How cotton stalk fired boilers work
Cotton stalk is the woody residue left in the field after picking, gathered in Gujarat, Maharashtra, Telangana and Madhya Pradesh in a short window after harvest. It is shredded or chipped and stored, or pressed into briquettes. As fired it carries 15 to 30% moisture, 3 to 7% ash including soil, and a calorific value of about 3,300 to 3,800 kcal/kg. The ash is high in potassium and the stalk contains chlorine, so deposits form on heat surfaces and alkali chlorides travel with the gas. Fired on travelling or reciprocating grates in process boilers for dyeing and textile plants, and in some AFBC units, the gas passes the boiler, economiser, air heater, multiclone and bag filter or ESP before the ID fan.
Where the ID fan sits
- Grate furnace
- Boiler bank
- Economiser
- Air heater
- Multiclone
- Bag filter or ESP
- ID fan
- Chimney
The fan is after the bag filter or ESP. Cotton stalk ash is fine and light, and much of it passes the multiclone, so the final collector does most of the work. Suction at the fan inlet is typically 180 to 260 mmWC when clean, rising by 30 to 60 mmWC as the economiser and air heater foul between cleanings. Keeping the fan downstream of the collector also keeps chloride-bearing dust from settling in the casing.
Gas at the ID fan
| Gas temperature, normal | 160 to 180°C after the air heater |
| Maximum or upset | Up to 230°C with fouled heat surfaces |
| Dust at the fan | Fine alkali-rich ash and fibrous char; low after the final collector |
| Moisture and dew point | Water dew point 50 to 60°C depending on stalk moisture |
| Corrosive species | Alkali chlorides, corrosive in condensate at the cold end |
| 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 cotton stalk fired boilers
| Blade form | Backward curved, with smooth blade surfaces and no pockets for deposits |
| Material of construction | Mild steel; corrosion-resistant coating or SS 316 at the inlet box and drains |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive, with pressure margin for the fouled resistance |
| Also specified | Access doors for cleaning, a casing drain and an insulated casing |
What this ID fan has to cope with
Fouling raises system resistance
Potassium and chlorine in cotton stalk form sticky deposits on the economiser and air heater. Between soot blowing or shutdowns, pressure drop climbs steadily and the fan must deliver more suction to hold draft. A fan selected at clean resistance runs out of reserve before the next cleaning, so the duty is set at the fouled state.
Chloride condensation
When the boiler stops or runs very low, gas at the fan can cool towards its dew point. Chloride-laden condensate pits mild steel in the inlet box and lower casing. Insulation, drainage and protective coating at these points, with operating practice that avoids long spells at very low load, limit the damage.
Moisture changes across the season
Fresh stalk can arrive at 30% moisture, while stock stored for months dries to around 15%. The flue gas volume per tonne of steam changes noticeably across the year. The fan must cover the wettest fuel while still running efficiently on dry stock, which favours speed control over fixed-speed throttling.
Energy and control
Process boilers in dyeing and textile plants run around the clock, but steam demand drops on weekends and during dye-house slowdowns. A variable speed drive on the ID fan, controlled from furnace draft, saves power through these periods. Regular soot blowing pays twice on cotton stalk: it restores heat transfer and lowers system resistance, so the fan runs slower for the same draft.
ID fan and FD fan in cotton stalk fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Ahead of the air heater |
| What it handles | Flue gas with sticky alkali ash | Ambient air |
| Temperature at the fan | 160 to 180°C | Ambient, 25 to 45°C |
| Main design concern | Fouling margin and chloride corrosion | Air distribution through a fibrous bed |
| Control | Furnace draft | Steam pressure with oxygen trim |
See also the Forced Draft (FD) Fan for cotton stalk fired boilers.
Questions buyers ask
Why does ID fan current rise steadily on a cotton stalk boiler?
Cotton stalk ash is sticky and fouls the economiser and air heater. As deposits build, both resistance and gas temperature rise, and the fan works harder to hold furnace draft. Cleaning the heat surfaces resets it. A steady rise between cleanings is normal; a sudden jump points to a blocked collector, hopper or damper.
Does cotton stalk cause corrosion in the ID fan?
It can, mainly through chlorides in condensate when the fan cools below the dew point during shutdowns or very low load. With gas at the normal temperature, a mild steel fan performs well. Coating, external insulation and drains at the low points are usually adequate protection.
What size ID fan is needed for a cotton stalk fired boiler?
It depends on steam output, fuel moisture, excess air and the resistance of the boiler, air heater and collectors. Cotton stalk produces more flue gas per tonne of steam than coal, and the fan must also allow for fouling. Share the boiler data and the fuel moisture range and we will calculate the duty.
Information needed to quote an ID fan for cotton stalk 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 form. Whether stalk is fired shredded, chipped or as briquettes, and its moisture range.
- Cleaning cycle. How often heat surfaces are cleaned and the draft loss seen between cleanings.
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.