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 ID fan sits
- Fluidised bed and freeboard
- Air preheater
- Waste heat boiler
- Bag filter
- ID fan
- Wet scrubber
- Stack
On many sludge plants the fan sits after the bag filter and before the wet scrubber, so it handles dry gas at 160 to 200°C and keeps the bed, preheater and filter under suction. The scrubber then runs at a slight positive pressure. Where the fan is after the scrubber, it handles saturated gas instead. Suction at the fan is typically 300 to 500 mmWC, because the air preheater and filter take a large share.
Gas at the ID fan
| Gas temperature, normal | 160 to 200°C after the bag filter |
| Maximum or upset | Up to 250°C with a fouled preheater or boiler |
| Dust at the fan | Fine phosphate-rich sludge ash and bed sand, 10 to 30 mg/Nm³ after the filter |
| Moisture and dew point | 30 to 45% water vapour by volume; water dew point about 70 to 80°C, acid dew point higher |
| Corrosive species | SO₂ and HCl from sludge sulphur and chloride; corrosive if the gas cools below dew point |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 sludge fluidised bed incinerators
| Blade form | Backward curved with wear allowance at the leading edge |
| Material of construction | Mild steel or corten class steel casing; impeller in wear-resistant plate where filter breakthrough is possible |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings outside the gas stream with a cooling disc |
| Flow control | Variable speed drive on freeboard pressure |
| Also specified | Insulated casing, casing drain, and inspection door |
What this ID fan has to cope with
Large, low-density gas volume
Sludge moisture makes the gas lighter and much larger in volume than coal or biomass flue gas at the same heat input. A fan sized on dry gas assumptions will run out of capacity. The duty is calculated from the wettest sludge expected, at the actual water vapour content.
Condensation at start-up and low load
With water dew point near 80°C and acid dew point higher, any cold surface in the fan or duct gets wet and corrodes. Insulation, a warm-up sequence that heats the train before sludge feed starts and a control that holds filter outlet temperature protect the fan.
Ash and sand erosion
All the ash leaves with the gas, and fine sand is elutriated from the bed. If a bag ruptures, abrasive dust reaches the fan. Wear-resistant blades and dust monitoring after the filter limit the damage.
Energy and control
Sludge incinerators run continuously, but sludge dryness changes with the dewatering plant. Wetter sludge means more gas and more ID fan power. A variable speed drive on freeboard pressure follows this. Improving dewatering by even a few percent of dry solids reduces gas volume, support fuel and ID fan power together, and is usually the best energy measure on the plant.
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 Forced Draft (FD) Fan for sludge fluidised bed incinerators.
Questions buyers ask
Why is the ID fan on a sludge incinerator so large?
Dewatered sludge is mostly water, and all of it becomes steam in the flue gas. The gas volume per tonne of dry solids is therefore much higher than for other fuels, and the ID fan must be sized for that volume at its low density.
Should the sludge incinerator ID fan be before or after the wet scrubber?
Before the scrubber keeps the fan dry and allows mild steel construction, but the scrubber then runs under pressure. After the scrubber keeps the whole system under suction but needs corrosion-resistant materials. Both are used; the choice depends on layout and emissions.
What causes corrosion in a sludge fluidised bed incinerator ID fan?
Condensation. The gas holds so much water that its dew point is close to normal temperatures in cold parts of the train. Insulation, warm-up procedures and temperature control at the filter outlet prevent it.
Information needed to quote an ID fan for sludge fluidised bed incinerators
- 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.
- Sludge properties. Dry solids content range, ash content, and sulphur and chlorine content.
- Gas train. Air preheater, boiler, filter and scrubber, in order, with pressure drops.
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.