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 ID fan sits
- Regenerator flue gas
- CO boiler furnace
- Superheater and evaporator
- Economiser
- ESP
- ID fan
- Wet gas scrubber or stack
Where fitted, the fan sits after the economiser and ESP, or between the ESP and a wet gas scrubber. Upstream, the regenerator gas enters at positive pressure, so the ID fan is not needed to draw gas from the regenerator; it overcomes the economiser, ESP and ducts, and keeps the CO boiler furnace at a balanced or slightly negative pressure so the boiler does not leak hot gas. Suction is typically 150 to 300 mmWC. On fully pressurised designs with no ID fan, the FD fan and regenerator pressure push the gas through to the stack.
Gas at the ID fan
| Gas temperature, normal | 190 to 240°C after the economiser and ESP |
| Maximum or upset | Up to 300°C or more on economiser bypass or loss of feedwater |
| Dust at the fan | Fine, hard alumina-silica catalyst; tens of mg/Nm³ after the ESP, much higher during ESP trips or regenerator upsets |
| Moisture and dew point | Moderate water vapour; acid dew point raised by SO₃ from feed sulphur |
| Corrosive species | SO₂ and SO₃ depending on feed sulphur; corrosive below the acid dew point |
| Density at the design temperature | About 0.73 kg/m³ at 215°C, 61% 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 FCC CO boilers
| Blade form | Backward curved with wear protection on the leading edges and pressure faces |
| Material of construction | Mild steel or high strength steel casing; impeller with wear-resistant plate or hard-faced leading edges |
| Drive and arrangement | Direct coupled, arrangement 8; steam turbine drive or dual drive on some refinery units |
| Bearings and cooling | Pedestal bearings with cooling disc, vibration and temperature monitoring |
| Flow control | Variable speed drive or inlet vane control on furnace pressure |
| Also specified | Casing insulation, drain, wear liners at the scroll cut-off, and documentation to refinery specifications where called for |
What this ID fan has to cope with
Catalyst fines erosion
FCC catalyst is a hard alumina-silica powder of a few tens of microns, and the fines that pass the third stage separator and ESP erode the blade leading edges and the scroll near the cut-off. A moderate tip speed, wear-resistant plate, and liners that can be changed at a turnaround keep the fan running between turnarounds.
ESP trips and dust surges
If the ESP trips or a regenerator upset sends a puff of catalyst, the dust at the fan rises by orders of magnitude for a while. The fan must survive these events without imbalance. Liners, an inspection door and vibration trips set at sensible levels limit the damage.
Turndown and boiler bypass
When the FCC unit is down or the CO boiler is bypassed, the boiler may fire refinery fuel alone or stand idle hot. The fan is checked for the low-volume, high-temperature case so it neither stalls nor overheats its bearings during these periods.
Energy and control
The CO boiler follows the FCC unit, which runs long campaigns at near steady rate. A variable speed drive gives savings mostly at turndown and during start-up. In many refineries the ID fan is steam turbine driven, using low pressure steam already available, and the choice between turbine and motor is an energy balance decision for the refinery rather than a fan choice.
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 Forced Draft (FD) Fan for FCC CO boilers.
Questions buyers ask
Does every FCC CO boiler have an ID fan?
No. Many CO boilers are pressurised, with the regenerator flue gas and FD fan pushing the gas through to the stack. An ID fan is added where an ESP, wet gas scrubber or long ducting adds resistance that the upstream pressure cannot cover, or where the boiler is designed for balanced draft.
How do you protect a CO boiler ID fan from catalyst erosion?
Keep the tip speed moderate, use wear-resistant plate or hard-facing on the blade leading edges and pressure faces, and fit replaceable liners at the scroll cut-off. Placing the fan after the ESP, and keeping the ESP in service, reduces the catalyst load at the fan most of all.
What temperature does the FCC CO boiler ID fan see?
Typically about 190 to 240°C after the economiser and ESP. Higher temperatures occur during economiser bypass, feedwater problems or start-up on supplementary fuel. The fan materials, shaft cooling and bearings are selected so it can run through these excursions without distortion, and the duty point is checked at both the normal and the hot case.
Information needed to quote an ID fan for FCC CO boilers
- 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.
- Regenerator flue gas. Flow, temperature, CO content and catalyst loading of the regenerator flue gas entering the boiler.
- Downstream equipment. ESP, wet gas scrubber or both, their pressure drop, and whether the boiler is to run balanced or pressurised.
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.