How hazardous waste rotary kiln incinerators work
Rotary kiln incinerators at common hazardous waste facilities and large chemical sites take solids, sludges, drummed waste and pumpable liquids. The kiln rotates slowly at about 900 to 1,200°C and discharges ash or slag at its lower end. Kiln gas passes to a secondary combustion chamber held at 1,100°C or above with two seconds residence time when halogenated waste is fired. The gas is then cooled rapidly, by a waste heat boiler and evaporative quench or by quench alone, to about 180 to 200°C, to limit dioxin re-formation. Lime and activated carbon are injected ahead of a bag filter, followed by a two-stage wet scrubber, and the ID fan pulls the whole train. Combustion air fans serve the kiln burner, the secondary chamber and the kiln hood.
Where the ID fan sits
- Rotary kiln
- Secondary combustion chamber
- Waste heat boiler or quench
- Dry reactor and bag filter
- Two-stage wet scrubber
- ID fan
- Stack
The fan is usually at the end of the train after the wet scrubber, so the kiln, chamber, filter and scrubber all run under suction. Total suction at the fan is typically 500 to 800 mmWC. Some plants place it between the bag filter and the scrubber to keep it dry, which then puts the scrubber under positive pressure. Air drawn in at the kiln seals, feed chute and ash discharge adds a large share to the gas the fan must handle.
Gas at the ID fan
| Gas temperature, normal | 65 to 80°C, saturated after the scrubber |
| Maximum or upset | Hot gas if quench or scrubber water fails; protected by emergency vent and temperature trips |
| Dust at the fan | Very fine salt fume and droplets; below 10 to 30 mg/Nm³ of solids |
| Moisture and dew point | Saturated, with free droplets carried past the demister |
| Corrosive species | Residual HCl, HF and SO₂; chlorides and fluorides in droplets |
| Density at the design temperature | About 1.02 kg/m³ at 75°C, 85% 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 hazardous waste rotary kiln incinerators
| Blade form | Backward curved, rugged blade sections for high pressure, with no liquid traps |
| Material of construction | Duplex stainless or high-alloy stainless for chloride service; FRP with a synthetic veil or rubber-lined steel where HF is present |
| Drive and arrangement | Direct coupled, arrangement 8, motor sized for cold-gas start-up |
| Bearings and cooling | Pedestal bearings with sealed shaft entry; no cooling needed on wet duty |
| Flow control | Variable speed drive on kiln hood pressure, with inlet isolation damper |
| Also specified | Standby fan or emergency drive, wash nozzles, casing drain, and vibration monitoring |
What this ID fan has to cope with
Kiln seal air ingress
Worn kiln seals let in large volumes of cold air, which the ID fan must carry through the whole cleaning train. The fan runs at higher speed and power, and kiln draft becomes hard to hold. Sizing the fan on measured gas volume with realistic seal leakage, and maintaining the seals, prevent loss of draft.
Puffing from drum feeding
Drums of high calorific waste release gas suddenly when they burst in the kiln. A pressure surge can push gas out at the feed chute and seals. Fan margin, a fast drive response and feed control that limits drum size and frequency keep the kiln negative.
Corrosion from mixed halogens
Waste mixes change daily, and HF from fluorinated waste attacks the glass in standard FRP and some stainless grades. Material is selected on the worst realistic waste blend, not the average, and inspected at each shutdown.
Salt deposits on the impeller
Sodium and potassium chlorides form sub-micron fume that passes the scrubber and deposits on the impeller. Wash nozzles used on a schedule and vibration monitoring catch build-up before it causes unbalance.
Energy and control
A rotary kiln ID fan moves gas against very high resistance, so it is often the largest motor on the plant. A variable speed drive on kiln pressure, rather than damper control, saves substantial power because feed rates and calorific value vary widely. Reducing air ingress at kiln seals and doors is the most effective saving, since every cubic metre of leakage air is carried through the quench, filter and scrubber.
ID fan and FD fan in hazardous waste rotary kiln incinerators
| ID fan | FD fan | |
|---|---|---|
| Position | End of the gas cleaning train | At the kiln firing end, feeding burners and lances |
| What it handles | Saturated gas with halogen acids | Ambient combustion air |
| Temperature at the fan | 65 to 80°C | Ambient, 25 to 45°C |
| Main design concern | High suction and corrosion | Fast response to variable waste |
| Control | Kiln hood pressure | Oxygen and chamber temperature |
See also the Forced Draft (FD) Fan for hazardous waste rotary kiln incinerators.
Questions buyers ask
Why does the ID fan on a hazardous waste kiln need such high pressure?
The gas passes the kiln, secondary chamber, waste heat boiler or quench, dry reactor, bag filter and a two-stage wet scrubber before the fan. Each adds resistance, and the total at the fan is typically 500 to 800 mmWC, more than most boiler ID fans.
Should the ID fan be before or after the wet scrubber?
After the scrubber is more common because it keeps the whole train under suction. Before the scrubber keeps the fan dry and allows cheaper materials, but puts the scrubber under positive pressure, so leaks release gas. The choice depends on the plant layout and the hazard.
What happens if the ID fan trips on a rotary kiln incinerator?
The kiln goes positive and gas escapes at the seals and feed chute. Plants use emergency vents, standby fans or emergency power, and interlocks that stop waste feed at once. The fan's reliability is therefore part of the plant's safety case.
Information needed to quote an ID fan for hazardous waste rotary kiln 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.
- Waste profile. Range of chlorine, fluorine and sulphur in the waste blend.
- Leakage. Measured or estimated air ingress at kiln seals and feed chute.
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.