How e-waste thermal treatment furnaces work
E-waste thermal treatment furnaces burn or pyrolyse shredded printed circuit boards, cables and electronic scrap to remove plastics and recover copper and precious metals. Feed is charged to a rotary kiln, a batch pyrolysis chamber or a small smelting furnace at 500 to 900°C. The plastics, mainly epoxy and ABS with brominated flame retardants, give off a gas rich in organics, HBr and HCl. A secondary combustion chamber at about 1,100°C destroys the organics, after which the gas must be cooled quickly, by quench or heat exchanger, to below about 200°C to limit dioxin and furan reformation. Dry lime and activated carbon injection with a bag filter, often followed by a wet scrubber, cleans the gas before the ID fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Air header
- Kiln primary air
- Afterburner burner
- Secondary air ports
The fan sits near the kiln and feeds a header with branches to the kiln burner and primary air, the afterburner burner and the afterburner's secondary air nozzles. On pyrolysis units, primary air is restricted or zero and most of the air goes to the afterburner. Pressure needed is typically 200 to 400 mmWC for the burners and mixing nozzles.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 200 to 400 mmWC |
| Air split | Varies between kiln and afterburner with the feed |
| Excess oxygen | Afterburner usually held at several percent oxygen |
| 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 e-waste thermal treatment furnaces
| Blade form | Backward curved, for steady pressure |
| Material of construction | Mild steel, painted |
| Drive and arrangement | Direct drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on header pressure, with branch control valves on oxygen and temperature |
| Also specified | Inlet filter and silencer, and air flow measurement on each branch |
What this FD fan has to cope with
Incomplete destruction of brominated organics
If the afterburner is short of air during a plastic-rich batch, brominated organics and soot pass through unburnt into the quench and gas cleaning, raising dioxin risk and fouling the bag filter. The fan is sized for the peak organic load, with secondary air nozzles that give strong mixing at about 1,100°C, not merely for the average feed.
Over-airing the pyrolysis chamber
On pyrolysis designs, too much primary air turns pyrolysis into burning inside the chamber. The temperature rises, metals oxidise and are harder to recover, and the gas volume becomes hard to control. Branch valves and flow measurement hold primary air at the intended low level, while the bulk of the air goes to the afterburner.
Branch flow imbalance
Without flow measurement on each branch, operators open dampers by habit and the afterburner can end up with the wrong air while the kiln is over-aired. Flow measurement on each branch, and automatic control of afterburner air from the exit oxygen analyser, keep the split right as the feed composition changes from batch to batch.
Energy and control
The afterburner burns support fuel to stay hot, and every cubic metre of excess air must be heated to about 1,100°C. Speed control on the combustion air fan, with oxygen trim in the afterburner, keeps excess air down and saves support fuel, which is the main running cost of the thermal stage. Fan power is a small part of the total.
ID fan and FD fan in e-waste thermal treatment furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After bag filter and scrubber | Ahead of kiln and afterburner |
| What it handles | Cleaned gas with HBr and HCl traces | Ambient air |
| Temperature at the fan | 140 to 180°C dry, 55 to 75°C wet | Ambient |
| Main design concern | Halide corrosion and high system resistance | Afterburner air for organics destruction |
| Control | Kiln pressure | Afterburner oxygen and temperature |
See also the Induced Draft (ID) Fan for e-waste thermal treatment furnaces.
Questions buyers ask
What does the combustion air fan do on an e-waste thermal treatment plant?
On an e-waste thermal treatment plant the combustion air fan supplies air to the kiln or pyrolysis chamber and to the afterburner, where organics released from burning circuit boards are destroyed. Correct air supply and mixing in the afterburner is essential to destroy brominated organics before the gas is quenched and cleaned.
Is pyrolysis of e-waste done without air?
Pyrolysis restricts or eliminates air in the primary chamber, so the plastics decompose into gas and char rather than burning, and the metals stay largely unoxidised. The pyrolysis gas is then burned in an afterburner with air from the combustion air fan, which therefore carries most of the air duty on such plants.
How much excess air does an e-waste afterburner need?
Afterburners are usually run with several percent oxygen in the exit gas, which means significant excess air over the theoretical requirement. The fan must supply this at peak organic load, when a plastic-rich batch releases volatiles quickly, and the control must trim it back during low load to avoid wasting support fuel.
Information needed to quote an FD fan for e-waste thermal treatment furnaces
- 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.
- Process type. Kiln incineration or pyrolysis, and the air split between primary and afterburner.
- Burner data. Burner capacities and the air pressure needed at the burners and nozzles.
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.