How fluidised bed hot air generators work
A fluidised bed hot air generator burns crushed coal, lignite, petcoke blends or biomass in a bed of sand at 800 to 900°C, fluidised by high pressure air through a nozzle distributor. It suits low grade, high-ash or high-moisture fuels that a grate handles poorly, and supplies large ceramic spray dryers, mineral and clay dryers and some kiln preheating duties. The gas leaving the freeboard passes a hot cyclone that returns or removes coarse ash and bed fines, then enters a mixing chamber where dilution air brings it to the dryer inlet temperature, typically 400 to 650°C. Limestone is often added to the bed to capture sulphur from lignite and coal. The dryer exhaust goes through cyclones and a bag filter or scrubber to the exhaust fan.
Where the ID fan sits
- Fluidised bed
- Hot cyclone
- Mixing chamber
- Dryer
- Cyclones
- Bag filter or scrubber
- ID (exhaust) fan
- Stack
The fan sits after the dryer collectors and holds the freeboard and dryer under suction while the FD fan pressurises the bed. The balance point is just above the bed, so the fan has to pull the whole train: freeboard, hot cyclone, mixing chamber, dryer and collectors, often 250 to 400 mmWC. The hot cyclone removes coarse ash, but much of the fine fly ash still reaches the dryer and adds to the dust load at the fan.
Gas at the ID fan
| Gas temperature, normal | 100 to 140°C after the dryer collectors |
| Maximum or upset | 300 to 450°C if dryer feed stops |
| Dust at the fan | Fine fly ash and spent limestone with product fines; heavier than on grate firing |
| Moisture and dew point | Humid exhaust; lignite firing adds fuel moisture, dew point 60 to 75°C |
| Corrosive species | Residual SO₂ after limestone capture; alkaline lime dust that hardens when wet |
| Density at the design temperature | About 0.88 kg/m³ at 130°C, 73% 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 fluidised bed hot air generators
| Blade form | Backward curved with wear liners behind a bag filter; radial tipped behind cyclones or a scrubber |
| Material of construction | Wear-resistant plate or lined impeller; stainless or lined casing if after a wet scrubber |
| Drive and arrangement | Direct coupled arrangement 8, moderate tip speed |
| Bearings and cooling | Pedestal bearings with cooling disc; vibration and temperature monitoring |
| Flow control | Variable speed drive on freeboard or dryer pressure |
| Also specified | Replaceable liners, inspection door and casing drain |
What this ID fan has to cope with
Lime and ash cementing on the blades
Spent limestone and FBC ash are partly calcium oxide and sulphate. When they meet condensation at the fan they set like cement on the blades. Keeping the exhaust above dew point at all loads, insulating the casing and running the fan on warm air before shutdown stop deposits forming.
Draft during bed upsets
Fuel feed interruptions, bed drains and sudden load changes swing the freeboard pressure. If the exhaust fan cannot respond fast enough, hot gas and sand puff out of the feed points. A speed drive with a fast draft loop on the freeboard pressure, and fan margin above full load, keep the furnace negative.
Heavy fine ash load
FBC fly ash is finer and more plentiful than grate ash, and much of it passes the hot cyclone into the product side. The dryer collectors must be sized for it, and the fan must tolerate what passes them. Liners and moderate speed are essential where the fan follows cyclones only.
Energy and control
An FBC hot air generator runs best near its design rate, and the dryer usually runs continuously at a steady throughput. The exhaust fan is a large motor with a long system, and a variable speed drive on dryer pressure saves much of the throttling loss of a damper. Keeping dilution air no higher than needed, and the exhaust temperature as low as the product and dew point allow, reduce the gas volume the fan must move.
ID fan and FD fan in fluidised bed hot air generators
| ID fan | FD fan | |
|---|---|---|
| Position | After the dryer collectors | Under the distributor plate |
| What it handles | Humid exhaust with FBC fly ash and product fines | Clean ambient air |
| Temperature at the fan | 100 to 140°C | Ambient |
| Static pressure | 250 to 400 mmWC suction | 700 to 1,100 mmWC |
| Main design concern | Fine ash, lime deposits and draft balance | High pressure and minimum fluidisation |
| Control | Freeboard or dryer pressure | Bed air flow with a minimum limit |
See also the Forced Draft (FD) Fan for fluidised bed hot air generators.
Questions buyers ask
Why does the exhaust fan on an FBC hot air generator need more pressure than on a grate unit?
It holds the freeboard under suction against a pressurised bed and pulls the gas through a hot cyclone, mixing chamber, dryer and collectors in series. The heavier fly ash load also needs more collection equipment. All of this adds static pressure, often 250 to 400 mmWC at the fan.
What causes hard deposits on the ID fan of a fluidised bed hot air generator?
Spent limestone and FBC ash contain lime and calcium sulphate, which set when wet. If the exhaust cools below its dew point at low load or at shutdown, they form cement-like deposits on the blades. Keeping the gas warm and running the fan dry before stopping prevent this.
Is FBC fly ash harder on the exhaust fan than ceramic dust?
It is finer and more plentiful, while ceramic dust contains more quartz and is individually harder. Together they form an abrasive load that wears blade tips and the scroll cut-off. Wear-resistant plate and liners are specified for both, and the efficiency of the collector in front of the fan matters more than the blade type.
Information needed to quote an ID fan for fluidised bed hot air generators
- 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.
- Fuel and bed. Fuel type and size, and whether limestone is added.
- Collection train. Hot cyclone, dryer cyclones, bag filter or scrubber, and the pressure drop of each.
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.