How petcoke fired CFBC boilers work
Petroleum coke from refinery delayed cokers has a high calorific value of about 7,800 to 8,300 kcal/kg, low volatile matter of 9 to 12%, ash below 1% and sulphur typically 4 to 7%, with vanadium and nickel in the ash. It is fired in CFBC boilers because the long residence time at 850 to 900°C burns this low reactivity fuel and limestone fed to the bed captures most of the SO₂. Limestone feed is large, often at a calcium to sulphur ratio of about 2 to 2.5, so most of the solids leaving the boiler are spent lime and calcium sulphate rather than fuel ash. After the cyclone, back pass and air heater the gas passes an ESP or bag filter and the ID fan.
Where the FD fan sits
- Inlet silencer
- FD (primary air) fan
- Air heater
- Air box
- Distribution grid
- Fluidised bed
The primary air fan stands at ground level and pushes ambient air through the air heater to the air box under the grid. A separate secondary air fan supplies air to ports higher in the furnace, and loop seal blowers fluidise the return leg. The PA fan outlet pressure is typically 1,000 to 1,500 mmWC, set mainly by the grid nozzles and the bed, whose make-up comes from limestone and added sand because petcoke contributes little ash.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 200 to 260°C, downstream of the fan |
| Pressure at the fan outlet | Typically 1,000 to 1,500 mmWC |
| Share of total air | Usually 45 to 60% as primary air |
| Dust | Clean air; limestone powder near the sorbent handling area |
| 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 petcoke fired CFBC boilers
| Blade form | Backward curved, high pressure, with a steep stable characteristic |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8, at high speed for the pressure needed |
| Bearings and cooling | Pedestal bearings, oil lubricated on larger fans |
| Flow control | Variable speed drive or inlet vanes, with a protected minimum fluidising flow |
| Also specified | Inlet silencer and filter; non-return damper for parallel fans |
What this FD fan has to cope with
Bed agglomeration from vanadium
Vanadium compounds in petcoke ash can form low melting phases that bind bed particles. Agglomeration is worst where fluidisation is weak. A PA fan that holds design air flow at all loads, and does not drop below the minimum fluidising flow, is one of the defences along with bed temperature control and bed drain.
Bed pressure drift with sorbent feed
When limestone feed changes, the bed inventory and particle size change, and so does bed pressure. The PA fan needs margin to keep air flow constant against a heavier bed, so that operators are not forced to drain the bed early. A bed pressure-based drain routine supports this.
Intake contamination from sorbent handling
Limestone powder handling plants are often near the boiler. If the PA fan intake is close by, fine limestone builds on the blades and in the air heater. Locating the intake away from the sorbent area and fitting an intake filter avoid this. The filter is checked for pressure drop.
Energy and control
The primary air fan must hold fluidisation, so its flow cannot follow load down very far. Energy savings come mostly from a variable speed drive replacing vane control, from keeping the bed inventory at its design pressure, and from shifting turndown to the secondary air fan. On refinery boilers that run steadily near full load, fan efficiency at the main operating point matters more than turndown range.
ID fan and FD fan in petcoke fired CFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Ahead of the air heater and air box |
| What it handles | High sulphur flue gas with lime-rich ash | Clean ambient air |
| Temperature at the fan | 150 to 170°C, above the acid dew point | Ambient, 25 to 45°C |
| Main design concern | Acid dew point and lime deposits | Stable fluidisation of an added-material bed |
| Control | Furnace draft | Bed fluidisation, with SA taking turndown |
See also the Induced Draft (ID) Fan for petcoke fired CFBC boilers.
Questions buyers ask
Is the FD fan the same as the primary air fan on a petcoke CFBC?
On most petcoke CFBC boilers, yes. The primary air fan acts as the forced draft fan, supplying fluidising air through the grid. A separate secondary air fan supplies the upper furnace. Some plants refer to both as FD fans, but their pressure and duty are quite different.
Why does a petcoke CFBC need added bed material?
Petcoke has under 1% ash, so it cannot build its own bed. The bed is made of sand and limestone products, which must be kept at the right size and inventory. The PA fan pressure depends on that inventory, so bed management and fan selection go together.
What pressure does a petcoke CFBC primary air fan need?
Typically 1,000 to 1,500 mmWC, including the air heater, ducts, grid nozzles and bed. The exact value comes from the boiler maker's grid and bed design. The fan needs margin over the normal bed pressure to cope with heavier beds and sorbent feed changes. Margin also covers grid nozzle wear.
Information needed to quote an FD fan for petcoke fired CFBC 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.
- Bed. Design bed pressure, bed material and the limestone feed range.
- Air split. Primary and secondary air at full and minimum load.
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.