How sludge fluidised bed incinerators work
Sewage sludge, ETP sludge and paper mill sludge are dewatered to about 20 to 35% dry solids and fed into a bubbling fluidised bed of hot sand at about 750 to 850°C, with the freeboard above at about 850 to 900°C. The water in the sludge evaporates almost at once, so the flue gas carries a very high water vapour content, often 30 to 45% by volume. All the ash leaves with the gas as fine particles, along with some bed sand. The gas passes a gas-to-air heat exchanger that preheats the fluidising air, often a waste heat boiler, then a bag filter or ESP, and usually a wet scrubber. The fluidising air blower is the FD fan, and the ID fan pulls the gas train.
Where the FD fan sits
- Inlet filter and silencer
- FD fan
- Air preheater
- Hot windbox
- Distributor nozzles
- Sand bed
The blower sits at ground level and delivers ambient air to the cold side of the gas-to-air heat exchanger, where it is preheated to typically 500 to 650°C, then into the windbox under the bed. It works against the preheater, distributor nozzles and the weight of the bed, typically 1,500 to 2,500 mmWC. Smaller units without preheaters use start-up burners in the windbox.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 500 to 650°C, downstream of the fan |
| Dust | Clean air; filtered inlet to protect preheater and nozzles |
| Pressure at the fan outlet | Typically 1,500 to 2,500 mmWC |
| Minimum flow | Must stay above minimum fluidising velocity at all loads |
| 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 sludge fluidised bed incinerators
| Blade form | High-pressure backward curved or radial, single stage or multistage depending on pressure |
| Material of construction | Mild steel; heavier casing for high pressure |
| Drive and arrangement | Direct coupled, arrangement suited to a large motor |
| Bearings and cooling | Pedestal bearings; no cooling needed on ambient air |
| Flow control | Variable speed drive with a minimum flow limit for fluidisation |
| Also specified | Inlet filter and silencer, non-return damper, and airflow measurement |
What this FD fan has to cope with
Loss of fluidisation
If air falls below minimum fluidising velocity, the bed slumps and hot spots form, leading to clinkering. The control system sets a floor on blower flow, and the blower is sized to keep fluidisation at minimum sludge feed.
Backflow of hot sand
When the blower stops, sand can fall back into the distributor nozzles and windbox. A non-return damper and nozzle design that resists backflow prevent this, and restarts follow a set sequence.
Noise
High-pressure blowers are noisy, particularly at the inlet, and sludge plants are often near residential areas or treatment plant staff. An inlet silencer, acoustic lagging on the casing and ducts, and an enclosure where needed bring noise within limits.
Energy and control
The fluidising blower works against high pressure all the time the bed is hot, so it is often the largest auxiliary motor. A variable speed drive holding flow just above the fluidisation minimum, and keeping bed depth at the design value rather than letting it build up with sand and ash, keep power down.
ID fan and FD fan in sludge fluidised bed incinerators
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, often before the scrubber | Ahead of the air preheater and windbox |
| What it handles | Humid flue gas with fine ash | Ambient fluidising air |
| Temperature at the fan | 160 to 200°C | Ambient, 25 to 45°C |
| Main design concern | Large wet gas volume and dew point | High pressure for the bed |
| Control | Freeboard pressure | Fluidising flow with minimum limit |
See also the Induced Draft (ID) Fan for sludge fluidised bed incinerators.
Questions buyers ask
What pressure does a fluidising air blower need for a sludge incinerator?
Typically 1,500 to 2,500 mmWC at the blower outlet, covering the air preheater, distributor nozzles and the weight of the sand bed. The exact value depends on bed depth and nozzle design.
Does the FD fan handle hot air in a sludge incinerator?
No. It handles ambient air and pushes it through the gas-to-air preheater, where it is heated to around 500 to 650°C before entering the windbox. The hot section starts after the blower, so it is sized at ambient density.
Can the fluidising blower use a VFD?
Yes, with a minimum flow limit in the control system to keep the bed fluidised. The drive lets the blower match the sludge feed rate and bed condition without throttling, which saves considerable power on this high-pressure duty.
Information needed to quote an FD fan for sludge fluidised bed incinerators
- 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 data. Bed area, depth, sand size and minimum fluidising velocity.
- Preheater. Air side pressure drop of the preheater at design flow.
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.