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 FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Grate zones
The FD fan sits beside the air heater and pushes ambient air through it into the plenum under the grate. Shredded stalk falls unevenly, with long pieces bridging and fines filling the gaps, so the fan supplies zoned undergrate air whose pressure must vary along the grate. A separate secondary air fan usually supplies overfire air to burn the volatiles that stalk releases quickly.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 140 to 190°C, downstream of the fan |
| Pressure at the fan outlet | Typically 100 to 180 mmWC |
| Bed condition | Fibrous and uneven, with resistance varying along the grate |
| Dust | Clean air; lint and stalk dust near the fuel shed |
| 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 cotton stalk fired boilers
| Blade form | Backward curved, with a stable pressure curve for zoned 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 zone dampers along the grate |
| Also specified | Inlet screen, and a silencer where the fan is near work areas |
What this FD fan has to cope with
Channelling through a matted bed
Long stalk pieces mat on the grate while fines pack the gaps between them. Air takes the easiest route, burning holes in the bed and leaving other areas smouldering. Zone dampers, and enough FD fan pressure to keep the plenum pressurised across the full grate width, even out the burn.
Lint on the impeller
Shredding cotton stalk releases fine dust and lint. An FD fan drawing air near the fuel shed collects lint on its inlet cone and blades, which unbalances the wheel and shows up as rising vibration. Taking suction from a clean location and fitting an inlet screen avoids this.
Excess air from operator caution
Operators often run extra air to keep the stack clear of smoke from the volatile stalk. That raises gas volume, increases ID fan power and lowers boiler efficiency. Oxygen measurement and a variable speed drive on the FD fan let excess air be held at a sensible figure without risking smoke.
Energy and control
Dyeing and textile plants usually draw steam continuously, with dips during batch changes. Speed control on the FD fan, tied to steam pressure with oxygen trim, saves power through those dips. Because cotton stalk arrives in different forms through the year, from loose shreds to briquettes, the fan also has to cope with very different bed resistances, which speed control handles more smoothly than a damper.
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 Induced Draft (ID) Fan for cotton stalk fired boilers.
Questions buyers ask
How should air be distributed on a cotton stalk fired grate?
More air usually goes to the middle zones where most of the burning takes place, less to the drying and burnout zones, with overfire air above the bed to burn volatiles. The FD fan supplies undergrate air at a pressure high enough for each zone damper to control its share.
Can a cotton stalk fired boiler run on briquettes?
Yes, and many plants switch between loose stalk and briquettes depending on supply. Briquettes form a denser bed with higher resistance, so the FD fan settings and zone dampers need adjusting when the fuel changes. A variable speed drive makes these changes easier and keeps the air-fuel ratio steady.
Why is my forced draft fan vibrating on a cotton stalk boiler?
Lint and fine dust from stalk handling often collect on the impeller and cause unbalance. Cleaning the wheel, fitting an inlet screen and moving the intake away from the fuel shed usually solve it. If vibration persists after cleaning, check the bearings, alignment and foundation bolts.
Information needed to quote an FD fan for cotton stalk 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 supply. Forms of cotton stalk used through the year and their moisture.
- Grate zones. Number of zones and whether there is a separate overfire air fan.
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.