How castor shell fired boilers work
Castor shell is the husk left when castor seed is decorticated, produced mostly in north Gujarat and parts of Rajasthan. It fires boilers in castor oil mills and castor derivative chemical plants, often on AFBC units or travelling grates and frequently blended with lignite, coal or other husks. The shell is crumbly and breaks into fines in handling. It carries 8 to 12% moisture, about 6 to 12% ash including sand from threshing floors, and a calorific value of about 3,500 to 4,000 kcal/kg. In an AFBC the shell burns in a bed of sand at about 800 to 900°C, and fines and bed material carried over reach the cyclones. The flue gas passes the boiler bank, economiser and air heater, then cyclones and a bag filter or ESP, before the ID fan.
Where the ID fan sits
- AFBC bed or grate
- Boiler bank
- Economiser
- Air heater
- Cyclones
- Bag filter or ESP
- ID fan
- Chimney
The fan sits after the bag filter or ESP. In an AFBC, much of the shell ash leaves as fly ash with fine sand worn from the bed, so the loading at the cyclones is heavy and gritty. Where the final collector is weak or bypassed, the fan sees that grit directly. Suction at the fan is typically 250 to 350 mmWC on an AFBC unit with cyclones and a bag filter, and 150 to 250 mmWC on a grate unit.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 220°C at peak load with fouled surfaces |
| Dust at the fan | Fine shell ash with bed sand and grit; low after a bag filter, abrasive after cyclones alone |
| Moisture and dew point | Water dew point about 45 to 55°C |
| Corrosive species | Low from castor shell alone; SO₂ rises when lignite or coal is blended |
| 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 castor shell fired boilers
| Blade form | Backward curved with wear liners; radial tipped where the fan follows cyclones only |
| Material of construction | Mild steel casing with cut-off liners; impeller in wear-resistant plate |
| Drive and arrangement | Direct coupled, arrangement 8, at moderate tip speed |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Replaceable blade liners, inspection door and casing drain |
What this ID fan has to cope with
Bed sand in the fly ash
Bed material in an AFBC wears into fine sand that leaves with the fly ash. Mixed with grit from threshing floors in the shell, it is far more abrasive than the shell ash alone. A working bag filter keeps most of it from the fan, and wear-resistant blades and liners give life through the occasional bypass.
Fuel fines and carry-over
Castor shell breaks into fines in handling and storage. Fines burn quickly above the bed or leave as char, raising fly ash and heat release in the freeboard. The collector load rises and gas temperature at the fan creeps up, so the fan selection covers the higher volume of a fines-rich fuel.
Blends with lignite or coal
Many plants blend castor shell with lignite or coal for stable firing. Lignite adds sulphur and raises the acid dew point; coal adds ash. The fan duty, material and insulation are checked for the highest blend share, not only for castor shell. Insulation that keeps the casing above the acid dew point matters most when lignite is in the fuel.
Energy and control
Castor oil and derivative plants run continuously, with steam demand that follows the reaction and distillation batches. A variable speed ID fan on furnace draft saves power through the lower-demand periods and copes with the steady rise in bag filter resistance between cleanings. On AFBC units, keeping the bed inventory near design also reduces the gas side pressure loss through the cyclones.
ID fan and FD fan in castor shell fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Before the air heater, feeding the bed |
| What it handles | Flue gas with shell ash and bed sand fines | Hot, dusty ambient air |
| Temperature at the fan | 150 to 180°C, up to 220°C | Ambient, 15 to 48°C |
| Main design concern | Abrasive fly ash | Fluidising pressure at summer density |
| Control | Furnace draft | Air flow with minimum fluidising interlock |
See also the Forced Draft (FD) Fan for castor shell fired boilers.
Questions buyers ask
Why does my castor shell AFBC boiler wear out the ID fan?
Fly ash from an AFBC contains fine bed sand, and castor shell brings threshing-floor grit. Whatever the collectors let through is highly abrasive. A good bag filter, wear-resistant blades and liners, and a moderate tip speed extend the life of the induced draft fan. Checking the cyclones for wear and leaks helps too, because a holed cyclone passes far more grit.
Can castor shell be blended with lignite in the same boiler?
Yes, and many Gujarat plants do. Lignite adds sulphur and ash, so the acid dew point and dust load at the ID fan change. The fan should be checked at the highest lignite share, with insulation to keep the casing above the dew point. Blending also changes the gas volume, so the fan's operating point is confirmed at each blend the plant uses.
What ID fan pressure does a castor shell AFBC boiler need?
Typically 250 to 350 mmWC at the fan, covering the boiler, economiser, air heater, cyclones and bag filter. Grate fired units need less. We size it from the boiler maker's data or measured values, with margin for bag filter loading and for the higher cyclone resistance when the bed inventory runs above design.
Information needed to quote an ID fan for castor 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 blend. Share of castor shell, lignite, coal or other husks, and the analysis of each.
- Collector. Cyclone and bag filter or ESP arrangement, and any bypass.
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.