How FCC CO boilers work
In a refinery fluid catalytic cracking unit, coke on the spent catalyst is burned off in the regenerator. When the regenerator runs in partial combustion, its flue gas carries typically 4 to 10% CO at about 650 to 730°C, along with catalyst fines that pass the cyclones and third stage separator. After the slide valves, orifice chamber or power recovery expander, the gas enters the CO boiler, where it is burned with supplementary refinery fuel gas or oil and combustion air to above about 950°C, completing CO burnout and raising high pressure steam. The flue gas then passes the economiser and an ESP or wet gas scrubber for catalyst fines and SO₂. An FD fan supplies combustion air; an ID fan is fitted where the downstream equipment needs it.
Where the FD fan sits
- Inlet filter and silencer
- FD fan
- Air preheater where fitted
- Windbox
- Supplementary burners and CO ports
- Furnace
The fan sits at grade and delivers ambient air to the windbox, sometimes through an air preheater. Air enters through the supplementary burners and through ports around the CO gas inlet, where it mixes with the regenerator gas. The fan must supply enough air for full CO burnout plus supplementary firing, at the windbox pressure needed for good mixing, typically 200 to 400 mmWC. When the FCC is shut down, the same fan supports full firing on refinery fuel gas or oil.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air preheater | Up to about 250°C where fitted, downstream of the fan |
| Dust | Refinery atmosphere; inlet filter and silencer usual |
| Pressure at the fan outlet | Typically 200 to 400 mmWC |
| Operating modes | CO gas plus supplementary fuel, and supplementary fuel alone |
| 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 FCC CO boilers
| Blade form | Backward curved or aerofoil |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled motor, or steam turbine drive; dual drive on some units |
| Bearings and cooling | Pedestal bearings with monitoring to refinery practice |
| Flow control | Inlet vanes or variable speed drive on combustion control |
| Also specified | Inlet filter and silencer, and often a spare fan or turbine-motor dual drive |
What this FD fan has to cope with
Two very different duties
With CO gas, much of the heat comes from the regenerator gas and the fan supplies burnout air; on supplementary fuel alone, it supplies full combustion air for the boiler rating. The fan is sized for the larger of the two and checked for stable operation at the smaller.
Incomplete CO burnout
If the air is short or poorly mixed with the regenerator gas, CO slips through and can burn in the convective pass or the ESP. The FD fan must hold steady windbox pressure across the load range so the air ports keep their penetration.
Availability
Losing the FD fan forces the CO boiler to be bypassed, sending CO to atmosphere and upsetting the FCC. Many refineries use a spare fan or a turbine and motor on the same shaft so the fan stays in service through a power dip.
Energy and control
The FD fan runs continuously for years between turnarounds, so efficiency over that period outweighs first cost. Inlet vanes or a variable speed drive follow the FCC load, and a turbine drive using available steam can reduce electrical load. Keeping excess air to what CO burnout needs, measured at the furnace exit, saves both fuel and fan power.
ID fan and FD fan in FCC CO boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the economiser and ESP, where fitted | At the windbox, ahead of burners and CO ports |
| What it handles | Flue gas with catalyst fines and SO₃ | Ambient air |
| Temperature at the fan | 190 to 240°C | Ambient |
| Main design concern | Catalyst erosion and acid dew point | Two firing modes and availability |
| Control | Furnace pressure | Combustion control on CO and supplementary fuel |
See also the Induced Draft (ID) Fan for FCC CO boilers.
Questions buyers ask
What does the FD fan do on an FCC CO boiler?
It supplies combustion air to burn the CO in the regenerator flue gas and any supplementary fuel. It handles ambient air only; the regenerator gas enters the boiler separately under its own pressure. When the FCC is down, the same fan supplies all the air for firing on refinery fuel alone.
Can the CO boiler FD fan be steam turbine driven?
Yes. Refineries often use steam turbine drives, or a motor and turbine on the same shaft, for critical fans. The turbine uses steam already available and keeps the fan running through electrical disturbances. The fan design is the same; the drive and coupling arrangement change.
Why must the FD fan keep running when the FCC trips?
The CO boiler often fires supplementary fuel to stay hot and keep steam supply to the refinery while the FCC restarts. The FD fan supplies that air, and it also provides the purge air needed before the regenerator gas is readmitted.
Information needed to quote an FD fan for FCC CO boilers
- 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.
- Operating modes. Air demand with CO gas plus supplementary fuel, and with supplementary fuel alone at full boiler load.
- Drive and standards. Motor, steam turbine or dual drive, and any refinery or international specification the fan must meet.
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.