How tamarind husk fired boilers work
Tamarind husk fuel comes from two streams: the reddish seed coat, or testa, removed when tamarind seed is roasted and decorticated to make tamarind kernel powder, and the brittle pod shells and fibre left from pulp processing. Both are produced in Tamil Nadu, Karnataka, Andhra Pradesh and Telangana. The seed coat is fine and flaky, the shells coarser. Moisture is low, about 8 to 12%, ash typically a few percent but higher where floor sweepings are included, and the calorific value about 3,500 to 4,000 kcal/kg. The fuel is fired in small boilers, often in the kernel powder plant itself, on fixed grates with overfire air or by blowing the fines into the furnace. The gas passes the boiler, a cyclone and often a bag filter before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Fuel injection and overfire air ducts
- Furnace and grate
The FD fan stands near the boiler front. On units where the fine husk is blown in, part of its air carries the fuel through an injector into the furnace, part goes under the grate to burn the coarser shells, and part goes to overfire nozzles. Few such boilers have an air heater. The fan pressure is typically 100 to 200 mmWC, set by the injector and nozzles.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 42°C |
| Air after the fan | Ambient; an air heater is uncommon |
| Dust | Fine reddish seed coat dust in the plant |
| Pressure at the fan outlet | Typically 100 to 200 mmWC |
| Air split | Conveying, undergrate and overfire air from one fan |
| 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 tamarind husk fired boilers
| Blade form | Backward curved, with a steep curve for steady injection |
| 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 with separate dampers for each air stream |
| Also specified | Inlet screen and dampers on conveying, undergrate and overfire ducts |
What this FD fan has to cope with
Wrong conveying air velocity
If conveying air is too low, fine husk settles in the injector duct and then surges into the furnace. If too high, it overshoots and burns against the back wall. A fan with a steep curve and a dedicated damper on the conveying duct keeps the velocity steady as the furnace pressure changes.
Overfire air short
Fine husk releases volatiles fast, and without enough overfire air they leave as smoke and tarry soot. The fan must supply the overfire share at full load with the conveying and undergrate air also open, which sets its total flow and pressure. Selecting the fan only on undergrate air, as for a lump fuel boiler, leaves the overfire nozzles starved.
Seed coat dust at the intake
Roasting and decorticating release fine dust that settles everywhere in the plant. An intake taken from outside the processing hall, with a screen, keeps the impeller and the injector clean and the air balance steady. Without it, dust builds on the blades and the inlet screen within weeks, cutting airflow and unbalancing the wheel, and the extra fines reaching the furnace raise spark carry-over.
Energy and control
Tamarind husk boilers run through the kernel powder season at fairly steady load. A variable speed FD fan, with the air split set once by dampers, saves power at part load and keeps combustion steady. Reducing excess air also reduces the gas volume, the ID fan power and the spark carry-over.
ID fan and FD fan in tamarind husk fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and bag filter | At the boiler front, feeding injector and nozzles |
| What it handles | Flue gas with fine char and sparks | Ambient air, part as fuel conveying air |
| Temperature at the fan | 170 to 210°C, up to 260°C | Ambient, 25 to 42°C |
| Main design concern | Steady draft and spark protection | Split of conveying, undergrate and overfire air |
| Control | Furnace draft | Steam pressure and air dampers |
See also the Induced Draft (ID) Fan for tamarind husk fired boilers.
Questions buyers ask
How is fine tamarind husk fed into the furnace?
Often by air. Part of the FD fan's output carries the fine seed coat through an injector into the furnace, while the rest goes under the grate and to overfire nozzles. The fan pressure must cover the injector as well as the nozzles, and the conveying duct needs its own damper so the fuel flow stays steady when the other dampers move.
Why does my tamarind husk boiler smoke?
Fine husk releases its volatiles quickly, and if there is not enough overfire air they leave unburnt. Increasing overfire air from the forced draft fan and steadying the fuel injection usually clears the smoke and reduces soot on the tubes. Check also that the ID fan is not pulling fuel out of the furnace too quickly.
What FD fan pressure is needed for a tamarind husk boiler?
Typically 100 to 200 mmWC, depending on whether the fuel is blown in and on the injector and nozzle design. We size it from the boiler data or the existing fan nameplate and site readings. Pneumatic feeding usually needs the higher end of the range, because the injector needs velocity to carry the fuel.
Information needed to quote an FD fan for tamarind husk 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.
- Feeding method. Pneumatic injection, screw feed or manual, and the conveying air quantity.
- Air distribution. Undergrate and overfire air arrangement and damper positions.
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.