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 FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Grate zones
The fan sits near the air heater at ground level. It takes ambient air, pushes it through the air heater and into compartments under the grate. Because loose shells lift easily, a good share of the air goes in above the grate as overfire air, from the FD fan or from a separate secondary air fan. The FD fan works against modest resistance: the air heater, dampers and a thin, open shell bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 130 to 180°C, downstream of the fan |
| Pressure at the fan outlet | Typically 80 to 160 mmWC |
| Bed resistance | Low, with a light open shell bed |
| Dust | Clean air; shell dust near the fuel feeders |
| 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 groundnut shell fired boilers
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled or impeller on the motor shaft |
| Bearings and cooling | Standard bearings; no cooling |
| Flow control | Variable speed drive on steam pressure, with zone dampers |
| Also specified | Inlet screen, and an overfire air branch with its own damper where no separate fan is fitted |
What this FD fan has to cope with
Shells lifted off the grate
Groundnut shells are light, and high undergrate velocity lifts them into the furnace before they ignite on the grate. Carry-over rises, unburnt carbon in the fly ash rises, and the collectors load up. A fan sized for the right undergrate share, with more air supplied as overfire air, keeps the bed where it belongs.
Uneven air through a thin bed
A light bed offers so little resistance that the plenum pressure barely builds, and air rushes through the zones nearest the duct while the far end starves. Zone dampers with enough pressure drop across them, and a fan with a stable curve at low flow, distribute the air evenly along the grate.
Shell dust at the intake
Decorticating and fuel handling raise fine dust that the FD fan can draw in. It builds on the blades and dampers and can block the air side of the air heater over a season. An intake away from the fuel area and an inlet screen solve this at very little cost.
Energy and control
Groundnut shell plants follow the crushing season, with high load for several months and lower load for the rest of the year. A variable speed drive on steam pressure, with oxygen trim, avoids running the FD fan throttled through the long low season. Holding excess air near 40 to 50% on the grate also cuts the gas the ID fan has to move.
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 Induced Draft (ID) Fan for groundnut shell fired boilers.
Questions buyers ask
How much undergrate air does a groundnut shell boiler need?
Typically 50 to 70% of the total air goes under the grate, with the rest as overfire air to burn volatiles and suspended shell. The exact split depends on the grate design and whether shells are fired whole, ground or as briquettes. Lighter, finer fuel needs a larger share of overfire air.
Why does my groundnut shell boiler show high carry-over?
Light shells lift from the grate when undergrate air velocity is too high. Reducing the undergrate share, raising overfire air, or briquetting the shells usually helps. The FD fan settings and zone dampers should be checked together with the fuel size, since ground shell behaves very differently from whole shell.
Can the FD fan of a coal boiler be reused for groundnut shells?
Sometimes. Coal needs higher undergrate pressure through a denser bed, so the fan may have more pressure than shells need, while its flow may be short because biomass needs more air per unit of heat. Checking the new duty against the fan curve shows whether reuse, a speed change or a new impeller is right.
Information needed to quote an FD fan for groundnut 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.
- Fuel form. Whether shells are fired whole, ground or as briquettes.
- Air split. Proportion of undergrate and overfire air, and whether a separate secondary air fan exists.
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.