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 ID fan sits
- Furnace with overfire air
- Boiler passes
- Spark arrestor
- Cyclone
- Bag filter
- ID fan
- Chimney
The fan sits after the bag filter where one is fitted, otherwise after the cyclone. Fine seed coat flakes burn quickly, but some leave the furnace still glowing, and the cyclone separates them poorly. Suction at the fan is typically 100 to 200 mmWC on a 1 to 5 TPH boiler. The fan must hold a steady suction because the light fuel follows the draft: too much and it is pulled through unburnt, too little and the furnace puffs.
Gas at the ID fan
| Gas temperature, normal | 170 to 210°C, economisers are uncommon on small units |
| Maximum or upset | Up to 260°C with heavy fines feed |
| Dust at the fan | Fine char and ash; low after a bag filter, a few hundred mg/Nm³ after a cyclone |
| Moisture and dew point | Water dew point about 45 to 55°C |
| Corrosive species | Minor; tannin-rich fuel leaves tarry soot if combustion is poor |
| Density at the design temperature | About 0.77 kg/m³ at 190°C, 64% 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 tamarind husk fired boilers
| Blade form | Backward curved, for efficiency on light dust |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled arrangement 8, or overhung on small units |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Spark arrestor upstream of the bag filter, temperature trip for the filter, casing drain |
What this ID fan has to cope with
Sparks reaching the bag filter
Glowing flakes of seed coat pass the cyclone and can burn holes in filter bags or start a fire in the hopper. A spark arrestor or cooling section ahead of the bag filter, a temperature trip that stops the ID fan and closes dampers on a fire, and bags of suitable material reduce the risk to the filter and the fan.
Unburnt fuel pulled through
Fine husk is light enough to be drawn out of the furnace before it burns if the draft is too strong. Fuel goes to the cyclone as char, efficiency drops and the fire risk rises. A drive that holds furnace suction at a low, steady value keeps the fuel in the furnace long enough to burn.
Tarry soot from poor combustion
Tamarind seed coat is rich in tannins and, burnt short of air, leaves sticky, tarry soot on the boiler tubes and the fan casing. It raises resistance and can catch fire. Good overfire air, regular tube cleaning, and a fan with access doors for cleaning prevent build-up.
Energy and control
Kernel powder plants run steadily through the processing season, often on one boiler. A variable speed ID fan saves power and gives steady draft, which improves combustion of the fine fuel and reduces unburnt carbon. Adding an economiser where gas leaves above 200°C recovers heat and lowers the gas volume at the fan.
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 Forced Draft (FD) Fan for tamarind husk fired boilers.
Questions buyers ask
Why does my tamarind husk boiler have bag filter fires?
Fine seed coat flakes can leave the furnace still glowing and pass the cyclone. A spark arrestor ahead of the bag filter, steady ID fan draft so the fuel is not pulled out of the furnace unburnt, and a temperature trip on the filter reduce the risk considerably.
Can tamarind seed husk be fired in a normal grate boiler?
Yes, usually on a fixed grate with good overfire air, or blown into the furnace. The fuel is fine and burns largely in suspension, so overfire air and steady draft from the induced draft fan are more important than grate area. Coarser pod shells can be fired with the seed coat to give a steadier bed.
What ID fan does a tamarind husk fired boiler need?
Usually a backward curved fan in mild steel, after a cyclone and bag filter, with a variable speed drive on furnace draft. Wear is low; the design points are steady draft, spark protection and access for cleaning soot from the casing. Where only a cyclone is fitted, the fan should also be checked for char build-up at each stop.
Information needed to quote an ID fan for tamarind husk 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 type. Seed coat, pod shell or a mix, and whether floor sweepings are included.
- Spark protection. Present collector, spark arrestor and any fire detection in the bag filter.
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.