How liquid waste incinerators work
Liquid waste incinerators burn pumpable wastes such as aqueous organic streams, mother liquors, still bottoms and contaminated oils, atomised into a refractory-lined chamber through steam or air atomised nozzles. Many process wastes are mostly water with a low calorific value, so support fuel is needed to hold the chamber at about 1,000 to 1,200°C. Wastes high in sodium or potassium salts form molten salt in the chamber, so many units are vertical down-fired, discharging directly into a water quench that dissolves the salts. The quenched, saturated gas passes a venturi and a packed alkali scrubber and a mist eliminator, and the ID fan delivers it to the stack. A combustion air fan supplies the burner and waste nozzles.
Where the FD fan sits
- Inlet filter
- FD fan
- Burner windbox
- Waste nozzle air
- Combustion chamber
The fan sits beside the burner and delivers ambient air to the windbox, which feeds the support fuel burner and the air registers around the waste nozzles. Atomising air for the nozzles usually comes from a separate compressor or blower. The combustion air fan works against the windbox and swirl register, typically 200 to 500 mmWC. On down-fired units the fan and burner are at the top of the chamber, so the fan is often on a platform.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the burner | Ambient; preheat is unusual on smaller units |
| Dust | Clean air; filtered inlet on chemical sites |
| Pressure at the fan outlet | Typically 200 to 500 mmWC |
| Load profile | Changes with the waste tank in service and its water content |
| 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 liquid waste incinerators
| Blade form | Backward curved or high-pressure radial, for stable pressure at low flow |
| Material of construction | Mild steel; coated where chemical site atmosphere is corrosive |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings or standard pedestal bearings |
| Flow control | Variable speed drive or inlet damper, trimmed on oxygen at the chamber outlet |
| Also specified | Air pressure switch interlocked with burner and waste feed, inlet filter |
What this FD fan has to cope with
Flame instability on wet waste
Aqueous waste cools the flame. If air is not reduced as calorific value falls, excess air cools the chamber further and the flame lifts. Combustion air trimmed on oxygen and chamber temperature keeps the flame stable.
Chamber temperature below minimum
Regulations require a minimum chamber temperature for destruction. Too much air drops the temperature and raises support fuel use. Accurate airflow measurement and fan control at low flow avoid this.
Corrosive site atmosphere
Chemical plants often have acid vapours in the air. An unprotected mild steel fan corrodes externally and internally. A coated fan and an inlet taken from a clean area extend its life.
Energy and control
The combustion air fan is small compared with the ID fan, but excess air from it has a large effect on support fuel use and on the gas volume the ID fan must move. Running at the lowest excess oxygen that keeps CO in limits, with a variable speed drive, saves fuel first and fan power second.
ID fan and FD fan in liquid waste incinerators
| ID fan | FD fan | |
|---|---|---|
| Position | After the scrubber and mist eliminator | At the burner windbox |
| What it handles | Saturated gas with salt fume | Ambient combustion air |
| Temperature at the fan | 60 to 85°C | Ambient, 25 to 45°C |
| Main design concern | Salt build-up and chloride corrosion | Stable flame on wet waste |
| Control | Chamber pressure | Oxygen and chamber temperature |
See also the Induced Draft (ID) Fan for liquid waste incinerators.
Questions buyers ask
What does the FD fan do on a liquid waste incinerator?
It supplies combustion air for the support burner and around the waste nozzles. Atomising air for the waste nozzles usually comes from a separate compressor or blower. The FD fan handles ambient air only.
How much excess air does a liquid waste incinerator use?
Typically enough to give 6 to 10% oxygen at the chamber outlet, depending on the waste and the design. Too little causes CO and soot; too much cools the chamber and increases support fuel.
Is atomising air from the FD fan?
Usually not. Waste nozzles need higher pressure for fine atomisation, typically from a compressor, a separate blower or steam. The FD fan supplies the bulk combustion air through the windbox and swirl register around the nozzles.
Information needed to quote an FD fan for liquid waste 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.
- Burner. Burner type, support fuel and required air pressure at the windbox.
- Waste range. Range of calorific value and water content of the wastes to be fired.
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.