How sugarcane trash fired boilers work
Sugarcane trash is the dry leaves and tops left in the field after harvest. It is collected, baled or chopped and fired with bagasse in sugar mill cogeneration boilers, which lets the boiler run longer into the off-season and raises export power. Trash usually arrives at 15 to 30% moisture, far drier than bagasse at about 50%, with 5 to 10% ash that includes field soil and sand, and a calorific value of about 3,200 to 3,600 kcal/kg. It is shredded and blended with bagasse, typically up to 20 to 30% of the fuel, and fed onto travelling or dumping grates. The gas path is the bagasse boiler's: superheater, economiser, air heater, ESP or wet scrubber, ID fan and chimney.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Travelling or dumping grate
The FD fan sits beside the air heater and pushes ambient air through it into the plenum under the grate. Hot air at 180 to 250°C dries wet bagasse on the grate; with a trash-rich blend less drying is needed, and more of the total air is moved to the overfire nozzles fed by the secondary air fan. The FD fan works against the air heater, ducts, dampers and the grate, typically 150 to 250 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 180 to 250°C, downstream of the fan |
| Dust | Bagasse pith and trash fines in the mill air |
| Pressure at the fan outlet | Typically 150 to 250 mmWC |
| Air split | Less undergrate air and more overfire air as the trash share rises |
| 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 sugarcane trash fired boilers
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 8 or overhung |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air flow with oxygen trim |
| Also specified | Inlet screen against bagasse pith and an inlet silencer |
What this FD fan has to cope with
Wrong air split with trash
A grate tuned for wet bagasse puts most of the air under the bed. Light trash lifts off the grate in that air and burns in the upper furnace, raising the furnace exit temperature and carry-over. Reducing undergrate air and raising secondary air, with an FD fan that stays stable at the lower flow, keeps combustion on the grate.
Bagasse pith at the inlet
Mill air carries bagasse pith and trash fines that clog screens and coat impellers and air heater tubes. An inlet screen sized for low velocity, placed away from fuel conveyors and the bagasse yard, and cleaned on a schedule, protects the fan and keeps the air heater from losing capacity during the season.
Off-season turndown
When the mill runs the boiler for power only on stored fuel, steam output may be well below the crushing rate. The FD fan must give stable pressure at low flow. Speed control, rather than a damper nearly closed, avoids pulsation, noise and the poor air distribution that follows from a fan working near its stall point.
Energy and control
During crushing the FD fan runs near its rating; in the off-season it may run at 50 to 70%. A variable speed drive tied to oxygen trim saves power through both periods, and holding excess air down as the fuel gets drier with more trash reduces the gas the ID fan must move. In a cogeneration plant these auxiliary savings appear directly as additional export units.
ID fan and FD fan in sugarcane trash fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or scrubber | Before the air heater, feeding the grate |
| What it handles | Flue gas with bagasse ash and field sand | Ambient air carrying bagasse pith |
| Temperature at the fan | 150 to 180°C, up to 220°C | Ambient, 25 to 45°C |
| Main design concern | Sand erosion | Air split between grate and overfire |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Induced Draft (ID) Fan for sugarcane trash fired boilers.
Questions buyers ask
Do I need a new FD fan to fire sugarcane trash in a bagasse boiler?
Usually not. Trash needs less total air per tonne of steam than wet bagasse, so the forced draft fan normally has spare capacity. The change is in the split between undergrate and overfire air, and in confirming the FD fan runs stably at the reduced undergrate flow.
Why does the upper furnace get hotter when firing cane trash?
Light, dry trash is lifted by the undergrate air and burns in suspension higher in the furnace. Reducing undergrate air and raising secondary air keeps more of it burning on the grate. The FD fan setting and the grate zone dampers need retuning for each blend.
Is an air heater still needed when trash is blended with bagasse?
Yes. The boiler still fires mostly bagasse, which needs hot air to dry on the grate. The air heater also recovers heat that would otherwise leave through the ID fan, so it stays in service whatever the trash share. With more trash the air heater outlet temperature can be trimmed with a bypass if the grate runs hot.
Information needed to quote an FD fan for sugarcane trash 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.
- Air split. Present undergrate and overfire air shares, and the planned shares with trash.
- Season profile. Steam output during crushing and in the off-season.
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.