How tyre pyrolysis plants work
Tyre pyrolysis plants heat whole or shredded end-of-life tyres in a closed reactor at about 400 to 500°C without air. The rubber breaks down into oil vapour, a non-condensable gas, carbon black and steel wire. Batch rotating drum reactors are common; continuous plants use shredded feed. The reactor is heated externally by a furnace burning the non-condensable gas and pyrolysis oil. Tyres contain about 1.5 to 2% sulphur, so the gas and oil carry hydrogen sulphide and sulphur compounds, and their combustion gives SO₂ in the furnace flue gas. Most plants pass this flue gas through a cyclone and an alkaline water scrubber, or desulphurisation tower, before the ID fan and stack. A combustion air fan supplies the heating burners.
Where the ID fan sits
- Heating furnace
- Reactor jacket
- Cyclone
- Desulphurisation scrubber
- Mist eliminator
- ID fan
- Stack
The fan is after the alkaline scrubber and mist eliminator, at the base of the stack, keeping the furnace, jacket, cyclone and scrubber under suction. Suction at the fan is typically 150 to 300 mmWC. Some plants place the fan before the scrubber to keep it dry, but then hot gas with SO₂ reaches the fan and the scrubber runs under pressure, which leaks sulphurous gas.
Gas at the ID fan
| Gas temperature, normal | 55 to 75°C, saturated after the scrubber |
| Maximum or upset | Hot gas if scrubber water fails |
| Dust at the fan | Fine soot and carbon black; scrubber droplets with sulphite and sulphate |
| Moisture and dew point | Saturated with free droplets |
| Corrosive species | Residual SO₂, sulphurous acid and sulphite; H₂S if burners run rich |
| Density at the design temperature | About 1.05 kg/m³ at 65°C, 88% 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 tyre pyrolysis plants
| Blade form | Backward curved or radial, with no liquid traps |
| Material of construction | SS 316, FRP or rubber-lined mild steel, chosen on scrubber chemistry |
| Drive and arrangement | Belt or direct drive, motor outside the wet zone |
| Bearings and cooling | Pedestal bearings with sealed shaft entry |
| Flow control | Variable speed drive or damper on furnace pressure |
| Also specified | Casing drain, wash nozzle, and temperature trip on scrubber water failure |
What this ID fan has to cope with
Sulphurous acid corrosion
Wet SO₂ forms sulphurous acid, which quickly corrodes mild steel. A mild steel fan after the scrubber may last only months. SS 316, FRP or rubber lining, with a good mist eliminator, gives long life.
Carbon black and soot deposits
Fine carbon from the burners, and from carbon black handling nearby, mixes with scrubber droplets to form a sticky sludge on the impeller. Wash nozzles and access for cleaning keep the fan balanced.
Odour and H₂S
If burners run rich or the gas is poorly burnt, H₂S passes to the stack, causing odour complaints. The ID fan must hold furnace suction so combustion stays complete, and the casing must be gas-tight.
Energy and control
As with plastic pyrolysis, the heating load changes through each batch, from heat-up to the main reaction and cool-down. A variable speed drive on furnace pressure follows these stages and saves power when firing is low. The scrubber and mist eliminator take a major share of fan pressure, so keeping their packing and pads clean keeps the fan at its design point and avoids a gradual rise in power.
ID fan and FD fan in tyre pyrolysis plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the desulphurisation scrubber | Beside the heating furnace |
| What it handles | Wet flue gas with SO₂ | Ambient combustion air |
| Temperature at the fan | 55 to 75°C | Ambient, 25 to 45°C |
| Main design concern | Sulphurous acid corrosion | Complete burning of sulphurous gas |
| Control | Furnace pressure | Oxygen and firing rate |
See also the Forced Draft (FD) Fan for tyre pyrolysis plants.
Questions buyers ask
Why does the ID fan corrode on a tyre pyrolysis plant?
Tyres contain sulphur, so the burnt pyrolysis gas and oil produce SO₂. After a wet scrubber the gas is saturated with sulphurous acid droplets, which corrode mild steel quickly. SS 316, FRP or rubber-lined fans are needed.
Where should the ID fan be on a tyre pyrolysis plant?
Usually after the desulphurisation scrubber, at the stack, so that the furnace and scrubber stay under suction and sulphurous gas does not leak. Corrosion-resistant materials are needed in this position.
Does the tyre pyrolysis ID fan handle carbon black?
Not the product carbon black, which is discharged from the reactor and handled by a separate dust extraction system. The ID fan sees only fine soot and carbon carried with the furnace flue gas.
Information needed to quote an ID fan for tyre 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.
- Scrubber. Scrubber type, liquor chemistry and pressure drop.
- Fuel sulphur. Expected sulphur in the pyrolysis gas and oil fired in the furnace.
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.