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 ID fan sits
- Down-fired chamber
- Water quench
- Venturi scrubber
- Packed scrubber
- Mist eliminator
- ID fan
- Stack
The fan sits after the mist eliminator, at the base of the stack. It works against the chamber, quench, venturi throat and packed column, typically 400 to 800 mmWC depending on the venturi pressure drop chosen for salt fume capture. Placing it after the scrubber keeps the hot chamber and quench under suction, which is important when wastes are toxic or odorous.
Gas at the ID fan
| Gas temperature, normal | 60 to 85°C, saturated after the quench and scrubber |
| Maximum or upset | Hot gas on quench water failure; protected by emergency vent |
| Dust at the fan | Sub-micron sodium and potassium salt fume, and scrubber droplets |
| Moisture and dew point | Saturated, with very high water vapour content from aqueous waste |
| Corrosive species | Chlorides and residual HCl; sulphates where sulphur-bearing waste is fired |
| 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 liquid waste incinerators
| Blade form | Radial or backward curved, with smooth surfaces and no ledges where salt can build |
| Material of construction | SS 316L or duplex stainless for chloride duty; FRP or rubber-lined steel where acid carry-over is severe |
| Drive and arrangement | Direct coupled or belt drive, motor outside the wet zone |
| Bearings and cooling | Pedestal bearings with sealed shaft entry |
| Flow control | Variable speed drive on chamber pressure |
| Also specified | Water wash nozzles on the impeller, casing drain, and inspection door |
What this ID fan has to cope with
Salt fume build-up
Sodium chloride and sulphate fume from salt-laden aqueous waste is too fine for a low-energy scrubber. It deposits on the impeller and casing, then sheds unevenly and causes vibration. A higher venturi pressure drop, a wet electrostatic precipitator on difficult wastes, and scheduled water wash keep the fan clean.
Chloride corrosion in wet gas
Wet chloride deposits pit stainless steel under the deposit. Duplex stainless steel, or FRP where it suits the temperature, along with regular washing to remove deposits, is the usual response.
Large gas volume from aqueous waste
Every kilogram of water in the waste becomes steam, so a mostly aqueous waste produces much more gas than a solvent waste of the same heat input. The fan is sized on the full water load at the wettest waste blend, not on the burner rating.
Energy and control
Liquid waste incinerators often run continuously with feed rates set by the production plant. A variable speed ID fan follows changes in waste water content and burner firing. The venturi is the largest single pressure drop, and reducing its pressure drop where emissions allow lowers fan power directly. Concentrating aqueous waste before incineration, where practical, reduces support fuel and ID fan duty together.
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 Forced Draft (FD) Fan for liquid waste incinerators.
Questions buyers ask
Why is my liquid waste incinerator ID fan vibrating?
On salt-laden wastes the most common cause is salt fume building up on the impeller and breaking off unevenly. Regular water washing, checking the demister and confirming the venturi pressure drop usually solve it. Bearing wear and corrosion are the next things to check.
What size ID fan is needed for an aqueous waste incinerator?
The fan is sized on the gas volume after the quench, including all the water evaporated from the waste and quench, at saturated conditions. Aqueous wastes produce far more gas per unit of heat than solvent wastes, so the fan is larger than the burner rating suggests.
Can an FRP ID fan be used on a liquid waste incinerator?
Yes, after the quench and scrubber, where gas is below about 80 to 90°C and protected against hot gas by a reliable quench and emergency vent. FRP resists chloride corrosion well, but a quench failure can damage it, so trips are essential.
Information needed to quote an ID fan for liquid waste 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 composition. Water content, salt content and halogen content of the waste streams.
- Quench and scrubber. Quench type, venturi pressure drop and mist eliminator design.
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.