How imported coal AFBC boilers work
Many AFBC boilers in textile, paper and process plants now fire imported coal, mostly Indonesian sub-bituminous grades with 4,200 to 5,500 kcal/kg, 20 to 35% total moisture, low ash of 3 to 12% and low sulphur, sometimes blended with South African or Australian coal of higher calorific value. Crushed to below about 6 mm, the coal is fed overbed into a bubbling bed at 800 to 880°C. The low ash cannot sustain the bed on its own, so sand is added as make-up, and the high volatile content means much of the coal burns in the freeboard above the bed. Flue gas passes the boiler bank, economiser and air heater, then a multiclone and ESP or bag filter, before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Plenum compartments
- Distributor nozzles
- Sand bed
The FD fan pushes ambient air through the air heater into the compartmented plenum. Total pressure is typically 750 to 1,050 mmWC. With high volatile imported coal, a share of air is often diverted to overbed nozzles to burn volatiles in the freeboard and to control freeboard temperature, either through a branch from the FD fan or through a separate overfire air fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 130 to 200°C, downstream of the fan |
| Pressure at the fan outlet | Typically 750 to 1,050 mmWC |
| Dust | Coal dust in the boiler house |
| Air split | Bed air plus overbed air for volatile burnout |
| 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 imported coal AFBC boilers
| Blade form | Backward curved with a stable, rising curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive with compartment dampers |
| Also specified | Inlet screen, silencer and an overbed air branch where used |
What this FD fan has to cope with
Freeboard burning and bed temperature
High volatile imported coal releases much of its heat above the bed, so the bed can run cool while the freeboard overheats and fouls the boiler bank. Shifting air from bed to overbed nozzles corrects this. The fan and ducts must allow that split while keeping enough air through the bed to fluidise it.
Bed make-up and pressure
With little coal ash, the bed depends on regular sand addition. If make-up is irregular, bed height drifts and so does the pressure the FD fan sees. A fan with a stable curve and moderate margin keeps fluidisation steady while the operator corrects bed height.
Bed agglomeration from low-fusion ash
Some imported coals have ash with low fusion temperatures, around 1,150 to 1,250°C. Local hot spots in a poorly fluidised bed can sinter ash and sand. Even fluidisation, from steady plenum pressure in every compartment, is the main defence, and the fan must hold it through the load range.
Energy and control
With imported coal the FD fan often runs at lower bed pressure than with Indian coal, because the sand bed is shallower and lighter. A fan originally sized for Indian coal may therefore be oversized and heavily throttled. A variable speed drive, or a new impeller matched to the present duty, recovers this margin as saved power.
ID fan and FD fan in imported coal AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter, before the chimney | Before the air heater, feeding the plenum and overbed air |
| What it handles | Wet flue gas with light ash and sand fines | Clean ambient air |
| Temperature at the fan | 140 to 165°C | Ambient |
| Main design concern | Gas volume from coal moisture | Bed and overbed air split |
| Control | Furnace draft | Bed temperature, steam pressure and oxygen |
See also the Induced Draft (ID) Fan for imported coal AFBC boilers.
Questions buyers ask
Do I need a new FD fan when an AFBC boiler changes from Indian to imported coal?
Not usually. The air required per tonne of steam is similar, and bed pressure is often slightly lower. Overbed air may need a new branch or fan for volatile burnout. A check of the existing fan curve against the new duty normally confirms whether a speed drive or impeller change would save power.
Why is overbed air needed with imported coal in an AFBC boiler?
High volatile imported coal releases much of its combustible gas soon after it lands on the bed, and a good part of that gas rises into the freeboard before it burns. Without enough air there, CO and unburnt volatiles leave the freeboard and burn in the boiler bank. Overbed air completes combustion where the volatiles are released and helps hold bed temperature down.
What FD fan pressure does an imported coal AFBC boiler need?
Typically 750 to 1,050 mmWC at the fan outlet, covering the air heater, ducts, dampers, distributor nozzles and a sand bed that is usually lighter than a high ash coal bed. We confirm the figure from the boiler design or measured plenum pressure before selecting or rerating a fan.
Information needed to quote an FD fan for imported coal AFBC 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.
- Air split. Bed air and overbed air quantities, and whether overbed air comes from the FD fan.
- Bed management. Sand make-up rate and design bed pressure drop.
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.