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 FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate or bed air plenum
- Grate or fluidised bed
The FD fan sits near the air heater and supplies the plenum under the grate or the air distributor of a fluidised bed. On grate units the overfire air comes from a separate secondary air fan or a damped branch of the FD fan. On a bubbling fluidised bed the FD fan supplies fluidising air at much higher pressure, typically 600 to 900 mmWC, against 100 to 200 mmWC on a grate.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 120 to 180°C, downstream of the fan |
| Dust | Meal dust from conveyors near the boiler |
| Pressure at the fan outlet | 100 to 200 mmWC for a grate; 600 to 900 mmWC for a fluidised bed |
| Air split | A large share of total air as overfire air on grate units |
| 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 de-oiled cake 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 with oxygen trim |
| Also specified | Inlet screen and silencer; a stable, rising pressure curve for fluidised bed duty |
What this FD fan has to cope with
Fines lifted off the grate
High undergrate air velocity picks up fine meal and carries it straight into the furnace, where it burns in suspension or leaves as char. Setting the FD fan for a modest primary air share, with the balance supplied as overfire air, keeps a burning layer on the grate and reduces carry-over to the collector and the ID fan.
Bed agglomeration in fluidised beds
On a bubbling bed firing de-oiled cake, phosphate and potassium in the ash can coat bed particles and make them sticky. If fluidising air falls, the bed agglomerates and slumps. The FD fan must hold minimum fluidising flow at all loads, with pressure margin for the bed resistance as the bed coarsens.
Meal dust at the intake
Conveying fine meal releases dust around the boiler. Drawn into the FD fan, it coats the impeller, unbalances it and settles in the air heater tubes. An intake taken from a clean point, a screen with ample area, and enclosed conveyors near the fan keep the air supply clean.
Energy and control
Steam demand in a solvent extraction plant is steady, so the FD fan runs near a fixed point most of the time. Oxygen trim matters more than speed range here: holding excess air low reduces stack loss and the gas volume the ID fan must handle with its heavy fly ash. A variable speed drive allows that trim without throttling, and gives a lower minimum when the plant runs reduced.
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 Induced Draft (ID) Fan for de-oiled cake fired boilers.
Questions buyers ask
How much primary air does a de-oiled cake fired boiler need?
Less than a lump fuel boiler. Fine cake burns largely in suspension, so a smaller share of the air goes through the grate and more through overfire nozzles. The exact split depends on the grate design; the FD fan is selected for the primary share and the resistance it meets.
Can a fluidised bed boiler burn de-oiled cake?
Yes, often blended with husk or coal. The forced draft fan then supplies fluidising air at much higher pressure than on a grate, and it must hold minimum fluidising flow at low load to prevent the bed agglomerating from the phosphate-rich ash. Many plants blend in husk or coal partly to keep the bed stable.
What FD fan pressure is needed for a de-oiled cake boiler?
About 100 to 200 mmWC for a grate fired unit including the air heater, and 600 to 900 mmWC for a bubbling fluidised bed. We confirm the figure from the boiler maker's data or site measurements before selecting the fan, and add margin for air heater fouling and bed resistance changes.
Information needed to quote an FD fan for de-oiled cake 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.
- Firing system. Grate or fluidised bed, and the air distributor resistance.
- Overfire air. Whether overfire air comes from the FD fan or a separate fan, and its share.
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.