How soya husk fired boilers work
Soya husk, or hull, is the thin seed coat removed when soybeans are dehulled before solvent extraction or in soya food plants. It is a fine, light fuel with 10 to 13% moisture, 4 to 6% ash and a calorific value of about 3,700 to 4,000 kcal/kg, available in large volumes in Madhya Pradesh and Maharashtra during the crushing season. Soya plants run continuously and need steam for desolventising, toasting and drying, so boilers of 5 to 30 TPH burn the husk on travelling grates, in AFBC beds or as pellets. Because the boiler often serves a hexane extraction plant, it stands at a safe distance with spark arresting in the gas path. Gas passes the economiser, air heater, multiclone with spark arrestor and a bag filter or ESP to the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Grate
- Overfire air nozzles
The FD fan takes ambient air near the air heater and pushes it through the air heater into the undergrate plenum. A large share of the air is diverted to overfire nozzles, either from the FD fan through a branch or from a separate secondary air fan. The undergrate resistance is moderate because the husk bed is thin, so on many units it is the overfire nozzles, not the grate, that set the pressure the fan must develop.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the air heater | 140 to 180°C, downstream of the fan |
| Pressure at the fan outlet | Typically 100 to 160 mmWC on a grate |
| Air split | A large share of total air above the grate |
| Dust | Clean air; husk dust near the dehulling section |
| 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 soya husk fired boilers
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on steam pressure, with separate dampers on the undergrate and overfire branches |
| Also specified | Inlet screen, and an intake located away from the dehulling and husk conveying area |
What this FD fan has to cope with
Poor burnout of fine husk
Fine husk needs overfire air with good mixing to burn completely before leaving the furnace. If the FD fan sends most of its air under the grate and little above it, char escapes to the collectors and loads the spark arrestor. A proper split, with overfire nozzles at high velocity, cuts unburnt carbon and protects the gas path.
Husk dust from dehulling
Dehulling and husk conveying raise a fine, light dust that drifts across the plant. Drawn into the FD fan, it coats the impeller and the air side of the air heater and slowly cuts both air flow and preheat. Locating the intake away from the dehulling section and fitting an inlet screen keep the air clean.
Switching to pelletised husk
Plants sometimes move to pelletised husk for easier handling and storage. Pellets form a denser bed that needs more undergrate air and more pressure, and less overfire air. A fan with pressure reserve and speed control handles both loose and pelletised husk without a change of impeller.
Energy and control
A soya plant draws steady steam through the crushing season, so the FD fan runs long hours at a similar load. That makes the fan efficiency itself matter: an oversized or worn fan running throttled wastes power every hour of the season. A correctly sized backward curved fan on a variable speed drive, with oxygen trim, keeps both fan power and excess air down.
ID fan and FD fan in soya husk fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Ahead of the air heater |
| What it handles | Flue gas with fluffy ash and fine char | Ambient air |
| Temperature at the fan | 155 to 180°C | Ambient, 20 to 45°C |
| Main design concern | Draft through the spark arrestor and collectors | Overfire air split for burnout |
| Control | Furnace draft | Steam pressure with oxygen trim |
See also the Induced Draft (ID) Fan for soya husk fired boilers.
Questions buyers ask
How much overfire air does a soya husk boiler need?
Often 30 to 50% of the total air, more than on a wood-fired grate, because fine husk burns largely in suspension. The exact share depends on furnace design and whether the husk is loose or pelletised. Overfire air must enter at high velocity so that it mixes well with the volatiles.
Is a separate secondary air fan needed for soya husk?
On larger boilers, usually yes, because overfire nozzles need higher pressure than the undergrate plenum. On smaller boilers a branch from the FD fan with its own damper can be enough. The choice depends on the nozzle pressure the furnace design calls for and on how independently the two air streams must be controlled.
Why is unburnt carbon high in soya husk fly ash?
Fine husk burns in suspension and can leave the furnace before burning out. Too little overfire air, poor mixing or too much undergrate air are the usual causes. Adjusting the air split and the nozzle velocity typically lowers unburnt carbon and the char load on the collectors.
Information needed to quote an FD fan for soya husk 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 split. Proportion of undergrate and overfire air, and whether a separate secondary air fan is fitted.
- Fuel form. Loose or pelletised husk, and any blend with other fuels.
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.