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 FD fan sits
- Inlet silencer
- FD (fluidising air) fan
- Air box
- Distributor nozzles
- Fluidised bed
- Freeboard
The fan feeds the air box under the distributor plate with ambient air. It works against the nozzles and the weight of the bed, typically 700 to 1,100 mmWC on a deep bed. Dilution air for the mixing chamber is a much larger volume at low pressure and is handled by a separate fan or drawn in by the exhaust fan, never by the fluidising fan, which would waste power raising it to bed pressure.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Typically 700 to 1,100 mmWC |
| Dust | Clean air; filtered to keep nozzles clear |
| Duty range | From minimum fluidising flow to full load |
| Dilution air | Separate low pressure fan or drawn in by the exhaust fan |
| 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 fluidised bed hot air generators
| Blade form | High pressure backward curved or radial bladed wheel |
| Material of construction | Mild steel, with the impeller designed for high tip speed |
| Drive and arrangement | Direct coupled, often at 2,900 rpm |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or inlet guide vanes, with a minimum flow limit |
| Also specified | Inlet silencer, lagging and non-return damper where two fans share an air box |
What this FD fan has to cope with
Bed defluidisation at low load
Cutting air below the minimum fluidising velocity collapses the bed, and the fuel stops mixing and clinkers. The fan control has a floor, and the fan curve must be stable at that floor. Turndown below it is taken by reducing bed temperature or by switching off a bed section on larger units.
Surge with a heavy bed
As the bed builds up with ash and limestone, its resistance rises and a high pressure fan can be pushed towards its stall point, causing pulsation that upsets fluidisation. Selecting the operating point well away from the peak pressure, with the heaviest bed allowed, and draining bed material regularly avoid this.
Nozzle blockage
Dust drawn in with the air, or bed sand falling back through damaged nozzles when the fan stops, blocks the distributor and raises pressure unevenly. Inlet filtration and nozzle designs that resist backflow keep the air distribution even and the fan on its design point.
Energy and control
The fluidising air fan is the largest power consumer on the generator because of its high pressure. Its flow can only be reduced to the fluidisation limit, so the saving from speed control is real but bounded. Keeping bed height at design, draining excess ash and limestone, and keeping dilution air off this fan entirely are the main ways to keep its power down.
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 Induced Draft (ID) Fan for fluidised bed hot air generators.
Questions buyers ask
What pressure does the fluidising air fan on an FBC hot air generator need?
Typically 700 to 1,100 mmWC, depending on bed depth, bed material and nozzle design. The figure comes from the generator supplier's data or from air box pressure measured with the bed at its normal operating height. The fan is selected with its operating point well clear of the peak of its pressure curve, so a heavier bed does not push it into surge.
Can the FD fan also supply dilution air on a fluidised bed hot air generator?
It should not. Dilution air is a large volume needed at low pressure. Raising it to bed pressure through the fluidising fan wastes a great deal of power. A separate low pressure dilution fan, or air drawn in by the exhaust fan, is the efficient arrangement.
Why does the fluidised bed lose fluidisation when the dryer load drops?
If the fluidising air is reduced along with the fuel, it can fall below the minimum fluidising velocity and the bed slumps, stops mixing and begins to clinker. The fan control must keep a minimum flow, and further turndown is taken through bed temperature and fuel rate, or by running a smaller bed section on larger generators.
Information needed to quote an FD fan for fluidised bed hot air generators
- 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.
- Bed and distributor. Bed depth, bed material, distributor pressure drop and design air box pressure.
- Turndown. Minimum fluidising air flow and the normal operating range.
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.