How calcined petroleum coke kilns work
Green petroleum coke from refinery cokers, with 8 to 12% volatile matter and 6 to 12% moisture, is calcined in a rotary kiln at about 1,200 to 1,350°C to drive off moisture and volatiles and raise its density and conductivity for anode and recarburiser use. The volatiles burn inside the kiln with air admitted through shell-mounted tertiary air fans, so the kiln needs little external fuel once running. The kiln exit gas, at about 900 to 1,100°C, still carries unburnt volatiles and coke fines, and passes to a pyroscrubber or incinerator where they burn out at 1,100 to 1,200°C. The gas then goes through a waste heat boiler, a bag filter or ESP, and often a desulphurisation unit, before the ID fan and stack. The calcined coke is cooled with water in a rotary cooler.
Where the FD fan sits
- Ambient air
- Shell-mounted tertiary air FD fans
- Kiln shell nozzles
- Kiln volatile burning zone
- Pyroscrubber air FD fans
- Pyroscrubber
Tertiary air fans are mounted on the kiln shell, typically several around the circumference at two or three axial positions, and blow air through nozzles into the kiln to burn the released volatiles. They rotate with the kiln and draw power through slip rings. Pyroscrubber air fans are conventional fans at ground level feeding air into the incinerator. Both handle ambient air at modest pressure, typically 100 to 300 mmWC for the shell fans.
Air at the FD fan
| Air temperature at the fan | Ambient, with radiant heat from the kiln shell at 250 to 350°C |
| Dust | Coke dust around the kiln; filters or protected intakes needed |
| Pressure at the fan outlet | Typically 100 to 300 mmWC for shell fans; higher for pyroscrubber air |
| Mounting | Shell fans rotate with the kiln at 1 to 3 rpm |
| Variation | Air set to the volatile release along the kiln |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 calcined petroleum coke kilns
| Blade form | Backward curved or radial, compact for shell mounting |
| Material of construction | Mild steel; heat shields between fan and shell |
| Drive and arrangement | Direct coupled, compact, with motor and fan on a common rigid base clamped to the shell |
| Bearings and cooling | Bearings and motor protected from shell radiation; no gas cooling needed |
| Flow control | Inlet damper or speed control per fan, set by the operator for each zone |
| Also specified | Robust mounting for continuous rotation, inspection access from the kiln platform |
What this FD fan has to cope with
Radiant heat from the shell
The kiln shell runs at 250 to 350°C. Fans and motors mounted close to it overheat unless shielded. Heat shields, a stand-off from the shell and motors rated for higher ambient temperature keep bearings and windings within limits.
Rotation and gravity load
A shell-mounted fan turns over once per kiln revolution, so its bearings and base see changing load direction. Rigid bases, bearings suited to the loading and careful balancing avoid fatigue cracks at the mounting.
Air distribution and coke burn-off
Too much tertiary air burns calcined coke instead of volatiles, reducing yield. Too little leaves volatiles for the pyroscrubber. Individual control of the fans at each axial position lets the operator match air to volatile release.
Energy and control
Each tertiary air fan is small, but there are many of them and they run continuously. Matching air to volatile release, rather than running all fans at full flow, saves power and, more importantly, reduces coke loss. Pyroscrubber air fans on speed control follow the volatile load and the boiler steam demand.
ID fan and FD fan in calcined petroleum coke kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the boiler and gas cleaning | On the kiln shell and at the pyroscrubber |
| What it handles | Sulphur-laden flue gas | Ambient air |
| Temperature at the fan | 180 to 250°C | Ambient, with shell radiation |
| Main design concern | Acid dew point and trip consequences | Rotating mount and shell heat |
| Control | Pyroscrubber or kiln draft | Air per zone for volatile burn-off |
See also the Induced Draft (ID) Fan for calcined petroleum coke kilns.
Questions buyers ask
What are tertiary air fans on a petcoke kiln?
They are small fans mounted on the shell of the calcined petcoke kiln, blowing ambient air into the kiln to burn the volatiles released from the coke. They rotate with the kiln and get power through slip rings.
Does a calcined petcoke kiln need a combustion air fan for the burner?
Only during start-up and for heat support. Once the kiln is running, the coke volatiles supply most of the heat, burnt with air from the tertiary fans and from the discharge end.
How hot does a shell-mounted fan get?
The fan handles ambient air, but it sits close to a kiln shell at 250 to 350°C. With heat shields and stand-off mounting, bearing and motor temperatures stay within normal limits.
Information needed to quote an FD fan for calcined petroleum coke kilns
- 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.
- Shell fans. Number of fans, axial positions, air per fan and available mounting space.
- Pyroscrubber. Combustion air flow and pressure required for the incinerator.
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.