How incinerator waste heat boilers work
Hazardous, industrial, biomedical or municipal waste is burned in a primary chamber, a rotary kiln or a grate, and the gases are completed in a secondary chamber held at typically 1,050 to 1,100°C with about two seconds of residence time. The waste heat boiler takes this gas down to about 250 to 300°C and raises steam for the plant. Below the boiler the gas is quenched quickly through the range where dioxins can re-form, then treated with lime and activated carbon in a dry or semi-dry reactor, filtered in a bag filter and often polished in a wet scrubber. The whole train runs under suction from a single ID fan near the stack; burner and combustion air come from separate blowers on the primary and secondary chambers.
Where the ID fan sits
- Primary chamber or kiln
- Secondary chamber
- Waste heat boiler
- Quench
- Lime and carbon reactor
- Bag filter
- ID fan
- Wet scrubber and stack
The fan usually sits after the bag filter, where the gas is dry, at 140 to 180°C and nearly free of dust, with the wet scrubber and stack after it under slight positive pressure. Some plants instead place it after the wet scrubber, where it handles saturated gas at 60 to 75°C carrying acid droplets. Either way it must overcome the boiler, quench, reactor, bag filter and scrubber, often 400 to 700 mmWC in total, while holding the primary chamber at a small negative pressure.
Gas at the ID fan
| Gas temperature, normal | 140 to 180°C after the bag filter |
| Maximum or upset | Up to 250°C on quench water failure, until the interlock trips the feed |
| Dust at the fan | Low after the bag filter, mainly lime and reaction salts, usually below 30 mg/Nm³ |
| Moisture and dew point | High moisture from quench water; acid dew point with HCl and SO₃ can be 130 to 150°C |
| Corrosive species | HCl, SO₂, traces of HF and heavy metal salts; strongly corrosive wherever the gas cools below its dew point |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 incinerator waste heat boilers
| Blade form | Backward curved, on dry gas after the bag filter; radial or backward curved with drainage on a wet duty after the scrubber |
| Material of construction | SS 316L or a higher alloy for wetted parts on dry gas with insulated casing; SS 316L, FRP or rubber-lined steel for saturated gas |
| Drive and arrangement | Direct coupled, arrangement 8, with the bearing pedestal outside the gas stream |
| Bearings and cooling | Pedestal bearings with a cooling disc; gas-tight shaft seal so no flue gas leaks into the building |
| Flow control | Variable speed drive holding primary chamber pressure, with an inlet damper |
| Also specified | Casing drain, standby fan or emergency vent arrangement, and casing insulation to keep metal above the dew point |
What this ID fan has to cope with
Acid condensation in the casing
Chlorides and SO₃ make the acid dew point far higher than on a fuel-fired boiler. Uninsulated casings, cold starts and long idle periods let acid condense on the scroll and impeller. Insulated casings, warm-up on clean air before waste feed and alloy steel for wetted parts give the fan a normal life.
Loss of draft and fugitive emissions
If the ID fan trips, hot toxic gas pushes out through feed chutes and seals. Plants use a standby fan on auto changeover, a UPS or diesel backed drive, or an emergency vent damper, and the fan control is tied into the feed cut-off interlock.
Salt and lime deposits
Reaction salts such as calcium chloride are hygroscopic and pick up moisture at low load, forming a sticky crust on the blades. An inspection door, wash connections and a blade shape that does not trap deposits keep vibration under control.
Large swings in gas volume
Waste calorific value varies from batch to batch, and the quench water flow follows it, so the gas volume at the fan changes quickly. The fan is sized on the maximum quench water and air ingress case, with a stable curve at low flow.
Energy and control
Incinerators run continuously but at a load set by the waste mix, and the ID fan rarely runs at its design point. A variable speed drive on primary chamber pressure saves considerable power over damper control and responds faster to waste surges. Cutting air ingress at the kiln seals and feed chute reduces both the fan load and the quench water needed, which in turn reduces the gas volume the fan must move.
Questions buyers ask
What material should an incinerator ID fan be made of?
It depends on where the fan sits. On dry gas after the bag filter, SS 316L wetted parts with an insulated casing are common. After a wet scrubber, the gas is saturated with acid droplets, and SS 316L, higher alloys, FRP or rubber lining are used. Mild steel rarely survives either duty for long.
Why does an incinerator waste heat boiler need a standby ID fan?
The ID fan is what keeps the plant under suction. If it stops while waste is burning, toxic gas leaks from the chambers. A standby fan, backed power supply or emergency vent ensures draft is never lost while the secondary chamber is hot.
Should the ID fan be before or after the wet scrubber?
Before the scrubber the fan handles dry, hotter gas and is easier to keep corrosion free with insulation. After the scrubber it handles cool, saturated gas, so it is smaller but must be fully corrosion resistant and drained. The choice depends on the treatment train and the stack design.
Information needed to quote an ID fan for incinerator waste heat 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.
- Waste and gas chemistry. Type of waste, chlorine and sulphur content, and the HCl and SO₂ levels measured before the gas treatment.
- Treatment train. Quench, reactor, bag filter and scrubber sequence, and whether the fan is to sit before or after the wet scrubber.
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.