How groundnut shell fired boilers work
Groundnut shells from decorticating units and oil mills are a light, dry fuel with 8 to 12% moisture, a calorific value of about 3,800 to 4,200 kcal/kg and 3 to 8% ash. Much of that ash is field soil and sand that stays with the pods after harvest and threshing. Whole shells are bulky and partly burn in suspension, so many plants grind them or buy them as briquettes. Small boilers use fixed or dumping grates and larger ones travelling grates, typically 2 to 20 TPH. Gas passes the boiler bank, economiser and air heater, a multiclone and usually a bag filter or ESP, then the ID fan. Supply follows the harvest, so shells are stocked for months and blended with other fuels as stock runs down.
Where the ID fan sits
- Grate furnace
- Boiler bank
- Economiser
- Air heater
- Multiclone
- Bag filter or ESP
- ID fan
- Chimney
The fan stands after the bag filter or ESP, so most of the sand is removed before the gas reaches it. The multiclone takes out coarse sand and burning fragments, protecting both the filter and the fan. Where an older plant has only the multiclone, the fan receives several grams of sand per cubic metre and needs wear protection throughout. Typical suction at the fan inlet is 160 to 240 mmWC on a 6 to 15 TPH unit.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | About 230°C with a fouled air heater or overfiring |
| Dust at the fan | Fine shell ash with sand; low after a bag filter, 2 to 5 g/Nm³ after a multiclone only |
| Moisture and dew point | Water dew point about 45 to 50°C; low sulphur |
| Fuel variability | Ash and sand content vary widely between lots and with stock age |
| Density at the design temperature | About 0.81 kg/m³ at 165°C, 68% 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 groundnut shell fired boilers
| Blade form | Backward curved with wear liners after a bag filter; radial tipped after a multiclone only |
| Material of construction | Mild steel casing; wear-resistant plate at the blade inlet and the scroll cut-off |
| Drive and arrangement | Direct coupled, arrangement 8, at moderate tip speed |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Bolted scroll liners, an inspection door and vibration pick-ups on both bearings |
What this ID fan has to cope with
Abrasion from field sand
Shells from threshing floors carry sand that ends up in the fly ash. Unlike the silica formed inside rice husk, it is coarser and its quantity varies, so wear comes in bursts tied to particular fuel lots. Wear plate at the blade inlet and scroll cut-off, with a wheel designed for liner replacement at site, keeps wear manageable.
Burning shell fragments
Light shell pieces lift from the grate and can still be alight when they reach the collectors. They hole filter bags and can ignite deposits in hoppers. A multiclone or spark trap ahead of the bag filter, and an ID fan rated for the bypass temperature, give a safe path when the filter has to be isolated.
Changing fuel blends
As the shell season ends, plants blend in wood, husk or briquettes. Each blend changes the gas volume and the ash, and the fan operating point moves with it. A fan with a broad efficiency range and a variable speed drive accommodates these changes far better than a fan chosen tightly for one fuel.
Energy and control
Groundnut shell boilers in oil mills often run hard through the crushing season and at reduced load afterwards. A variable speed drive follows these changes without the losses of a nearly closed damper. Worn impellers are a hidden energy cost: an eroded wheel moves less gas per kilowatt, so replacing it with a lined backward curved impeller often restores draft and efficiency together.
ID fan and FD fan in groundnut shell fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Ahead of the air heater |
| What it handles | Flue gas with sand-laden shell ash | Ambient air |
| Temperature at the fan | 150 to 180°C | Ambient, 25 to 45°C |
| Main design concern | Sand abrasion and sparks | Air distribution through a light bed |
| Control | Furnace draft | Steam pressure |
See also the Forced Draft (FD) Fan for groundnut shell fired boilers.
Questions buyers ask
Why does the ID fan wear so fast on a groundnut shell boiler?
Groundnut shells carry soil and sand from harvesting and storage. The sand passes into the fly ash and erodes the impeller, especially where the fan follows only a multiclone. Better fuel screening, an efficient final collector and wear liners on the blades reduce the problem. Wear usually tracks fuel quality closely, so it varies from season to season.
What type of ID fan suits a groundnut shell fired boiler?
Behind a bag filter or ESP, a backward curved wheel with wear liners gives good efficiency and life. Where the fan follows only a multiclone, a radial tipped wheel in wear-resistant plate is more robust. In both cases a moderate tip speed and a variable speed drive are advisable.
Can a boiler designed for groundnut shells burn other biomass?
Often yes, within limits. Wood chips, husk and briquettes differ in moisture and ash, so gas volume and draft change. The ID fan should be checked at the worst fuel in the planned blend, because a wetter fuel can make it the limit on steam output even when the grate copes.
Information needed to quote an ID fan for groundnut shell 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 lots. Ash and sand content of typical and worst shell lots, and any blending fuels.
- Present wear. Running hours between impeller repairs and photographs of the worn areas.
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.