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 ID fan sits
- Reciprocating grate
- Boiler bank
- Economiser
- Air heater
- Multiclone
- Bag filter or ESP
- ID fan
- Chimney
The fan is placed after the bag filter or ESP. Potassium chloride leaves the furnace as a sub-micron fume that a multiclone cannot catch, so the final collector matters more here than for coarse ash. Suction at the fan inlet is typically 200 to 300 mmWC, rising as the economiser and air heater foul with alkali deposits. The air heater must stay well above the water dew point of the wet gas, which keeps the fan inlet warm.
Gas at the ID fan
| Gas temperature, normal | 170 to 190°C after the air heater |
| Maximum or upset | Up to 240°C with fouled heat surfaces |
| Dust at the fan | Fine potassium chloride fume and light ash; low after a bag filter |
| Moisture and dew point | Water vapour of 20 to 28% by volume; water dew point about 62 to 68°C |
| Corrosive species | Potassium chloride and other alkali salts; very aggressive in condensate |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 palm empty fruit bunch fired boilers
| Blade form | Backward curved, sized for the wet gas volume, with smooth surfaces and no ledges |
| Material of construction | Mild steel with protective coating; SS 316 at the inlet box, lower casing and drains |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive with margin for the fouled resistance |
| Also specified | Insulated casing, drain, access door and wash connections |
What this ID fan has to cope with
Gas volume from wet bunches
Pressing performance varies, and EFB reaching the furnace can range from 40 to 55% moisture over a day. Each step in moisture adds water vapour that the ID fan must move. A fan sized on the design moisture runs out of capacity on wetter fuel, so the selection is made at the worst moisture the mill's press actually delivers.
Salt deposits on the impeller
Potassium chloride fume that passes the collector condenses on any cooler surface. It forms a hard, hygroscopic crust on the blades that grows unevenly and takes up moisture when the fan stands. Smooth blades, insulation, washing at shutdowns and vibration trending keep the wheel balanced.
Corrosion during mill stops
Palm oil mills stop at weekends and on no-fruit days. Each stop lets the fan and duct cool, and the salt deposits draw in moisture and form a corrosive brine on the steel. Cleaning before long stops, keeping the casing closed and dry, and stainless steel at the low points slow this attack.
Energy and control
EFB is usually fired alongside fibre and shell, and the share changes with how much bunch the mill wants to dispose of. More EFB means more water vapour and more ID fan power for the same steam. A variable speed drive handles this range. Better pressing of the bunches before firing is the single most effective energy measure, as every point of moisture removed reduces both fuel use and fan load.
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 Forced Draft (FD) Fan for palm empty fruit bunch fired boilers.
Questions buyers ask
Why does the ID fan limit steam output on an EFB fired boiler?
EFB brings far more moisture than shell or fibre, and that water leaves as vapour in the flue gas. If the fan was sized for drier fuel, it cannot move the extra volume at full firing. Measuring gas flow and suction at the fan, and checking the EFB moisture, shows whether pressing or the fan is the limit.
What causes deposits on the ID fan impeller of a palm EFB boiler?
EFB ash is very rich in potassium and chlorine, which leave the furnace partly as a fine salt fume. Fume that passes the collector condenses on cooler surfaces, including the fan blades. Good collection, a warm casing and regular washing keep the deposits in check.
Is a stainless steel ID fan needed for EFB firing?
Not usually for the whole fan. A mild steel fan with good insulation and coating works while the gas stays well above its dew point. Stainless steel at the inlet box, lower casing and drains, where salt brine collects during stops, is a sensible minimum on EFB-fired boilers.
Information needed to quote an ID fan for palm empty fruit bunch 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.
- EFB preparation. Shredding and pressing arrangement, and the EFB moisture as fired.
- Fuel mix. Proportion of EFB, fibre and shell at full load.
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.