How eucalyptus bark fired boilers work
Eucalyptus bark is stripped from pulpwood logs in the drum debarkers of paper mills and fired, often with wood chips, sawdust and other biomass, in the mill's power boiler. It is wet, at 45 to 55% moisture, with a calorific value of about 1,800 to 2,400 kcal/kg as fired. Clean bark has 3 to 6% ash, rich in calcium, but logs dragged and stacked on bare ground bring sand and soil that can push total ash to 10% or more. The long, stringy bark is shredded before firing on a travelling or reciprocating grate, or in a bubbling fluidised bed. The flue gas passes the superheater, economiser and air heater, then an ESP, before the ID fan and chimney.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum or air distributor
- Grate or fluidised bed
The FD fan sits at ground level near the air heater and pushes ambient air through it, heating it to about 200 to 280°C, into the grate plenum or the bed air distributor. On grate units the overfire air comes from a separate fan. On a bubbling fluidised bed the FD fan supplies fluidising air at much higher pressure, typically 700 to 1,000 mmWC, compared with 150 to 250 mmWC on a grate.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 200 to 280°C, downstream of the fan |
| Dust | Wood and bark dust from the fuel yard |
| Pressure at the fan outlet | 150 to 250 mmWC for a grate; 700 to 1,000 mmWC for a fluidised bed |
| Standby | Often duplicated on paper mill boilers |
| 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 eucalyptus bark fired boilers
| Blade form | Backward curved, for efficiency at high pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air flow with oxygen trim |
| Also specified | Inlet silencer for high pressure bed fans, inlet screen and acoustic lagging |
What this FD fan has to cope with
Dense, wet bed
Wet, stringy bark forms a dense bed that resists air. As moisture rises, the fan needs more pressure to keep air flowing and drying going. Without margin, combustion falls back along the grate, unburnt bark reaches the ash pit and the boiler loses output just when the mill needs steam.
Sand building up in the fluidised bed
Sand brought in with bark adds to the bed and changes its size distribution. Bed resistance rises, and the FD fan has to hold fluidising flow against it. Regular bed draining and screening, with a fan whose curve stays stable at the higher pressure, keep the bed fluidised and the fan clear of its stall point.
Noise from high pressure fans
Fluidising air fans at 700 mmWC and above are loud, mostly at the inlet. An inlet silencer, lagging on the casing and a flexible connection to the duct keep noise in the boiler house within limits and stop vibration travelling into the ductwork. Choosing a fan size that meets the pressure at moderate tip speed helps more than any silencer added later.
Energy and control
The FD fan on a paper mill boiler runs around the clock. A variable speed drive with oxygen trim saves power and fuel as bark moisture changes. On fluidised beds, keeping the bed inventory and pressure drop near design by regular draining reduces fan power directly, since every extra millimetre of bed resistance is paid for continuously at the fan.
ID fan and FD fan in eucalyptus bark fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP | Before the air heater |
| What it handles | Wet flue gas with ash and sand | Ambient air |
| Temperature at the fan | 160 to 190°C, up to 230°C | Ambient, 25 to 45°C |
| Main design concern | Sand erosion and wet gas volume | Pressure through air heater and wet bed |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Induced Draft (ID) Fan for eucalyptus bark fired boilers.
Questions buyers ask
How hot should the undergrate air be for a bark fired boiler?
Wet bark at 45 to 55% moisture usually needs air at about 200 to 280°C to dry on the grate. The FD fan pushes ambient air through the air heater; the hot air temperature depends on the air heater surface and the flue gas temperature entering it.
What FD fan pressure is needed for a bark fired fluidised bed boiler?
Typically 700 to 1,000 mmWC, covering the air heater, ducts, distributor and the bed itself. A grate fired boiler needs far less, typically 150 to 250 mmWC. The forced draft fan for a bed is therefore a high pressure fan, often needing an inlet silencer.
Can the FD fan handle more bark in the fuel mix?
Bark is wetter than chips, so more bark means a wetter bed, higher bed resistance and more air per tonne of steam. Check the FD fan's pressure and flow margin at the highest bark share expected, especially if the boiler was designed mainly for chips.
Information needed to quote an FD fan for eucalyptus bark 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. Grate or fluidised bed, with the air heater outlet temperature.
- Bark share. Highest share of bark in the fuel mix and its moisture.
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.