How Arecanut husk fired boilers work
Arecanut husk is the fibrous outer layer of the areca fruit, removed when the nut is dehusked, mainly in Karnataka, Kerala and Assam. Fresh husk carries 40 to 60% moisture; sun-dried husk falls to 15 to 25%. Ash is low, typically 2 to 5%, and dried husk has a calorific value of about 3,500 to 4,000 kcal/kg. The husk is coarse, fibrous and springy, so it bridges in hoppers and packs unevenly on the grate. It is fired on fixed or inclined grates of smoke tube and small water tube boilers in areca processing and food plants, often blended with firewood or coconut shell. The flue gas passes the boiler and a cyclone or multicyclone before the ID fan.
Where the ID fan sits
- Fixed or inclined grate
- Boiler passes
- Cyclone or multicyclone
- ID fan
- Chimney
The fan sits after the cyclone, typically on boilers of 1 to 5 TPH. Ash is low, but fibrous flakes of half-burnt husk are carried from the furnace to the cyclone, and some pass through. Suction at the fan is typically 80 to 160 mmWC. Because the gas volume follows the fuel moisture, a fan sized for dry husk runs short in the harvest season, and one sized for wet husk runs far back on its curve for the rest of the year.
Gas at the ID fan
| Gas temperature, normal | 170 to 220°C, often with no economiser |
| Maximum or upset | Up to 260°C with dry husk and fouled tubes |
| Dust at the fan | Low ash, with fibrous char flakes after a cyclone |
| Moisture and dew point | Water dew point about 50 to 65°C, at the top of the range with fresh husk |
| Corrosive species | Minor |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 Arecanut husk fired boilers
| Blade form | Backward curved, selected to stay efficient over a wide flow range |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled arrangement 8, or belt driven on small units |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing drain, inspection door, and insulation where wet husk lowers the gas temperature |
What this ID fan has to cope with
Seasonal change in gas volume
Fresh husk in the harvest months carries far more water than stored husk, and the extra vapour raises the gas volume by a third or more for the same steam. A fan selected at the wet-husk point, with a variable speed drive, covers both seasons; a fixed-speed fan with a damper wastes power for most of the year or starves the boiler in the harvest.
Bridging and uneven firing
Springy husk bridges in the hopper and then drops in a mass. The bed starves, then floods, and the gas volume swings with it. A responsive draft loop on the ID fan keeps the furnace at suction through these swings, while hopper agitators or a wider throat reduce how often they happen.
Condensation with wet husk
Wet husk cools the furnace and raises the dew point of the gas. At low load the gas at the fan can approach condensation, and the fine ash in the casing turns to a paste that dries hard. Casing insulation, a drain, and a minimum firing rate when burning fresh husk prevent these deposits.
Energy and control
Areca processing is seasonal, with the heaviest steam demand in the harvest months when husk is wettest. For the rest of the year the boiler runs on drier husk, often at lower load. A variable speed drive suits this pattern: the fan runs fast only when wet husk needs it. Drying husk in the sun, or with flue gas, cuts fan power and raises boiler output at the same time.
ID fan and FD fan in Arecanut husk fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the chimney | At the boiler front, feeding the grate |
| What it handles | Flue gas with fibrous char flakes | Ambient air |
| Temperature at the fan | 170 to 220°C, up to 260°C | Ambient, 20 to 40°C |
| Main design concern | Seasonal gas volume range | Pressure through an uneven bed |
| Control | Furnace draft | Steam pressure |
See also the Forced Draft (FD) Fan for Arecanut husk fired boilers.
Questions buyers ask
Why does my arecanut husk boiler struggle in the harvest season?
Fresh husk is wet, so the flue gas volume rises and the furnace runs cooler. If the ID fan was sized for dry husk, it cannot move the extra gas and the boiler loses output. An induced draft fan sized for wet husk, with a variable speed drive, covers both seasons without wasting power.
Can arecanut husk be fired with firewood?
Yes, and many plants blend husk with firewood or coconut shell to steady the fire. Blending changes the gas volume and temperature at the fan, so the ID fan should be checked at the wettest blend the plant expects to fire, usually fresh husk with little wood during the harvest.
What dust collector is used before the ID fan on an arecanut husk boiler?
Usually a cyclone or multicyclone on smaller boilers, with a bag filter where emission limits require it. Husk ash is low, but fibrous char flakes escape the furnace, so a collector protects the fan from build-up and cuts visible emission from the chimney. The collector's pressure drop is part of the fan duty.
Information needed to quote an ID fan for Arecanut husk 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.
- Fuel moisture range. Moisture of fresh and stored husk, and any blend with wood or shell.
- Season profile. Steam demand in the harvest and off-season months.
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.