How palm fibre and shell fired boilers work
Palm oil mills burn their own mesocarp fibre and kernel shell to raise steam for the sterilisers, digesters and back-pressure turbine. Fibre leaves the press at 35 to 45% moisture, with 5 to 8% ash and a calorific value of about 2,600 to 3,000 kcal/kg as fired; shell is drier and denser at about 4,000 kcal/kg with 2 to 4% ash. They are fired together, typically about 70 parts fibre to 30 parts shell, on fixed or reciprocating grates in water tube boilers of 10 to 45 TPH. Fibre burns largely in suspension, shell on the grate. Gas passes the boiler bank, economiser or air heater where fitted, a multiclone and a wet scrubber, ESP or bag filter, then the ID fan. Steam demand swings with batch sterilisation cycles.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater, where fitted
- Undergrate plenum
- Grate
The fan sits beside the furnace and pushes ambient air, through an air heater where one is fitted, into the plenum under the grate. Shell settles on the grate and burns there, while fibre is thrown into the furnace by pneumatic spreaders and burns largely in suspension. A separate secondary air fan feeds overfire nozzles, and on many units also supplies the spreader air. The FD fan works against the grate, the shell bed and the air heater.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 38°C, humid |
| Air after the air heater | 150 to 200°C where fitted, downstream of the fan |
| Pressure at the fan outlet | Typically 80 to 160 mmWC |
| Dust | Clean air; fibre fluff around the boiler house |
| Climate | Hot and humid, often outdoors or under a roof only |
| 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 palm fibre and shell fired boilers
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel with a tropical paint system |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on steam pressure, coordinated with the secondary air fan |
| Also specified | Inlet screen against fibre fluff and a weatherproof motor |
What this FD fan has to cope with
Wet fibre needs heat
Fibre at 40% moisture ignites slowly. Without preheated air, the furnace temperature falls whenever wet fibre arrives, burnout suffers and smoke rises. An air heater with an FD fan sized for its resistance raises the air temperature and steadies combustion; without one, the fan and secondary air must be set to keep the furnace hot.
Lagging air on steriliser demand
When a steriliser opens, steam pressure drops and fuel feed rises. If the air does not follow quickly, the furnace smokes and burnout falls. Linking FD and secondary air fan speeds to steam pressure, with oxygen trim, keeps the air in step with fuel. A fixed damper cannot follow these swings.
Fibre fluff at the intake
Fibre handling releases fluff that drifts around the boiler house. An FD fan drawing nearby air collects it on the inlet and blades, which unbalances the impeller and chokes the inlet screen. A clean intake location and regular screen cleaning prevent both problems and keep the air side of any air heater clear..
Energy and control
The FD and secondary air fans in a palm oil mill follow the steriliser cycle all day. Speed control on both, linked to steam pressure, avoids the losses of fixed-speed fans throttled most of the hour. In the lean crop months, when the mill runs short weeks, the drives also let the fans idle at low speed while the boiler is kept warm.
ID fan and FD fan in palm fibre and shell fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multiclone and final collector | Beside the furnace, under the grate |
| What it handles | Flue gas with fibre ash and shell grit | Ambient humid air |
| Temperature at the fan | 180 to 230°C | Ambient, 25 to 38°C |
| Main design concern | Shell grit erosion and fast draft response | Following steriliser load swings |
| Control | Fast furnace draft loop | Steam pressure with secondary air |
See also the Induced Draft (ID) Fan for palm fibre and shell fired boilers.
Questions buyers ask
What is the difference between the FD fan and secondary air fan on a palm oil mill boiler?
The FD fan supplies air under the grate to the burning shell. The secondary air fan supplies higher-pressure air to overfire nozzles and often to the fibre spreaders, where most of the fibre burns. Both have to follow steam demand together. On larger boilers the two are often of similar size.
Does a palm fibre boiler need an air heater?
Not always, but it helps. Fibre at 35 to 45% moisture ignites more readily with hot air, and an air heater raises efficiency. The FD fan must then be sized for the air heater resistance. Many older mills run without one and rely on a hot furnace and good secondary air.
Why does my palm oil mill boiler smoke when the steriliser opens?
Steam demand rises suddenly, fuel feed increases, and the air supply lags. Linking fan speeds to steam pressure with a variable speed drive and oxygen trim lets air follow the load and reduces smoke. Checking the secondary air nozzles for blockage also helps. A steady fibre feed reduces the swings further.
Information needed to quote an FD fan for palm fibre and shell 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 system. Whether there is a secondary air fan and whether it also supplies the fibre spreaders.
- Air heater. Whether an air heater is fitted and its air side pressure drop.
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.