ID fan sits
- Spreader-fired furnace
- Superheater and bank
- Economiser
- Air heater
- Mechanical collector
- ESP
- ID fan
- Chimney
The fan sits after the ESP, with suction typically 200 to 300 mmWC. Ahead of it, carry-over from suspension firing is caught by the mechanical collector and ESP. Some of this material is unburnt fibre that can be smouldering, and it collects in hoppers and duct corners. If carry-over is high, the fan sees more fines and the occasional spark, so it is built with access, drains and protection against both wear and fire.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 230°C with afterburning in the air heater or collector |
| Dust at the fan | Fine ash and unburnt fibre; higher than bed-burning grates if collection is weak |
| Moisture and dew point | About 20 to 25% water vapour; dew point near 60 to 65°C |
| Fire risk | Glowing fibre can reach collectors and occasionally the fan |
| 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 bagasse spreader stoker boilers
| Blade form | Backward curved with liners, or radial tipped if collection is only mechanical |
| Material of construction | Mild steel with wear liners; casing designed for brief temperature excursions |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with cooling disc sized for the upset temperature |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Inlet temperature alarm, access doors, casing drain |
What this ID fan has to cope with
Afterburning of carry-over
Unburnt fibre that settles in the air heater or collector hoppers can ignite, raising gas temperature well above normal. The ID fan must tolerate this until the operators act. We select bearings, shaft cooling and casing for the upset temperature and recommend a temperature alarm at the fan inlet.
Higher fine dust load
Suspension firing sends more fine ash and light fibre to the collectors than bed-burning grates do. If the mechanical collector is worn or its hoppers overfill, the fan sees much more dust within days. Liners on the blades and scroll cut-off, regular hopper evacuation and an inspection at each stoppage keep wear in hand.
Spreader air settings change carry-over
Too much distributor or spreader air throws more fines into suspension and carries them up the furnace before they burn. The ID fan then sees more dust and more gas. Correct spreader air pressure, set with the combustion engineer for the bagasse being fired, reduces the load on both the fan and the collectors.
Energy and control
Spreader stoker boilers respond quickly to load changes in the mill, so the ID fan sees frequent swings. A variable speed drive on furnace draft follows these swings without wasting power across a damper. Reducing carry-over through good spreader adjustment also lowers excess air and the gas volume the fan must move.
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 Forced Draft (FD) Fan for bagasse spreader stoker boilers.
Questions buyers ask
Why do bagasse spreader stoker boilers have more fly ash?
Bagasse is thrown into the furnace and much of it burns while falling, so fine ash and light fibre are carried up with the gas instead of staying on the grate. The collectors and ID fan therefore handle more dust and some unburnt fibre than on bed-burning grates.
Can a fire start in the ID fan on a bagasse boiler?
It can, though rarely. Glowing fibre that passes the collectors can settle in the fan casing and ignite deposits, particularly during a shutdown when the gas flow stops. A casing drain, access doors for cleaning, an inlet temperature alarm and good collector maintenance reduce the risk considerably.
What ID fan is used on a bagasse spreader stoker boiler?
Usually a backward curved ID fan with wear liners after an ESP, or a radial tipped fan if collection is mechanical only. It is designed for about 150 to 180°C normal and higher upset temperatures from afterburning, with a variable speed drive on furnace draft.
Information needed to quote an ID fan for bagasse spreader stoker 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.
- Spreader type. Mechanical rotor or pneumatic spreaders, and distributor air pressure.
- Carry-over. Unburnt carbon in fly ash and any history of collector fires.
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.