How palm empty fruit bunch fired boilers work
Empty fruit bunches are the stalks left after fruit is stripped in a palm oil mill. They arrive at 55 to 65% moisture with long, tough fibres, and as received have a calorific value under 2,000 kcal/kg. Before firing they are shredded and pressed to about 40 to 45% moisture, which raises the calorific value to roughly 2,500 to 3,000 kcal/kg. EFB ash, about 4 to 7% of dry weight, is extremely rich in potassium and carries chlorine, so slagging and fouling are severe. Firing is on reciprocating grates with a strong air preheat, or in fluidised beds, often with fibre and shell as support fuel. Gas passes the boiler bank, economiser, air heater, multiclone and bag filter or ESP to the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Hot air duct
- Undergrate zones
- Reciprocating grate
The fan draws ambient air and pushes it through a tubular air heater, where it is heated to about 200 to 250°C, then into zoned compartments under a reciprocating grate. The front zones dry the wet shredded bunches; later zones burn them out. The FD fan works against the air heater, the hot air duct and dampers, and a deep bed of fibrous, wet fuel whose resistance is higher than on fibre and shell firing.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 38°C |
| Air after the air heater | 200 to 250°C, downstream of the fan |
| Pressure at the fan outlet | Typically 150 to 250 mmWC |
| Bed condition | Thick, wet and fibrous; resistance varies with pressing |
| Dust | Clean air; shredder dust and fibre near the EFB line |
| 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 empty fruit bunch fired boilers
| Blade form | Backward curved, with pressure reserve for a wet, thick bed |
| 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, with zone dampers for drying and burning |
| Also specified | Inlet screen, and an intake away from the EFB shredder |
What this FD fan has to cope with
Not enough preheat
If the air heater fouls on its gas side or the FD fan cannot push enough air through it, the hot air temperature falls. Wet bunches then dry slowly, the fire retreats along the grate and unburnt fuel reaches the ash pit. A fan sized for the air heater resistance at its fouled state keeps the drying zone working.
Fibre balls and bridging
Shredded EFB can form balls and mats that bridge across the grate. Air rushes through the gaps around them, leaving the mats unburnt inside. Higher plenum pressure, grate movement and zone control help, but the fan has to have the pressure reserve to make the zone dampers effective.
Variable pressing performance
When the press wears or the feed changes, EFB moisture rises and the bed becomes heavier and denser. The FD fan has to supply both more air and more pressure for the same steam. Speed control with reserve capacity copes with this until the press is serviced.
Energy and control
The FD fan on an EFB boiler carries more pressure than most biomass grates because of the large air heater and the wet bed. Speed control on steam pressure, with oxygen trim, saves power at part load and during mill stops when the boiler is held warm. Keeping the air heater clean is equally important, since a fouled air heater forces the fan to work harder and still delivers cooler air.
ID fan and FD fan in palm empty fruit bunch fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Ahead of the air heater |
| What it handles | Wet flue gas with potassium chloride fume | Ambient air, preheated downstream |
| Temperature at the fan | 170 to 190°C | Ambient, 25 to 38°C |
| Main design concern | Wet gas volume and salt deposits | Pressure through air heater and wet bed |
| Control | Furnace draft | Steam pressure |
See also the Induced Draft (ID) Fan for palm empty fruit bunch fired boilers.
Questions buyers ask
Why does EFB need hot combustion air?
Even after pressing, EFB carries 40% or more moisture. Hot undergrate air dries it on the front zones of the grate so it can ignite and burn. Without enough preheat the fuel burns poorly, steam output falls and unburnt bunches leave with the ash. The air heater is therefore central to EFB firing.
What pressure does the FD fan need on an EFB fired boiler?
Typically 150 to 250 mmWC, covering the air heater, hot air duct, zone dampers and a thick, wet bed. The figure depends on air heater size and grate design. The boiler maker's plenum pressure and air heater pressure drop at full load are the starting points.
Can an existing fibre and shell boiler fire EFB?
Often, in part. Adding EFB increases moisture, so the furnace needs more preheated air and the ID fan has more gas to move. The FD fan, air heater and ID fan should all be checked for the planned EFB share before it is introduced. Increasing the EFB share in steps shows where the limit lies.
Information needed to quote an FD fan for palm empty fruit bunch 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 heater. Hot air temperature and the air side pressure drop at full load.
- Grate zones. Number of undergrate zones and the plenum pressure for each.
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.