How garden and green waste fired boilers work
Garden and green waste boilers burn shredded tree trimmings, leaves, grass cuttings and park waste collected by municipalities and estates. The fuel is very variable: woody prunings may carry 30 to 40% moisture, fresh grass and leaves 50 to 65%, and the whole stream brings soil, grit and stray plastics. Calorific value is low, often 1,800 to 2,800 kcal/kg as fired. Most plants use a reciprocating or step grate that tolerates oversize and wet fuel, with a tall refractory-lined furnace and preheated undergrate air to dry the fuel on the first grate section. Flue gas passes the boiler, economiser and air heater, then a multiclone and a bag filter, sometimes with lime injection for acid gases, before the ID fan and chimney.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate air ducts
- Zone dampers
- Reciprocating grate
The fan takes ambient air at ground level, pushes it through the air heater, where it is heated to 180 to 250°C, and delivers it to the grate zones. The drying zone at the feed end gets the largest share of hot air. Pressure covers the air heater, ducts, zone dampers, the grate bars and a fuel bed that can be thick and patchy, typically 150 to 300 mmWC. Secondary air for volatile burnout comes from a separate fan or a branch.
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 | Typically 150 to 300 mmWC |
| Dust | Dusty fuel yard; leaf fragments can be drawn in |
| Air distribution | Most hot air to the drying zone; less to the burnout zone |
| 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 garden and green waste fired boilers
| Blade form | Backward curved with a stable, rising curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8 or direct mounted |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on steam pressure with oxygen trim |
| Also specified | Coarse inlet screen against leaves and plastic film, inlet silencer |
What this FD fan has to cope with
Wet fuel needs drying air
A bed of wet grass and leaves smothers combustion if the drying zone is short of hot air. The fan must deliver enough air at the pressure needed to pass through a dense, wet layer, and the air heater must give it enough temperature. Sizing on a dry fuel case starves the grate when the wet season arrives.
Patchy bed and channelling
Green waste does not spread evenly; clumps of grass and branches leave thick and thin patches. Air takes the easy path through thin areas and the thick patches smoulder. A fan with pressure in hand, and zone dampers set from furnace observation, let the operator push air through the thicker sections.
Leaves and plastic film at the inlet
Light leaves and plastic film from the fuel yard blow into unscreened fan inlets, block the screen or wrap around the impeller and shaft. The result is falling air flow, unbalance and unplanned stops. A coarse screen with easy access for cleaning, and an inlet duct drawn from a cleaner, enclosed area of the building, avoid most of these problems.
Energy and control
The FD fan on a green waste boiler often runs with plenty of excess air because operators fear smouldering wet fuel. Oxygen trim on a speed-controlled fan cuts excess air to what the grate really needs, which reduces both FD and ID fan power and the stack loss. When the fuel is dry the saving is largest.
ID fan and FD fan in garden and green waste fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the chimney | Before the air heater, feeding the grate |
| What it handles | Wet flue gas with soil grit and chloride | Ambient air from a dusty yard |
| Temperature at the fan | 150 to 180°C, up to 230°C | Ambient |
| Main design concern | Seasonal gas volume and acid condensation | Drying air through a wet, patchy bed |
| Control | Furnace pressure | Steam pressure and oxygen |
See also the Induced Draft (ID) Fan for garden and green waste fired boilers.
Questions buyers ask
Why does a green waste boiler need hot undergrate air?
Garden and green waste is wet, and the moisture has to be driven off before the fuel can ignite on the grate. Preheated undergrate air, typically 180 to 250°C, dries the fuel on the first grate zone. The FD fan pushes ambient air through the air heater to produce it.
How much pressure does the FD fan on a green waste grate boiler need?
Usually 150 to 300 mmWC at the fan outlet, covering the air heater, ducting, zone dampers, the grate bars and a fuel bed that may be thick and wet. Reciprocating grates with deeper beds need the higher end of the range. We confirm the figure from the grate design and site data.
Is the FD fan exposed to hot air or flue gas?
No. In the usual layout the FD fan on a green waste boiler handles only ambient air and pushes it into the air heater. The air is heated downstream of the fan, so the fan stays cool, in plain mild steel, and is selected at ambient density. Only the ID fan sees the wet flue gas.
Information needed to quote an FD fan for garden and green waste 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.
- Grate zones. Number of grate zones and the share of air each receives.
- Air preheat. Target hot air temperature and air heater air side pressure drop.
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.