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 ID fan sits
- Furnace and grate
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan sits after the final collector and ahead of the chimney. Dust loading at the boiler outlet on a spreader stoker is commonly two to four times that of a chain grate burning the same coal, so the multiclone does real work and the ESP or bag filter takes the fines. The suction at the fan inlet covers furnace draft, boiler and economiser, air heater and both collectors, typically 180 to 280 mmWC. Placing the fan downstream of both collectors is what makes a backward curved wheel practical here.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 230°C with fouled surfaces or when firing wet bagasse alongside coal |
| Dust at the fan | Fine ash and porous char; 50 to 150 mg/Nm³ after an ESP or bag filter, 1 to 3 g/Nm³ if the final collector is out of service |
| Moisture and dew point | Coal moisture of 8 to 15% keeps the water dew point near 45 to 50°C; much higher when bagasse is co-fired |
| Corrosive species | Sulphur of 0.4 to 0.8% in Indian coal gives a modest acid dew point, usually 110 to 130°C |
| 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 coal spreader stoker boilers
| Blade form | Backward curved with wear liners behind an ESP or bag filter; radial tipped where the fan follows a multiclone only |
| Material of construction | Mild steel casing with replaceable liners at the cut-off; impeller in wear-resistant plate or mild steel with hard-faced leading edges |
| Drive and arrangement | Direct coupled, arrangement 8, with moderate tip speed to limit erosion |
| Bearings and cooling | Pedestal bearings outside the gas stream with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft, inlet damper kept for isolation and start-up |
| Also specified | Inspection door, casing drain and a spare impeller where the plant runs through a long crushing season |
What this ID fan has to cope with
Char erosion when the collector falters
Spreader stoker char is coarse and hard. When an ESP field trips or bags fail, the dust at the fan jumps by an order of magnitude and a backward curved blade can thin in weeks. Liners on the pressure face and leading edge, plus a dust alarm on the collector, give the operators time to react before the wheel is damaged.
Cinder reinjection raising the dust load
Reinjection improves carbon burnout but recirculates fine ash through the boiler, so the gas leaving the furnace carries more dust than the coal ash alone suggests. The fan and collector are sized on the dust load measured with reinjection in service, not on the fuel analysis.
Draft swings from feeder and grate changes
Changes in throw distance, a choked feeder or a dumping grate cycle alter the furnace pressure within seconds. A sluggish damper lets the furnace go positive and puff ash at the doors. A drive with a fast draft loop and a correctly tuned furnace pressure transmitter keeps the furnace steady.
Higher gas volume when bagasse replaces coal
On sugar mill boilers that swap between coal and bagasse, bagasse with 50% moisture produces much more flue gas per tonne of steam. The fan must be sized for the bagasse case at full steam, or the boiler will be draft limited in the season.
Energy and control
Spreader stoker boilers in sugar mills run hard through the crushing season and at low load or on coal alone in the off-season. A fan sized for the bagasse case runs well below its design flow when firing coal, so damper throttling wastes a large share of its power. A variable speed drive on furnace draft recovers most of that loss, and checking the fan inlet temperature against design shows when the air heater or economiser needs cleaning.
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 Forced Draft (FD) Fan for coal spreader stoker boilers.
Questions buyers ask
Why is fly ash so high on a spreader stoker boiler?
Because much of the coal burns in suspension above the grate, the fine ash and unburnt char are carried away with the gas instead of staying on the grate. Cinder reinjection adds a recirculating load. The result is a dust burden at the boiler outlet several times that of a chain grate, which the collectors and ID fan must be designed for.
Should a spreader stoker ID fan be radial or backward curved?
If the fan sits behind a working ESP or bag filter, a backward curved wheel with wear liners is the efficient choice. If it follows only a multiclone, or the final collector is often bypassed, a radial tipped wheel tolerates the coarse char far better and is easier to repair.
Can the same induced draft fan handle coal and bagasse on a sugar mill boiler?
Yes, if it is sized for the larger case, which is almost always bagasse because of its moisture. The fan then runs at reduced speed on coal. Send us the fuel analysis for both fuels and the steam output for each and we will check the existing fan against both duties.
Information needed to quote an ID fan for coal spreader stoker 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.
- Reinjection. Whether cinder reinjection is in service and the measured dust at the boiler outlet and at the fan inlet.
- Fuels. Coal analysis and, if co-fired, the bagasse or biomass share and moisture for each season.
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.