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 ID fan sits
- Heating furnace
- Reactor jacket
- Heat exchanger
- Cyclone
- ID fan
- Stack
The fan sits after a heat exchanger or air preheater and a cyclone, which remove heat and coarse ash. It keeps the furnace and jacket under slight suction so that flame and gas flow evenly around the reactor. Suction at the fan is typically 80 to 200 mmWC. Some plants add a water scrubber before the fan, which cools the gas further and makes it wet.
Gas at the ID fan
| Gas temperature, normal | 200 to 300°C after heat recovery |
| Maximum or upset | Up to 450°C without heat recovery or with fouled surfaces |
| Dust at the fan | Soot from oil and gas firing; wood or coal ash during start-up |
| Moisture and dew point | Moderate; well above dew point in dry systems |
| Corrosive species | HCl if PVC enters the feed and its gas is burned; normally minor |
| Density at the design temperature | About 0.68 kg/m³ at 250°C, 57% 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 plastic pyrolysis plants
| Blade form | Radial or backward curved, tolerant of soot |
| Material of construction | Mild steel; heat-resisting steel impeller where the gas can exceed about 350°C |
| Drive and arrangement | Belt drive or direct coupled, motor away from the hot casing |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive or damper on furnace pressure |
| Also specified | Cleaning door, temperature monitoring, and casing drain |
What this ID fan has to cope with
Uneven reactor heating
If furnace draft is unstable, flame and hot gas favour one side of the reactor, causing local overheating, coking and lower oil yield. A fan with stable control on furnace pressure keeps heat distribution even.
Soot from non-condensable gas and oil
Burning the non-condensable gas and pyrolysis oil, especially with poor air control, produces soot that coats heat surfaces and the fan impeller. Good combustion control and access for cleaning keep the fan in balance.
PVC contamination
PVC in the feed produces HCl in the pyrolysis gas and oil, which then appears in the furnace flue gas when they are burned. This corrodes heat surfaces and the fan below the acid dew point. Feed sorting to exclude PVC is the main protection.
Energy and control
Batch pyrolysis plants have a heat-up, reaction and cool-down cycle, so the heating furnace load and ID fan duty change through each batch. A variable speed drive on furnace pressure saves power during cool-down and low-fire periods. Recovering heat from the jacket flue gas, for example to preheat combustion air or dry the feed, reduces fuel use and lowers gas volume at the fan.
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 Forced Draft (FD) Fan for plastic pyrolysis plants.
Questions buyers ask
Does the ID fan on a pyrolysis plant handle the pyrolysis gas?
No. The pyrolysis vapours go to the condensers and the non-condensable gas to the burners, under slight positive pressure. The ID fan handles flue gas from the furnace that heats the reactor, after the burners have burnt the fuel.
What temperature is the flue gas at the ID fan on a plastic pyrolysis plant?
Typically 200 to 300°C after a heat exchanger or air preheater, and up to 450°C on plants without heat recovery or with fouled surfaces. Fans for gas above about 350°C need heat-resisting steel impellers and bearings protected by a cooling disc.
Why does the reactor heat unevenly on my pyrolysis plant?
One common reason is unstable or insufficient furnace draft, which lets the flame and hot gas favour one side of the jacket. Checking the ID fan capacity against the actual suction needed, its control, and the furnace gas passages usually finds the cause.
Information needed to quote an ID fan for plastic pyrolysis plants
- 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.
- Heating fuel. Fuels used in the heating furnace, including start-up fuel.
- Batch cycle. Batch size, cycle time and heating furnace firing rate.
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.