How supercritical pulverised coal power units work
A supercritical unit, typically 660 to 800 MW, grinds coal in vertical roller mills and blows it to tangential or wall-fired burners with hot primary air. Indian coal at 30 to 45% ash and 3,000 to 4,000 kcal/kg means very large gas and ash volumes. Water passes once through the furnace walls above the critical pressure, so there is no drum. Flue gas leaves the economiser at about 330 to 380°C, crosses a rotary regenerative air preheater, then a multi-field ESP. Two ID fans follow the ESP and discharge to the wet limestone FGD absorber, often without a separate booster fan, and then the chimney. Two FD fans supply secondary air and two PA fans the mill air, both through the air preheater.
Where the FD fan sits
- Air intake and silencer
- FD fan
- Steam coil air preheater
- Rotary air preheater
- Secondary air duct
- Windbox and burners
Two FD fans sit at ground level near the air preheaters, each normally sized for 50 to 60% of full-load air so one fan can carry reduced load. They discharge through a steam coil air preheater, where fitted for start-up, into the secondary air sector of the rotary air preheater and then to the windbox. The fan works against the preheater, ducts, windbox dampers and burner registers, and its flow must also cover the air that leaks across the preheater seals to the gas side.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the air preheater | Secondary air about 300 to 340°C, downstream of the fan |
| Pressure at the fan outlet | Typically 300 to 500 mmWC at full load |
| Leakage allowance | Air preheater leakage, typically 6 to 10% of air flow, added to the duty |
| Dust | Clean air; coal and fly ash dust around the plant kept out by the intake screen |
| 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 supercritical pulverised coal power units
| Blade form | Variable pitch axial, the normal choice at this size, with backward curved centrifugal on some designs |
| Material of construction | Mild steel casing; aluminium alloy or steel blades on axial fans |
| Drive and arrangement | Direct coupled medium-voltage motor with hydraulic pitch actuator |
| Bearings and cooling | Oil-lubricated bearings with combined lube and hydraulic oil unit, vibration and temperature monitoring |
| Flow control | Blade pitch on total air demand, trimmed by excess oxygen |
| Also specified | Inlet silencer, outlet isolation damper, stall warning |
What this FD fan has to cope with
Unequal loading between the two fans
Two FD fans sharing a common secondary air duct must be matched in pitch and curve. If one leads, the other can be pushed toward stall at low load or when windbox resistance rises. Pitch balancing in the combustion control and a duty point well clear of the stall line at minimum load keep both fans stable.
Growing preheater seal leakage
Radial and axial seals of the rotary air preheater wear between overhauls, and leakage can rise well above design. The FD fan then runs at higher pitch for the same furnace air, and the ID fan carries the extra air. Measuring O₂ before and after the preheater tells the plant when seal repair, not a bigger fan, is the answer.
Intake noise
Large FD fans produce strong blade-pass noise at the intake, which carries across the plant and to the boundary. Inlet silencers tuned to the blade-pass frequency, acoustic lagging on the casing and an enclosure on the motor where needed meet plant noise limits without adding much pressure loss to the air path.
Energy and control
Under flexible operation the FD fans run much of the year between 50 and 75% of design air. Variable pitch control holds efficiency through this range; where a unit uses centrifugal FD fans with inlet guide vanes, a VFD gives a clear saving. Excess air is the other lever: each point of oxygen trimmed at the economiser outlet reduces FD and ID fan power together, and reduces stack loss as well.
ID fan and FD fan in supercritical pulverised coal power units
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, before the FGD absorber | At ground level, ahead of the air preheater |
| What it handles | Flue gas with residual fly ash | Clean ambient air |
| Temperature at the fan | 125 to 145°C | Ambient, 20 to 45°C |
| Main design concern | Head through FGD, stall and ash erosion | Wide-range efficiency and parallel stability |
| Control | Furnace pressure | Total air flow and excess oxygen |
See also the Induced Draft (ID) Fan for supercritical pulverised coal power units.
Questions buyers ask
What is the difference between the FD fan and PA fan in a supercritical boiler?
The FD fan supplies secondary air at moderate pressure to the windbox and burners. The PA fan supplies primary air at much higher pressure through the air preheater to the mills, where it dries the coal and carries it to the burners. Most of the combustion air comes from the FD fans, so they move the larger flow at lower pressure.
Can one FD fan run a supercritical unit?
With one FD fan out of service the unit normally runs back to about 50 to 60% load, depending on how the fans were sized. The remaining fan must reach that point without stall, which is checked on the fan curve against the single-fan system resistance, including preheater leakage, before the runback logic is set.
Why is FD fan current higher than design on my power unit?
Common causes are worn air preheater seals passing extra air to the gas side, high excess air, a choked steam coil air preheater, or windbox damper positions that add resistance. Measuring oxygen across the preheater and the fan pressure at site usually finds the cause before any change to the fan is considered.
Information needed to quote an FD fan for supercritical pulverised coal power units
- 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 preheater. Type of rotary air preheater and the leakage measured at the last overhaul.
- Single-fan operation. The load the unit must carry with one FD fan out of service.
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.