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 ID fan sits
- Grate furnace
- Superheater
- Economiser
- Air heater
- ESP or wet scrubber
- ID fan
- Chimney
The fan follows the ESP on newer cogeneration plants, or a wet scrubber or multicyclone on older mills. Field sand is coarse and heavy and the collector removes most of it, but what reaches the fan is much more abrasive than bagasse fly ash. Suction at the fan is typically 200 to 300 mmWC. Many mills run two ID fans in parallel; changing from straight bagasse to a trash-rich blend moves the operating point on both fan curves, so the selection is checked at each fuel case.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 220°C with dry, trash-rich fuel and fouled surfaces |
| Dust at the fan | Bagasse ash with field sand and soil; far more abrasive behind a scrubber or multicyclone than behind an ESP |
| Moisture and dew point | Lower than on pure bagasse; water vapour falls from about 25% toward 15 to 18% as the trash share rises |
| Corrosive species | Trash carries more chlorine and potassium than bagasse, which show up as deposits on the superheater and air heater |
| 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 sugarcane trash fired boilers
| Blade form | Backward curved with wear liners; radial tipped where the fan follows a scrubber or multicyclone only |
| Material of construction | Mild steel casing, blades in wear-resistant plate, and liners at the scroll cut-off |
| Drive and arrangement | Direct coupled, arrangement 8, impeller tip speed kept moderate |
| Bearings and cooling | Pedestal bearings outside the gas stream with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Bolted liners replaceable at site, inspection doors and a casing drain |
What this ID fan has to cope with
Sand erosion from field trash
Trash gathered by rakes and balers brings soil with it. A blend with 20% trash can double the ash reaching the collector, and the sand fraction cuts the blade inlet and the casing at the cut-off. Wear-resistant plate, a lower tip speed and liners that can be changed during the off-season shutdown give a predictable impeller life.
Fan selected only for bagasse
A fan sized for wet bagasse has volume margin that dry trash does not need, but a thicker fuel bed, sand in the hoppers and faster air heater fouling raise the pressure it must develop. Checking the fan curve at the trash-blend duty, rather than assuming the bagasse margin covers it, avoids running two parallel fans near their stall region.
Hotter furnace and superheater fouling
Drier fuel burns hotter. With trash in the blend the furnace exit temperature rises, chlorine and potassium deposits build faster on the superheater, and gas at the fan climbs between sootblowing cycles. The impeller, shaft and bearings are designed for this higher upset temperature, not the normal bagasse value.
Energy and control
Sugar mill cogeneration plants run hard through the crushing season and at reduced or variable load in the off-season on stored bagasse and trash. Variable speed ID fans keep auxiliary power down when the boiler runs below rating, and each kilowatt saved is a kilowatt exported. Where the old fans were sized for wet bagasse with generous margins, a re-rated impeller matched to the actual blend often saves more than speed control alone.
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 Forced Draft (FD) Fan for sugarcane trash fired boilers.
Questions buyers ask
Can a bagasse boiler ID fan handle sugarcane trash?
Usually yes for gas volume, because trash is drier than bagasse. The concern is wear: trash brings field sand and adds ash. Check the dust reaching the fan, fit wear liners, and confirm the induced draft fan's operating point at the trash-blend gas volume and pressure before raising the trash share.
Why does ID fan wear increase when we fire cane trash?
Field trash carries soil and sand picked up during raking and baling. Sand is much harder than bagasse ash, and whatever passes a scrubber or multicyclone erodes the blade inlet and casing. Better trash cleaning before shredding, a working ESP and wear-resistant blades all extend impeller life.
How much trash can a sugarcane bagasse boiler burn?
Many mills fire up to 20 to 30% trash blended with bagasse without major changes. Beyond that, slagging, chlorine deposits and ash load rise quickly. The ID and FD fans should be checked at the planned blend, because gas volume, temperature and dust at the fan all change with the trash share.
Information needed to quote an ID fan for sugarcane trash fired 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 blend. Bagasse and trash share in season and off-season, with the moisture and ash of each.
- Wear record. Life of the present impeller and casing liners, and the collector ahead of the fan.
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.