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 ID fan sits
- Furnace and travelling grate
- Superheater and bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan sits after the ESP or bag filter with suction typically 200 to 300 mmWC. A multiclone ahead of the final collector removes coarse sand and char, and with a bag filter a spark arrestor is usually fitted as well. When the fuel is mostly clean chips, the fan sees little dust. When bark share rises, so does sand at the fan, especially if the multiclone is worn.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 220°C with fouled air heater or char burning in hoppers |
| Dust at the fan | Low on clean chips; sand-rich with high bark share; 30 to 100 mg/Nm³ after ESP or bag filter |
| Moisture and dew point | 20 to 28% water vapour; dew point about 60 to 70°C |
| Co-fired fuels | Sludge or other biomass in paper mills can add chlorides and fine ash |
| Density at the design temperature | About 0.81 kg/m³ at 165°C, 68% 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 wood chip travelling grate boilers
| Blade form | Backward curved with leading edge liners, for efficiency with protection against bark sand |
| Material of construction | Mild steel; wear-resistant liners on blades and scroll cut-off |
| Drive and arrangement | Direct coupled, arrangement 8; two fans in parallel on large units |
| Bearings and cooling | Pedestal bearings with cooling disc; vibration monitoring |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Inlet temperature alarm, casing drain, access door |
What this ID fan has to cope with
Bark sand erosion
Bark from debarking drums often carries sand and soil, which passes the furnace and erodes the fan far faster than clean wood ash. Fans selected for clean chips wear out when bark share rises. Leading edge liners, moderate tip speed and a well-maintained multiclone ahead of the final collector are the answer.
Char sparks reaching the bag filter
Spreader firing throws light chips and char into the furnace, and some leave still glowing. Without a spark arrestor, bags burn and the fan receives char and burnt bag material. The fan casing needs a drain and access door so smouldering deposits can be cleared before restart.
Changing fuel mix
Paper mills vary the mix of bark, chips and sludge with supply and price, and gas volume, moisture and ash change with it. The fan is sized for the wettest, highest volume mix, lined for the sandiest one, and checked for efficiency at the usual mix so it does not spend most of the year throttled.
Energy and control
Wood chip travelling grate boilers usually run near steady load in paper mills and power plants, so ID fan efficiency at the normal point matters most. A variable speed drive handles daily changes in fuel moisture without throttling. Covered fuel storage, lower moisture and controlled excess air reduce gas volume and fan power more than any change to the fan itself.
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 Forced Draft (FD) Fan for wood chip travelling grate boilers.
Questions buyers ask
Why does the ID fan wear faster when we burn more bark?
Bark carries sand and soil from harvest and handling, which is much more abrasive than wood ash. As bark share rises, so does sand in the fly ash reaching the fan. Liners, lower tip speed and better mechanical collection ahead of the ESP or bag filter control the wear.
What ID fan is used on a wood chip travelling grate boiler?
Usually a backward curved ID fan with leading edge liners after an ESP or bag filter, designed for about 150 to 180°C and wet flue gas with 20 to 28% water vapour. Large boilers often use two fans in parallel. A variable speed drive on furnace draft is standard, with dampers for isolation.
How do we stop fires in the bag filter on a wood chip boiler?
Fit a spark arrestor or a multiclone ahead of the filter, keep combustion air and spreader settings right to reduce char carry-over, and monitor filter inlet temperature with an alarm. The ID fan should have a drain and access door so any smouldering deposits in the casing are cleared before restarting.
Information needed to quote an ID fan for wood chip travelling grate 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 mix. Shares of chips, bark and other fuels, with moisture and ash for each.
- Collector. Multiclone and ESP or bag filter, with spark arrestor details.
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.