How Juliflora wood fired boilers work
Prosopis juliflora is a thorny invasive shrub harvested across the dry districts of Tamil Nadu, Gujarat and Rajasthan, and is the main fuel for many biomass power plants of about 7.5 to 20 MW in Tamil Nadu. The wood is chipped to 25 to 50 mm, with 20 to 35% moisture as fired, 2 to 4% ash plus soil and sand from uprooted stumps, and a calorific value of about 3,000 to 3,500 kcal/kg. Power plant boilers of 30 to 90 TPH run at high pressure and temperature with superheaters, firing on travelling grates or in AFBC and CFBC furnaces. Gas passes the superheater, boiler bank, economiser, air heater and ESP to the ID fans, usually two in parallel, then the chimney.
Where the FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Hot air duct
- Grate zones or bed air box
The FD fan, often one of two, takes ambient air through an inlet silencer and pushes it through the air heater into the hot air duct. On a travelling grate, the air goes to zoned compartments under the grate; on an AFBC or CFBC unit, to the air box under the distributor. Separate secondary air fans supply overfire air. The FD fan works against the air heater and either the grate or the much higher bed resistance.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 42°C |
| Air after the air heater | 180 to 250°C, downstream of the fan |
| Pressure at the fan outlet | Typically 150 to 250 mmWC on a grate; 700 to 1,100 mmWC on a fluidised bed |
| Dust | Clean air; chip yard dust can be drawn in without a screened intake |
| Operating pattern | Continuous base load |
| 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 Juliflora wood fired boilers
| Blade form | Backward curved or aerofoil, for efficiency at base load |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, with medium voltage motor on larger units |
| Bearings and cooling | Pedestal bearings with oil lubrication and temperature monitoring |
| Flow control | Variable speed drive on air flow or steam demand |
| Also specified | Inlet silencer and screen, outlet isolating damper for parallel fans |
What this FD fan has to cope with
Grate and bed need different pressures
A travelling grate needs about 150 to 250 mmWC, while a fluidised bed needs several times that. Plants sometimes convert one firing system to the other; the FD fan then has the wrong duty. The fan selection is based on the actual firing system and any planned conversion.
Air heater leakage at high pressure
The FD fan pushes air at a higher pressure than the flue gas on the other side of the air heater. Worn or corroded tubes let air leak across, wasting FD fan power and loading the ID fan. Regular air heater inspection and including leakage in the FD fan duty avoid surprises.
Parallel fans out of balance
Where two FD fans feed a common duct, one can take more load than the other. Stable pressure curves and balanced control keep the load shared, and the selection confirms how much air one fan can supply if the other trips. Isolating dampers prevent backflow.
Energy and control
In a power plant the FD fan runs every hour the plant generates, and its power comes off the export. Efficient aerofoil or backward curved wheels and a variable speed drive pay back quickly. Reducing excess air with good oxygen control, and keeping air heater leakage low, lower the flow the FD fan must deliver and also reduce ID fan power.
ID fan and FD fan in Juliflora wood fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, usually two in parallel | Ahead of the air heater, often two in parallel |
| What it handles | Flue gas with fine ash and sand | Ambient air |
| Temperature at the fan | 140 to 160°C | Ambient, 25 to 42°C |
| Main design concern | Parallel stability and erosion life | Efficiency at base load |
| Control | Furnace draft | Air flow or steam demand |
See also the Induced Draft (ID) Fan for Juliflora wood fired boilers.
Questions buyers ask
What pressure does the FD fan need on a juliflora power plant boiler?
It depends on the firing system. A travelling grate needs about 150 to 250 mmWC at the fan outlet. An AFBC or CFBC boiler needs 700 to 1,100 mmWC because the air must pass through the distributor and the bed. The boiler maker's air system data is the starting point.
Is a VFD worth fitting to the FD fan on a biomass power plant?
Usually, yes. Even at base load, the fan rarely runs exactly at its design point, and a variable speed drive trims power where a damper wastes it. Over thousands of hours a year, the saving in auxiliary power typically pays for the drive. The motor must be inverter rated.
How does air heater leakage affect the FD fan?
Air leaking from the air side to the gas side of the air heater is supplied by the FD fan but never reaches the furnace. The FD fan must deliver more air, and the ID fan must remove it. Checking leakage at overhauls and including it in the fan duty keeps both fans properly sized.
Information needed to quote an FD fan for Juliflora wood 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.
- Firing system. Travelling grate, AFBC or CFBC, and the pressure required at the air box or plenum.
- Fan configuration. Number of FD fans and the share of full load each must carry.
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.