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 FD fan sits
- Inlet screen
- FD fan
- Air header
- Kiln burner and lances
- Secondary chamber burners
- Kiln and SCC
The combustion air fan sits near the kiln firing end, feeding a header that branches to the kiln burner, waste lances, kiln hood and secondary chamber. Larger plants use separate fans for the kiln and the secondary chamber. Pressure needed is typically 300 to 600 mmWC, set by burners and atomising lances. Kiln shell cooling fans, where fitted, are a separate duty.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the burners | Ambient on most plants; preheated where heat recovery allows |
| Dust | Clean air; screen against site dust and debris |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Load profile | Rapid changes with drum feeding and changing liquid waste |
| 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 hazardous waste rotary kiln incinerators
| Blade form | Backward curved or high-pressure radial, for stable delivery to burners and lances |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive with branch dampers, trimmed on oxygen and chamber temperature |
| Also specified | Airflow measurement on each branch, inlet silencer and air pressure interlocks |
What this FD fan has to cope with
Oxygen starvation from drum feeding
A drum of high calorific waste can use up the available oxygen in seconds, producing CO and soot. Combustion air must rise quickly in response to oxygen, so a fast drive and airflow measurement on the kiln branch are needed.
Secondary chamber temperature control
The secondary chamber must hold its minimum temperature regardless of waste. Too much air cools it and increases support fuel; too little leaves CO. Separate control of secondary chamber air allows the operator to set it independently of the kiln.
Burner and lance pressure
Waste lances and dual-fuel burners need steady air pressure across a wide turndown. A fan with a stable, steep pressure curve avoids flame instability and poor atomisation when demand falls during low feed or between drum charges.
Energy and control
Combustion air fans on a rotary kiln follow the waste feed, which varies hour to hour. Variable speed drives with oxygen trim lower fan power and also lower total gas volume, which reduces ID fan power, quench water and reagent use. Holding excess oxygen at the minimum that keeps CO in limits is the most valuable control setting on the plant.
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 Induced Draft (ID) Fan for hazardous waste rotary kiln incinerators.
Questions buyers ask
How many combustion air fans does a rotary kiln incinerator have?
Smaller plants use one fan with branches to the kiln and secondary chamber. Larger plants usually have separate fans for each so they can be controlled and maintained independently. Kiln shell cooling fans, if fitted, are a separate system.
What pressure is needed for kiln burner combustion air?
Typically 300 to 600 mmWC at the fan, depending on burner design, waste lance atomisation and duct losses. The burner supplier states the air pressure needed at the burner, and the fan is sized to that with margin.
Does the FD fan on a hazardous waste incinerator need special materials?
No. It handles ambient air and does not see flue gas, so mild steel construction is normal. The corrosion-resistant materials are needed on the ID fan after the scrubber.
Information needed to quote an FD fan for hazardous waste rotary kiln 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.
- Air branches. Air flow and pressure to the kiln burner, lances, kiln hood and secondary chamber.
- Turndown. Range of air flow between minimum and maximum waste firing.
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.