How coal spreader stoker boilers work
A spreader stoker throws crushed coal, usually sized below 25 to 30 mm, across the furnace with rotating feeder rotors or pneumatic distributors. Fines burn in suspension while the coarser pieces land on a travelling, dumping or reciprocating grate and burn out as a thin bed. Because so much of the coal burns in flight, a large share of the ash and unburnt carbon leaves with the gas, and many units return the cinders from the boiler and economiser hoppers to the furnace by a reinjection system. The gas passes the boiler bank, economiser and air heater, then a multiclone and an ESP or bag filter before the ID fan. The FD fan supplies undergrate air; a separate overfire air fan feeds high velocity jets above the grate. Many sugar mill units burn bagasse on the same stoker.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate air duct
- Grate plenums
- Fuel bed
The fan stands at ground level beside the air heater and pushes ambient air through it into the undergrate duct. On a spreader stoker the bed is thin, so grate resistance is low, but the air heater and ducts add enough that the fan outlet pressure is typically 80 to 150 mmWC. The overfire air fan is separate and runs at a much higher pressure, often 400 to 700 mmWC, to drive jets across the furnace. Air for pneumatic distributors may come from yet another blower.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 130 to 180°C, downstream of the fan |
| Dust | Clean air; coal and bagasse dust in the boiler house can load an unscreened inlet |
| Pressure at the fan outlet | Typically 80 to 150 mmWC including air heater and grate |
| Share of total air | Usually 60 to 75% of combustion air, the rest as overfire air |
| 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 coal spreader stoker boilers
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled; direct mounted on the motor shaft for smaller units |
| Bearings and cooling | Standard pedestal bearings; no cooling needed |
| Flow control | Variable speed drive or inlet vanes, trimmed on steam pressure and excess oxygen |
| Also specified | Inlet screen and silencer; outlet damper for isolation during grate maintenance |
What this FD fan has to cope with
Wrong split between undergrate and overfire air
If the FD fan supplies too much air through a thin bed, it lifts fines off the grate and increases carry-over. If too little, the coarse coal clinkers. The FD duty is set together with the overfire air fan so that each can be trimmed independently.
Clinkering from uneven air distribution
Spreader throw is never perfectly even, and heavy patches on the grate need more air. A fan with a steadily rising pressure curve holds plenum pressure as dampers move, so the zones that need air receive it. Separate zone dampers, set during commissioning and checked at each grate inspection, do the fine adjustment.
Air heater leakage
Leaking tubes in an older air heater bypass FD air straight into the gas path. The FD fan then works harder for the same undergrate flow, and the ID fan carries the extra volume. Leakage is measured and included when the FD fan is reselected.
Energy and control
The FD fan on a spreader stoker follows steam demand closely, and the excess air drifts high when the operators open the dampers to avoid clinkers. A variable speed drive tied to steam pressure, trimmed by flue gas oxygen, keeps air close to what the fuel needs. On sugar mill boilers the change between bagasse and coal shifts the air demand, so the controls should be set for each fuel.
ID fan and FD fan in coal spreader stoker boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multiclone and ESP or bag filter | Ahead of the air heater and grate plenums |
| What it handles | Flue gas with heavy fly ash and char | Clean ambient air |
| Temperature at the fan | 150 to 180°C, up to 230°C upset | Ambient, 25 to 45°C |
| Main design concern | Char erosion and dust load | Air split with overfire air |
| Control | Furnace draft | Steam pressure and excess oxygen |
See also the Induced Draft (ID) Fan for coal spreader stoker boilers.
Questions buyers ask
What is the difference between the FD fan and overfire air fan on a spreader stoker?
The FD fan supplies low pressure air through the grate to the burning bed. The overfire air fan delivers a smaller volume at much higher pressure through nozzles above the grate, where it mixes with the suspended fines and volatiles. Both are needed for good burnout on a spreader stoker.
Does a forced draft fan on a spreader stoker boiler handle hot air?
Usually not. The FD fan pushes ambient air into the air heater, and the air is heated downstream. If your plant has a hot air recirculation line to the fan inlet, the fan must be selected at the mixed temperature instead. Hot air recirculation is unusual on spreader stokers but is sometimes added to dry wet bagasse.
Why do my grate zones get uneven air at low load?
At low load the FD fan runs further back on its curve and plenum pressure falls, so zones with heavier coal receive proportionally less air. A drive that holds plenum pressure, rather than a fixed damper position, keeps the distribution stable. Checking zone plenum pressures at low load confirms whether this is the cause.
Information needed to quote an FD fan for coal spreader stoker 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. Share of total air supplied as undergrate, overfire and distributor air.
- Grate type. Travelling, dumping or reciprocating grate, and the number of plenum zones.
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.