How petcoke AFBC boilers with limestone injection work
Bubbling bed AFBC boilers can fire petcoke if limestone is fed with it to capture sulphur. Petcoke is crushed fine and is usually fed under the bed by pneumatic transport, with limestone fed either with the fuel or separately. Because the bubbling bed has a short gas residence time and coarse bed, sulphur capture is less efficient than in a CFBC, and calcium to sulphur ratios of about 2.5 to 3.5 are common. Petcoke burns slowly, so a share of unburnt carbon and limestone fines leaves the bed and is often returned from the economiser and multiclone hoppers by a fly ash recirculation system. The gas then passes an ESP or bag filter and the ID fan. Operating bed temperatures are typically 840 to 880°C.
Where the ID fan sits
- Bubbling bed and freeboard
- Superheater and bank
- Economiser
- Air heater
- Multiclone with recirculation
- ESP or bag filter
- ID fan
- Chimney
The fan sits after the final collector. Ahead of it, the multiclone returns carbon-rich fines to the bed while the ESP or bag filter takes the rest. Suction at the fan inlet is typically 220 to 350 mmWC. Because AFBC sulphur capture is less complete, the gas at the fan contains more SO₂ and SO₃ than a CFBC firing the same petcoke, so the air heater outlet temperature is held higher to protect the collector and fan.
Gas at the ID fan
| Gas temperature, normal | 160 to 180°C, held above the acid dew point |
| Maximum or upset | Up to 220°C with fouled back pass |
| Dust at the fan | Lime, calcium sulphate and carbon fines; 50 to 100 mg/Nm³ after the final collector |
| Moisture and dew point | Low fuel moisture; water dew point about 40 to 50°C |
| Corrosive species | Higher SO₂ and SO₃ slip than a CFBC; acid dew point typically 130 to 150°C |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 petcoke AFBC boilers with limestone injection
| Blade form | Backward curved, with liners at the leading edges for carbon and lime fines |
| Material of construction | Mild steel impeller with insulated casing; corrosion-resistant steel at casing bottom, drains and inlet cone |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | External insulation, drains, inspection doors and warm air purge for shutdowns |
What this ID fan has to cope with
Acid corrosion from SO₃ slip
Bubbling beds capture less sulphur than circulating beds, and the SO₃ that remains raises the acid dew point. A cold casing wall at low load or during a slow start condenses acid, which attacks mild steel and binds ash. Insulation, a minimum air heater outlet temperature and corrosion-resistant steel at the casing bottom give protection.
Carbon and lime from ash recirculation
Fly ash recirculation improves burnout and lime use, but it raises the dust load through the back pass and the collectors. When the multiclone or recirculation system is upset, more of this material reaches the final collector and the fan. Leading edge liners and a dust alarm protect the impeller.
Deposits on blades after shutdown
Lime-rich ash left on the impeller absorbs moisture during shutdowns and hardens. The fan then restarts out of balance. Running the fan briefly on warm air before stopping, and inspecting the blades at each outage, prevent heavy build-up. Shutdown purging takes only a few minutes and is easily written into the procedure.
Energy and control
Petcoke AFBC boilers in cement, chemical and textile plants often run at steady load with occasional compartment slumping. A variable speed ID fan on furnace draft saves energy at reduced load. Because the gas at the fan is kept relatively hot to avoid acid condensation, gas volume is higher than on a coal AFBC, so a fan selected close to the actual operating point, rather than with excessive margin, pays back over the year.
ID fan and FD fan in petcoke AFBC boilers with limestone injection
| ID fan | FD fan | |
|---|---|---|
| Position | After the final collector | Ahead of the air heater and air boxes |
| What it handles | Flue gas with SO₃ slip, lime and carbon fines | Clean ambient air |
| Temperature at the fan | 160 to 180°C | Ambient, 25 to 45°C |
| Main design concern | Acid dew point and deposits | Pressure margin for a limestone-heavy bed |
| Control | Furnace draft | Steam pressure and bed fluidisation |
See also the Forced Draft (FD) Fan for petcoke AFBC boilers with limestone injection.
Questions buyers ask
Why is acid corrosion worse on a petcoke AFBC than a petcoke CFBC?
A bubbling bed captures sulphur less efficiently than a circulating bed, even with more limestone. More SO₂ and SO₃ remain in the flue gas, raising the acid dew point. The ID fan and collector see a gas that condenses acid more readily at low load or on cold surfaces.
Does a petcoke AFBC boiler need a stainless steel ID fan?
Not usually throughout. A mild steel impeller with an insulated casing is normal if the gas stays above the acid dew point. Corrosion-resistant steel at the casing bottom, drains and inlet cone, where condensate collects, is a sensible addition on plants with frequent low load or start-stop operation.
How does fly ash recirculation affect the induced draft fan?
It does not add to the gas volume, but it raises the dust load in the back pass and collectors. The fan sees more dust only when the collectors are upset. Fan wear protection and collector condition monitoring therefore matter more on a petcoke AFBC with recirculation.
Information needed to quote an ID fan for petcoke AFBC boilers with limestone injection
- 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.
- Sulphur capture. Petcoke sulphur, Ca/S ratio and the measured SO₂ at the stack.
- Recirculation. Whether fly ash recirculation is used and the recirculation rate.
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.