How jute stick fired boilers work
Jute sticks are the woody core left after the fibre is stripped from retted jute stalks, mainly in West Bengal, Assam and Bihar. They are light and hollow, with a bulk density of about 100 to 150 kg/m³, 10 to 15% moisture when sun-dried, 1 to 3% ash and a calorific value of about 3,800 to 4,200 kcal/kg. Volatile matter is high, so the sticks flame quickly and burn mostly above the bed. They are chopped and charged by hand or by conveyor onto fixed or inclined grates of smoke tube and small water tube boilers in jute mills, rice mills and food units. The flue gas passes the boiler, sometimes an economiser, and a cyclone or multicyclone before the ID fan and chimney.
Where the FD fan sits
- Inlet screen
- FD fan
- Air damper
- Undergrate plenum and overfire nozzles
- Fixed or inclined grate
The FD fan is mounted beside the boiler front and supplies the plenum under the grate through a damper, while a branch or a small separate fan feeds overfire nozzles in the furnace wall. Few small jute stick boilers have an air heater, so the air enters the furnace at ambient temperature. The fan pressure is modest, typically 60 to 120 mmWC, set by the grate, the thin bed and the nozzles.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 40°C |
| Air after the fan | Ambient; an air heater is uncommon |
| Dust | Jute fibre and stick dust in the boiler house air |
| Pressure at the fan outlet | Typically 60 to 120 mmWC |
| Humidity | High through the monsoon; negligible effect on the duty |
| 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 jute stick fired boilers
| Blade form | Backward curved, for steady flow at modest pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Variable speed drive or inlet damper linked to steam pressure |
| Also specified | Inlet screen against jute fibre and a separate damper for overfire air |
What this FD fan has to cope with
Too much air through the grate
Excess air under the grate lifts the light sticks and blows burning flakes into the boiler passes, raising char carry-over and gas temperature. Most of the air should enter above the bed. Setting the FD fan for a smaller undergrate share, with more overfire air, keeps the flame in the furnace and the char on the grate.
Fibre on the inlet screen
Jute mills and stick yards shed fine fibre that wraps around screens and impellers. As it builds, the fan delivers less air over the shift and smoke increases. A screen with generous area, an intake taken from outside the fuel area, and a cleaning routine keep the airflow from falling.
Smoke after each charge
The burst of volatiles after charging needs extra air above the bed for a short time. An FD fan with spare overfire capacity, or a timed boost after each charge, burns the volatiles completely. Without it, the boiler smokes for a minute after every charge however well the rest of the cycle runs.
Energy and control
Small jute stick boilers usually run one or two shifts, with load following the mill. A variable speed drive on the FD fan, matched to the ID fan, saves power and allows an overfire boost after each charge without wasting air the rest of the time. Sealing air leaks at the firing and ash doors is often the first and cheapest step, because leakage air does nothing useful.
ID fan and FD fan in jute stick fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the chimney | At the boiler front, feeding grate and overfire nozzles |
| What it handles | Flue gas with light char flakes | Ambient air with jute fibre |
| Temperature at the fan | 180 to 230°C, up to 280°C | Ambient, 20 to 40°C |
| Main design concern | Draft swings and sparks | Overfire air for volatiles |
| Control | Furnace draft | Steam pressure |
See also the Induced Draft (ID) Fan for jute stick fired boilers.
Questions buyers ask
Why does my jute stick boiler give black smoke after charging?
Jute sticks release volatiles in a burst when charged, and there is not enough air above the bed to burn them. Raising overfire air from the FD fan, or boosting it for a short time after each charge, usually clears the smoke without raising excess air for the rest of the cycle.
How much air should go under the grate on a jute stick boiler?
Less than for wood logs or coal. Jute stick burns mostly as flame above the bed, so a large share of the air from the forced draft fan should enter through overfire nozzles. The undergrate air keeps the grate bars cool and burns the char left on the bed.
Does the FD fan of a jute stick fired boiler need a filter?
A screen is usually enough, provided the intake is not in the stick yard or the mill. Jute fibre in the air wraps around unscreened impellers and cuts airflow, so the screen must be checked and cleaned often, especially in the stripping season when fibre is everywhere.
Information needed to quote an FD fan for jute stick 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 distribution. Undergrate and overfire air arrangement, and any separate overfire fan.
- Duty cycle. Shifts per day and the steam demand pattern.
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.