How wood chip travelling grate boilers work
Wood chip travelling grate boilers, typically 20 to 100 TPH, are used in paper mills, panel plants and biomass power stations. Chips and bark are spread pneumatically or mechanically onto a slowly moving grate; fines burn in suspension and the rest burns on the grate as it travels to the ash discharge. Paper mills often fire bark from debarking drums mixed with chips and sometimes sludge, and bark carries sand and soil picked up during harvest and handling. Moisture is typically 35 to 55%. The gas passes superheater, boiler bank, economiser and air heater, then a multiclone and ESP or bag filter before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate duct
- Zone dampers
- Travelling grate
The FD fan pushes ambient air through the air heater, heating it to about 180 to 250°C, then into compartments under the grate. Each compartment damper sets the air along the grate length, with more toward the feed end where wet chips land and dry. Secondary or overfire air, and spreader air where pneumatic spreaders are used, come from separate fans. Undergrate air is typically 40 to 60% of the total.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 180 to 250°C, downstream of the fan |
| Pressure at the fan outlet | About 150 to 250 mmWC |
| Share of total air | About 40 to 60% as undergrate air |
| Dust | Wood dust and fibre from fuel handling near the intake |
| 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 wood chip travelling grate boilers
| Blade form | Backward curved or aerofoil, efficient on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled with pedestal bearings |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive with oxygen trim |
| Also specified | Inlet screen, silencer where needed |
What this FD fan has to cope with
Air starvation at the grate edges
Wide travelling grates lose air at the edges and the far zones if plenum pressure falls at part load, and wet chips then pile up unburnt at those points. Control to plenum pressure, rather than a single throttling damper, and a fan with a steadily rising curve keep air distribution even across the whole grate.
Air heater leakage
Air heater tubes eroded by bark sand on the gas side start to leak FD air directly into the flue gas, increasing both the FD and the ID duties as the year goes on. The selection includes a realistic leakage allowance, and the leakage is worth measuring at each overhaul to decide on retubing.
Wood dust at the intake
Fuel handling areas and chip screens carry wood dust and fibre in the air, which builds up on the FD impeller and blinds inlet screens. A screened intake from a clean area, away from the chip conveyors and screens, prevents unbalance and keeps the fan delivering its full flow.
Energy and control
The FD fan follows load and fuel moisture through the day. A variable speed drive with oxygen trim keeps excess air low, which saves FD power and, more importantly, reduces the wet gas volume through the ID fans. Correct air split between undergrate and overfire air also cuts char carry-over to the collectors.
ID fan and FD fan in wood chip travelling grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After ESP or bag filter | Before the air heater |
| What it handles | Wet flue gas with bark sand and char | Ambient air |
| Temperature at the fan | 150 to 180°C | Ambient, 25 to 45°C |
| Main design concern | Bark sand erosion and fire risk | Even air across the grate |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Induced Draft (ID) Fan for wood chip travelling grate boilers.
Questions buyers ask
What FD fan pressure suits a wood chip travelling grate boiler?
Typically about 150 to 250 mmWC at the fan outlet, covering the air heater, ducts, zone dampers and the grate with its chip bed. The exact value depends on grate size, air heater design and the share of undergrate air. On existing boilers we confirm it from measured plenum pressure at full load.
Why is air preheat important for wood chip boilers?
Wood chips at 35 to 55% moisture must dry before they burn. Air preheated to 180 to 250°C under the grate speeds drying and ignition, and keeps the bed stable when wetter chips arrive. The FD fan itself handles ambient air; the heating is done in the air heater downstream of the fan.
Is the FD fan the only air fan on a wood chip boiler?
No. Most travelling grate boilers have a separate secondary or overfire air fan for the volatiles and fines burning above the grate and, where pneumatic spreaders are used, a distributor air fan. Each is sized for its own pressure and flow, and all are controlled together to hold total air and the split.
Information needed to quote an FD fan for wood chip travelling grate 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 split. Shares of undergrate, overfire and spreader air.
- Grate zones. Number of undergrate compartments and design pressure.
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.