How wood chip reciprocating grate boilers work
Reciprocating grate boilers push wood chips down an inclined grate of alternating fixed and moving bars, which stroke back and forth to tumble the bed forward. The action suits wet, uneven chips and bark, typically 30 to 55% moisture, that would burn poorly on a static grate. Chips dry on the upper section, burn in the middle and burn out at the bottom before ash falls to a wet or dry ash conveyor. Wood ash is low, usually 0.5 to 3%, but rich in potassium and calcium, and bark adds sand. Many units recirculate cooled flue gas under the grate or into the furnace to control temperature. The gas passes the boiler, economiser and often an air heater, then a multiclone and bag filter or ESP.
Where the ID fan sits
- Reciprocating grate furnace
- Boiler
- Economiser
- Multiclone
- Bag filter or ESP
- ID fan
- Chimney
The fan sits after the bag filter or ESP, with suction typically 150 to 250 mmWC. On plants with flue gas recirculation, the recirculated gas is often taken from the ID fan discharge or the duct just after the filter, so the ID fan carries the chimney flow plus the recirculated share. Wet chips produce flue gas with 20 to 30% water vapour, so the gas temperature at the fan must be kept clear of the water dew point.
Gas at the ID fan
| Gas temperature, normal | 150 to 190°C after economiser or air heater |
| Maximum or upset | Up to 230°C with fouled economiser or very dry fuel |
| Dust at the fan | Fine alkaline wood ash and some bark sand, typically below 50 mg/Nm³ after a bag filter |
| Moisture and dew point | 20 to 30% water vapour on wet chips; water dew point about 60 to 70°C |
| Recirculated gas | FGR flow, where taken after the fan, adds to the fan duty |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 reciprocating grate boilers
| Blade form | Backward curved, efficient on low dust, with smooth surfaces that resist deposits |
| Material of construction | Mild steel; stainless steel at the casing bottom and drain where condensation is frequent |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft, coordinated with the FGR fan |
| Also specified | Casing drain, insulated casing, inspection door |
What this ID fan has to cope with
Condensation during low load
When load drops or the boiler starts cold, wet wood flue gas can cool to near its dew point in the filter and fan. Moisture and alkaline ash form a paste on the blades and casing. Insulating the fan and duct, warming the system before full chip feed, and a casing drain keep the fan dry.
Alkali ash build-up
Wood ash is fine and rich in potassium and calcium compounds. On damp surfaces it hardens into deposits that form unevenly across the blades and throw the impeller out of balance within weeks. Smooth blade surfaces, an access door for cleaning, and regular vibration checks catch build-up before it damages the bearings.
FGR flow left out of the fan duty
Where recirculated gas is taken downstream of the ID fan, the fan must move chimney flow plus recirculation. Fans sized only on chimney flow run short of draft when FGR is increased to control grate temperature on dry fuel. We size the fan on the total flow it actually carries.
Energy and control
Reciprocating grate boilers in paper mills and process plants often run steadily, but chip moisture changes daily and alters the gas volume. A variable speed drive on draft follows those changes. Where FGR is used, coordinating the ID and FGR fans avoids one fan fighting the other. Drying chips in covered storage reduces gas volume and fan power together.
ID fan and FD fan in wood chip reciprocating grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After bag filter or ESP | Before the grate zones |
| What it handles | Wet flue gas with fine alkaline ash, plus FGR | Ambient air, sometimes mixed with FGR |
| Temperature at the fan | 150 to 190°C | Ambient, warmer if FGR is mixed in |
| Main design concern | Condensation and deposits | Drying air for wet chips |
| Control | Furnace draft with FGR coordination | Steam pressure and zone dampers |
See also the Forced Draft (FD) Fan for wood chip reciprocating grate boilers.
Questions buyers ask
Why does wet wood fuel need a bigger ID fan?
Water in the fuel leaves as vapour in the flue gas, so a boiler burning 50% moisture chips produces considerably more gas per tonne of steam than one on dry fuel. The ID fan must be sized for the wettest fuel the plant expects, with the motor rated for that volume.
What causes deposits on the ID fan of a wood chip boiler?
Fine, alkaline wood ash sticks where the fan runs damp, typically at start-up, low load or when the gas cools close to its dew point. Keeping the gas temperature above the dew point, insulating the fan and duct, and cleaning at shutdowns prevents hard build-up.
Should the FGR fan be separate from the ID fan on a reciprocating grate boiler?
Usually yes. A separate FGR fan gives independent control of recirculation for grate and furnace temperature, at the pressure the FGR nozzles need. Where the recirculated gas is taken from the ID fan discharge instead, the ID fan duty includes that flow, and the ID fan and FGR dampers are controlled together from one loop.
Information needed to quote an ID fan for wood chip reciprocating 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.
- Chip moisture range. Driest and wettest fuel moisture, and share of bark.
- Flue gas recirculation. Whether FGR is used, its take-off point and flow.
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.