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 FD fan sits
- Inlet screen
- FD fan
- Air heater or direct
- Zone dampers
- Reciprocating grate
- Furnace
The FD fan delivers primary air to compartments under the drying, burning and burnout sections of the grate. Where an air heater is fitted, air passes through it first; many smaller reciprocating grate boilers run without one. Secondary air from a separate fan enters through nozzles above the grate. On plants with FGR, recirculated gas may be mixed into the primary air duct, either before or after the FD fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C, or 60 to 100°C if FGR is mixed into the fan inlet |
| Pressure at the fan outlet | About 100 to 200 mmWC |
| Air preheat | Where fitted, 120 to 200°C after the air heater |
| Zone split | More air to drying and burning zones on wet chips |
| Dust | Clean air, or fine ash if FGR is mixed in |
| 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 reciprocating grate boilers
| Blade form | Backward curved, stable over the range of zone resistance |
| Material of construction | Mild steel; heat and ash tolerant if FGR enters the fan inlet |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; cooling disc if FGR raises inlet temperature |
| Flow control | Variable speed drive on steam pressure, zone dampers for distribution |
| Also specified | Inlet screen; FGR mixing box design where applicable |
What this FD fan has to cope with
Wet fuel needs more drying air
Chips at 50% moisture need strong air flow on the upper grate to dry before burning. A fan sized for dry fuel leaves the drying zone short and unburnt chips reach the burnout zone. We size for the wettest expected chips and allow the drying zone damper to take a larger share.
FGR mixed into the fan inlet
If recirculated flue gas is mixed into the FD fan suction, the fan handles warmer, dusty, moist gas and must be designed for that temperature and density. A mixing box ahead of the fan, and a check of condensation at the mixing point, protect it.
Grate bar overheating on dry fuel
On very dry fuel the bed burns hotter and the grate bars can overheat and warp. More primary air, or more recirculated gas under the grate, cools them. The FD fan and FGR fan must together cover this dry-fuel condition, not only the wet-fuel design case, so both are checked at each end of the moisture range.
Energy and control
The FD fan follows steam demand and chip moisture, which can change from one truckload to the next. A variable speed drive with oxygen trim reduces excess air, which lowers fan power and also the wet gas volume through the ID fan. Correct zone damper setting has a large effect on combustion and costs nothing to adjust.
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 Induced Draft (ID) Fan for wood chip reciprocating grate boilers.
Questions buyers ask
How is air distributed on a reciprocating grate?
The FD fan feeds compartments under the drying, burning and burnout zones through separate dampers. Wet chips need more air in the drying zone, dry chips more in the burning zone. Secondary air from a separate fan completes combustion of the volatiles above the bed.
Does the FD fan handle flue gas on a wood chip boiler?
Only where flue gas recirculation is mixed into the FD fan inlet. In that case the fan sees a warmer, moist and slightly dusty mixture, typically 60 to 100°C, and is designed for that temperature and density, with a check on condensation. Otherwise it handles clean ambient air and is a plain mild steel fan.
What FD fan pressure does a reciprocating grate boiler need?
Typically about 100 to 200 mmWC at the fan outlet, depending on grate length, bed depth and whether an air heater is fitted. Wet chips with a thick bed sit at the upper end of that range. On existing boilers we confirm the figure from measured zone pressures at full load before selecting a fan.
Information needed to quote an FD fan for wood chip reciprocating 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.
- Grate zones. Number of primary air zones and design pressure for each.
- FGR arrangement. Whether recirculated gas enters the FD fan inlet or a separate duct.
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.