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 FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Air box
- Nozzles
- Bubbling bed
The FD fan sits at ground level and pushes ambient air through the air heater into the compartment air boxes. Separately, a high pressure transport air fan carries crushed petcoke through underbed feed pipes, and another blower conveys limestone and recirculated ash. Together these streams make up the air entering the bed. The FD fan pressure is typically 800 to 1,100 mmWC, set by the nozzles and a bed built largely from limestone products.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 150 to 200°C, downstream of the fan |
| Pressure at the fan outlet | Typically 800 to 1,100 mmWC |
| Dust | Clean air; limestone and petcoke dust near the feed system |
| Other air streams | Underbed transport air and recirculation air from separate fans |
| 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 AFBC boilers with limestone injection
| Blade form | Backward curved, high pressure, stable down to low compartment flows |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8, at high speed |
| Bearings and cooling | Pedestal bearings; no cooling needed |
| Flow control | Variable speed drive or inlet vanes on steam pressure, with compartment dampers |
| Also specified | Inlet silencer and filter; acoustic lagging |
What this FD fan has to cope with
Bed inventory growing with limestone
With Ca/S ratios of 2.5 or more, spent limestone builds the bed quickly. Bed pressure rises and the FD fan must push harder for the same air. Without regular bed drain, the fan reaches its limit and fluidisation weakens. Fan margin plus a bed pressure-based drain routine keep the duty within the fan's design.
Air balance with transport air
Underbed feed and recirculation air enter the bed through separate pipes, and their flow changes with feed rate. If the FD control does not account for them, excess air drifts. Measuring all air streams and trimming the FD fan on flue gas oxygen keeps combustion stable.
Limestone dust at the intake
Limestone handling near the boiler produces fine dust that the FD fan draws in if its intake is badly placed. The dust coats the impeller and the air heater tubes. Taking air from a clean area and fitting an inlet filter prevents it. The filter is cleaned on a schedule.
Energy and control
The FD fan on a petcoke AFBC carries a high pressure duty all the time. Keeping bed inventory at its design level, rather than letting spent limestone accumulate, is the simplest way to cut its power. A variable speed drive in place of vane control saves energy at partial load, and trimming excess oxygen avoids heating air the slow burning petcoke does not need.
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 Induced Draft (ID) Fan for petcoke AFBC boilers with limestone injection.
Questions buyers ask
How many fans does a petcoke AFBC boiler need?
Usually at least four: the FD fan for fluidising air, a transport air fan for underbed petcoke feed, a blower for limestone and recirculated ash, and the ID fan. Larger plants may add a separate overbed air fan. Each has a different pressure and duty.
Why does the forced draft fan pressure rise over time on a petcoke AFBC?
Spent limestone and calcium sulphate accumulate in the bed and increase its weight. If bed drain does not keep pace, the FD fan must develop more pressure for the same air flow. Monitoring bed pressure and draining to a set value keeps fan pressure steady.
Does the FD fan handle hot air on a petcoke AFBC?
No. The FD fan takes ambient air and pushes it through the air heater, which heats it before it reaches the bed. The fan is selected at ambient density. Only if hot air is recirculated to its inlet would it need selection at a higher temperature.
Information needed to quote an FD fan for petcoke AFBC boilers with limestone injection
- 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.
- Air streams. Fluidising, transport and recirculation air flows at full and minimum load.
- Bed management. Design bed pressure and the bed drain arrangement.
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.