How sunflower husk fired boilers work
Sunflower husk is the hull separated from the seed before oil extraction, produced in quantity at oil mills in Karnataka, Maharashtra and Andhra Pradesh. It carries 8 to 12% moisture, 2 to 4% ash and a calorific value of about 4,000 to 4,400 kcal/kg, but its ash is rich in potassium and softens from about 850 to 1,000°C. Bulk density is low, around 100 to 150 kg/m³. The husk is blown or screw-fed onto a travelling or reciprocating grate, usually in the mill's own boiler, and a large part of it burns in suspension. The flue gas passes the boiler bank, economiser and air heater, then a multicyclone and bag filter, before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Travelling or reciprocating grate
The FD fan pushes ambient air through the air heater into the grate plenum, with a damper for each zone. Secondary air comes from a separate fan, and on many boilers part of it is the conveying air that blows husk into the furnace. Because the ash melts at a low temperature, the air heater outlet is kept moderate, about 120 to 160°C, and the FD fan pressure is typically 120 to 200 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 120 to 160°C, downstream of the fan |
| Dust | Husk fines from the dehulling section |
| Pressure at the fan outlet | Typically 120 to 200 mmWC |
| Bed temperature limit | Kept below the ash softening range of about 850 to 1,000°C |
| 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 sunflower husk fired boilers
| Blade form | Backward curved, with a stable curve for zone control |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or overhung |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive with oxygen trim |
| Also specified | Inlet screen against husk fines and individual zone dampers |
What this FD fan has to cope with
Clinker from low-melting ash
If primary air is short or badly distributed, hot spots form in the bed and potassium-rich ash fuses into clinker that blocks the grate air holes. Once holes block, fan pressure rises and flow falls further, so the problem feeds itself. Steady air to every zone, and a fan with pressure in hand to hold flow, prevent the start of it.
Husk fines at the intake
Mills that dehull seed release fine husk dust into the air. A fan drawing from the dehulling area collects it on the screen and impeller and passes it to the air heater, where it settles on the tubes. A screened intake from a clean area, with a cleaning routine, keeps the airflow and heat transfer steady.
Conveying air upsets the balance
Husk blown in pneumatically brings its own conveying air into the furnace. If the FD fan setting ignores it, excess air rises, the gas volume through the fouling boiler grows and the ID fan reaches its limit sooner. The FD and secondary air settings are trimmed to allow for the conveying air.
Energy and control
Steady oil mill operation lets the FD fan run near one point for most of the year. Oxygen trim with a variable speed drive cuts excess air, the main avoidable loss on a husk boiler, and reduces the gas volume through the fouled boiler, which slows the climb in ID fan power between cleanings. The two fans are best tuned together.
ID fan and FD fan in sunflower husk fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Before the air heater, feeding the grate |
| What it handles | Flue gas with low, potassium-rich ash | Ambient air |
| Temperature at the fan | 150 to 170°C, up to 240°C fouled | Ambient, 25 to 45°C |
| Main design concern | Rising draft loss from fouling | Primary air to prevent clinker |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Induced Draft (ID) Fan for sunflower husk fired boilers.
Questions buyers ask
How do I stop clinker in a sunflower husk fired boiler?
Keep the bed below the ash softening point with steady, well-distributed primary air, and avoid too hot an air heater outlet. Check that the FD fan delivers design air to each grate zone and that the grate air holes are clear, then trim the fuel feed so the bed does not grow too deep.
Does the pneumatic feeding air count as combustion air?
Yes. Air used to blow husk into the furnace adds to the total air. The forced draft fan and secondary air fan settings must allow for it, otherwise excess air rises and the ID fan handles more gas than the combustion needs, which shows as high oxygen at the economiser outlet.
What FD fan pressure does a sunflower husk boiler need?
Typically 120 to 200 mmWC, depending on the air heater, ducts, zone dampers and grate resistance. We size it from the boiler maker's data and confirm against site readings where the boiler is running, with margin for partly blocked grate holes and for the resistance rise as the air heater collects husk dust.
Information needed to quote an FD fan for sunflower husk 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.
- Feed system. Pneumatic or screw feeding, and the conveying air quantity if pneumatic.
- Grate zones. Number of zones and the design plenum pressure.
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.