How plastic pyrolysis plants work
Plastic pyrolysis plants heat shredded waste plastics, mainly polyethylene and polypropylene, in a closed reactor at about 400 to 500°C without air. The plastic cracks into vapours that are condensed to pyrolysis oil, a non-condensable gas, and a small residue of char. Batch plants use a rotating drum reactor; continuous plants use screw or kiln reactors. The reactor is heated externally by a furnace or jacket burning the non-condensable gas, pyrolysis oil, or wood and coal at start-up. The product vapours pass through condensers under slight positive pressure and do not go through a fan. The draft fans serve the heating furnace: a combustion air fan for its burners and an ID fan that draws the furnace flue gas from the reactor jacket to the stack.
Where the FD fan sits
- Inlet screen
- FD fan
- Air preheater
- Burner
- Heating furnace
- Reactor jacket
The fan sits beside the heating furnace and supplies ambient air to the burners, through an air preheater where fitted. It works against burner and duct resistance, typically 150 to 400 mmWC. On plants that use wood or coal at start-up, the fan also supplies air under a small grate. The pyrolysis vapour path has no fan, so this fan affects only the heating side.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 150 to 250°C where fitted, downstream of the fan |
| Dust | Plastic flakes and dust from feed handling; inlet screen needed |
| Pressure at the fan outlet | Typically 150 to 400 mmWC |
| Load profile | Changes through the batch as gas production rises and falls |
| 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 plastic pyrolysis plants
| Blade form | Backward curved for efficiency |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings |
| Flow control | Variable speed drive or damper, linked to burner firing and oxygen |
| Also specified | Inlet screen, air pressure interlock with burners |
What this FD fan has to cope with
Changing gas composition
The non-condensable gas changes in flow and calorific value as the batch progresses. Fixed air settings produce smoke or excess air. Air control linked to burner firing or flue oxygen keeps combustion clean.
Plastic flakes at the inlet
Light flakes from shredding are drawn into the fan, block the inlet screen and build up on the impeller. Taking air away from the feed area keeps the fan clean.
Backfire risk
Burning a gas of variable composition needs a reliable air supply. A fan trip while gas is still flowing can lead to an unburnt gas build-up in the furnace, so air pressure interlocks close the gas supply at once, and the furnace is purged before relighting.
Energy and control
The combustion air fan is small, but good air control saves heating fuel, which on many plants is the process's own oil. Excess air carries heat out with the flue gas and increases the gas the ID fan must move. A variable speed drive with oxygen trim through the heat-up, reaction and cool-down stages of each batch cuts both fuel and fan power.
ID fan and FD fan in plastic pyrolysis plants
| ID fan | FD fan | |
|---|---|---|
| Position | After heat recovery and cyclone, at the stack | Beside the heating furnace |
| What it handles | Heating furnace flue gas | Ambient combustion air |
| Temperature at the fan | 200 to 300°C, up to 450°C upset | Ambient, 25 to 45°C |
| Main design concern | Stable draft and soot | Air for variable gas |
| Control | Furnace pressure | Burner firing and oxygen |
See also the Induced Draft (ID) Fan for plastic pyrolysis plants.
Questions buyers ask
Does the FD fan supply air to the pyrolysis reactor?
No. The reactor runs without air, because air would burn the plastic instead of cracking it. The FD fan supplies combustion air only to the burners in the heating furnace, which heat the reactor from outside.
What fuel do pyrolysis plant burners use?
Usually the non-condensable gas from the process, with pyrolysis oil, diesel, or wood and coal at start-up. The FD fan is sized for the highest air demand among these fuels.
Why does the heating furnace smoke at certain times in the batch?
Gas production and calorific value change through the batch, rising during the main reaction and falling towards the end. If combustion air does not follow these changes, smoke forms. Air control linked to flue oxygen solves most cases.
Information needed to quote an FD fan for plastic pyrolysis plants
- 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.
- Burners. Number of burners and fuels, including start-up fuel.
- Gas production. Non-condensable gas flow range through the batch.
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.