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 ID fan sits
- Grate or fluidised bed furnace
- Boiler bank
- Economiser
- Air heater
- Multiclone and spark arrestor
- Bag filter or ESP
- ID fan
- Chimney
The fan is placed after the bag filter or ESP. Soya husk fly ash is light and carries unburnt char, so a multiclone with a spark arrestor removes burning particles ahead of the filter. Suction at the fan inlet is typically 200 to 280 mmWC including the spark arrestor, a component that boilers in other industries may not have. The fan is sized for that full collector train, with no allowance for bypassing it.
Gas at the ID fan
| Gas temperature, normal | 155 to 180°C after the air heater |
| Maximum or upset | About 230°C with fouled surfaces |
| Dust at the fan | Light, fluffy ash with fine char; low after the final collector |
| Moisture and dew point | Water dew point about 45 to 50°C |
| Corrosive species | Low sulphur; some potassium and chlorine in the ash |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 soya husk fired boilers
| Blade form | Backward curved, since dust after the filter is light and soft |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Access door to clear char deposits, a casing drain and a pressure tapping across the spark arrestor |
What this ID fan has to cope with
Sparks near a solvent plant
A solvent extraction plant handles hexane, and burning particles leaving the chimney are a recognised hazard. The gas path includes a spark arrestor and an efficient final collector, and the ID fan must hold draft through their combined resistance at full load. Since bypassing them is not acceptable, the fan is selected for the full collector train with margin for fouling.
Char settling in the fan
Light char and fluffy ash settle in the fan inlet box and scroll during low-load running. A layer of char can smoulder after the fan stops, and if it breaks away on restart it unbalances the wheel. Regular cleaning through access doors, and avoiding long periods at very low speed, prevent build-up.
Carry-over from suspension burning
Soya husk is fine and much of it burns above the grate. Carry-over is high, and incomplete burnout raises the char in the fly ash and the load on the spark arrestor. The ID fan duty includes the extra air needed for good burnout, and better overfire air mixing upstream cuts the char that reaches the collectors.
Energy and control
Soya plants run continuously through the crushing season, often at a steady load. A variable speed drive on the ID fan still saves energy, because most fans carry generous margin and spend their life throttled. When crushing slows and the boiler runs on other fuels or at lower load, the drive follows that change too. Watching the pressure drop across the spark arrestor shows when cleaning will lower fan power.
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 Forced Draft (FD) Fan for soya husk fired boilers.
Questions buyers ask
Why do soya husk boilers need spark arrestors?
Soya husk burns largely in suspension and releases fine burning char into the gas. Many soya plants include hexane solvent extraction, where any spark escaping from the chimney is a fire risk. A spark arrestor and an efficient final collector keep burning particles out of the atmosphere, and the ID fan is sized for their resistance.
What type of ID fan suits a soya husk fired boiler?
Behind a bag filter or ESP, a backward curved fan in mild steel is normal, because the dust is light and wear is low. The fan must be sized for the full system resistance, including the spark arrestor, and fitted with access doors so char deposits can be removed.
Can soya husk be burnt in a coal boiler?
It can, often blended with coal or other biomass. Soya husk produces more flue gas per unit of heat than coal, so the ID fan capacity must be checked. Fine husk also burns differently, raising carry-over and the need for overfire air, so the furnace air system is reviewed at the same time.
Information needed to quote an ID fan for soya 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.
- Spark arrestor. Type and pressure drop of the spark arrestor in the gas path.
- Fuel form. Whether husk is fired loose, pelletised or blended 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.