How de-oiled cake fired boilers work
De-oiled cake is the meal left after solvent extraction of oil from rice bran, oilseed cakes and non-edible seeds such as neem, karanja and mahua. Grades unfit for feed are fired as boiler fuel, often in the extraction plant's own boiler. De-oiled rice bran, the most common, carries 8 to 12% moisture, 15 to 25% ash rich in silica and phosphorus, and a calorific value of about 3,000 to 3,500 kcal/kg. It is a fine, free-flowing meal, fed by screw or rotary valve onto a travelling grate, a fixed grate with overfire air, or into a fluidised bed, and much of it burns in suspension. The flue gas passes the boiler bank, economiser and often an air heater, then a multicyclone and a bag filter or ESP before the ID fan.
Where the ID fan sits
- Grate or bed furnace
- Boiler bank
- Economiser
- Air heater
- Multicyclone
- Bag filter or ESP
- ID fan
- Chimney
The fan sits after the bag filter or ESP. Because so much of the fine meal leaves the furnace as fly ash, the collector inlet loading is high, well above that of a husk or wood boiler of the same steam output, and the fan should not follow a multicyclone alone. The suction covers the furnace, boiler bank, economiser, air heater and two stages of collection, typically 180 to 280 mmWC. The boiler usually stands outside the solvent plant's hazardous area, linked by enclosed conveyors.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the economiser or air heater |
| Maximum or upset | Up to 230°C when deposits build on the economiser |
| Dust at the fan | Fine silica and phosphate ash with some char; low after a bag filter, very high if the fan follows a multicyclone only |
| Moisture and dew point | Water dew point about 45 to 55°C |
| Corrosive species | Protein-rich seed cakes add nitrogen and some sulphur; SO₂ is moderate |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 de-oiled cake fired boilers
| Blade form | Backward curved with wear liners after a bag filter; radial tipped behind a multicyclone |
| Material of construction | Mild steel casing; impeller in wear-resistant plate or with hard-faced blade inlets |
| Drive and arrangement | Direct coupled, arrangement 8, moderate tip speed |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Cleaning doors on the casing and inlet box, a casing drain and liners at the cut-off |
What this ID fan has to cope with
Cohesive deposits on the impeller
Phosphorus and potassium in rice bran ash form low-melting compounds that stick to the economiser and carry through to the fan as a fine, cohesive dust. It builds unevenly on the blades and causes vibration. Smooth blade surfaces, a door large enough to clean the wheel by hand, and vibration trending to set the cleaning interval keep the fan in service.
Bag filter bypass
During start-up, or when bags fail, the fan sees the full fly ash load leaving the multicyclone. With 20% ash fuel burning largely in suspension, this is a heavy and abrasive stream. Wear liners and a fan selected with the bypass case in mind stop a short bypass from ending the impeller's life.
Feed surges and draft swings
Fine meal can flush through a screw feeder when the hopper bridges and then collapses, releasing a puff of volatiles that pushes the furnace briefly positive. A fast draft loop on the ID fan drive, and a rotary valve or metering screw that limits flushing, keep the furnace at steady suction.
Energy and control
Solvent extraction plants run continuously with steady steam demand for desolventising and distillation, so the ID fan sits near one operating point for long periods. Getting the selection right for that point gives the main saving. A variable speed drive still pays where the boiler is oversized, during plant maintenance when steam demand drops, and as the bag filter resistance rises between cleaning cycles.
ID fan and FD fan in de-oiled cake fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Before the air heater, feeding the grate or bed |
| What it handles | Flue gas with fine phosphate and silica ash | Ambient air |
| Temperature at the fan | 150 to 180°C, up to 230°C | Ambient, 25 to 45°C |
| Main design concern | Deposits and heavy fly ash | Primary air share and fluidising flow |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Forced Draft (FD) Fan for de-oiled cake fired boilers.
Questions buyers ask
Can de-oiled rice bran be used as boiler fuel?
Yes. Grades not sold as cattle feed are commonly fired in extraction plant boilers. The meal is fine and high in ash, so it burns largely in suspension and needs good overfire air, a multicyclone followed by a bag filter, and a boiler ID fan designed for high fly ash loading and deposit cleaning.
Why is ID fan vibration frequent on de-oiled cake fired boilers?
Fine ash containing phosphorus and potassium sticks to the impeller and builds unevenly, or falls away in patches. Either way the rotor goes out of balance. Regular cleaning through an access door, smooth blades and online vibration monitoring keep it under control between shutdowns. Cleaning before balancing matters: balancing a dirty wheel only lasts until the deposit moves.
Where should the ID fan be placed on a de-oiled cake boiler?
After the final collector, normally a bag filter or ESP, so the fan handles the least dust. On de-oiled cake the multicyclone alone leaves too much fine ash for an impeller to last, so the induced draft fan should not be placed straight after it.
Information needed to quote an ID fan for de-oiled cake 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.
- Cake type. Rice bran, oilseed or non-edible seed cake, with ash, moisture and the fuel analysis.
- Collector bypass. Whether the bag filter has a bypass and how often it is used.
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.