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 ID fan sits
- Furnace
- Economiser
- Air preheater
- ESP
- ID fan
- FGD absorber
- Chimney
The ID fans sit between the ESP outlet and the FGD inlet, usually two at 50 to 60% duty each, so the unit can run at reduced load on one fan. Upstream are the furnace, the convection pass, the economiser, the rotary air preheater and the ESP; downstream are the absorber and the wet stack. Where the booster fan has been dropped, the ID fan develops the full system head, typically 600 to 900 mmWC at full load against roughly 400 to 500 mmWC without the absorber. Placing it after the ESP keeps the dust load to tens of milligrams per cubic metre.
Gas at the ID fan
| Gas temperature, normal | 125 to 145°C after the air preheater |
| Maximum or upset | Up to 180 to 200°C with an air preheater trip or poor sootblowing |
| Dust at the fan | Fly ash, typically 30 to 100 mg/Nm³ after the ESP; grams per Nm³ if ESP fields trip |
| Moisture and dew point | About 8 to 10% water vapour; acid dew point typically 110 to 130°C with low-sulphur Indian coal |
| Flow margin | Air preheater leakage of about 6 to 10% and ESP in-leakage add to the gas volume at the fan |
| Density at the design temperature | About 0.87 kg/m³ at 135°C, 72% 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 supercritical pulverised coal power units
| Blade form | Variable pitch axial on most units of this size, or double inlet backward curved or radial centrifugal with VFD on others |
| Material of construction | Mild steel or low alloy structural steel casing; wear protection on centrifugal blade leading edges, hard coating on axial blades |
| Drive and arrangement | Direct coupled medium-voltage motor, usually 6.6 or 11 kV; hydraulic blade pitch on axial fans, VFD on centrifugal fans |
| Bearings and cooling | Oil-lubricated bearings with a lube oil unit; temperature and vibration signals to the DCS |
| Flow control | Blade pitch or VFD on furnace pressure, with load-sharing logic between the two fans |
| Also specified | Stall margin and stall warning on axial fans, isolation gates at inlet and outlet, fan-out-of-service runback |
What this ID fan has to cope with
Stall in parallel operation
Two axial ID fans in parallel can stall if one runs at much higher pitch than the other, or if system resistance rises from a fouled air preheater or absorber. The duty points at full load, in single-fan operation and in the worst fouling case must all sit well inside the stall line, with pitch balancing logic and a stall alarm from pressure fluctuation.
Head rise when FGD is added
Retrofitting an absorber to an existing unit adds roughly 150 to 250 mmWC of resistance with its ducts and dampers. Original ID fans rarely have that margin. The options are a booster fan, new impellers or blades on the existing casings with larger motors, or replacement fans; foundations, motor capacity and duct space at the plant decide which is practical.
Fly ash erosion after ESP upsets
When an ESP field trips or rapping is poor, the ash load at the fan can jump by two orders of magnitude. Indian fly ash, rich in silica and alumina, then erodes leading edges and blade roots quickly. Wear-protected leading edges, thickness checks at each overhaul and an alarm on ESP field status in the fan control room protect the fans.
Wet gas and acid at the fan outlet
At low load the gas at the ID fan moves toward the acid dew point, and an FGD upset can push saturated gas back toward the fan outlet. Outlet ducts sloped away from the fan, casing drains, tight outlet gates and a minimum gas temperature limit in the control keep corrosion away from the impeller and casing.
Energy and control
Under flexible operation a supercritical unit now spends long hours at 55 to 70% load, which makes ID fan control the main energy question. Variable pitch axial fans hold good efficiency across this range; centrifugal fans on inlet dampers do not, and a VFD retrofit usually pays back quickly on them. Cutting air preheater leakage and tramp air into the ESP also reduces the gas the fans must move, often a cheaper saving than any change to the fan.
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 Forced Draft (FD) Fan for supercritical pulverised coal power units.
Questions buyers ask
Why do supercritical units use axial ID fans instead of centrifugal?
At the very large flows and high heads of a 660 or 800 MW unit, variable pitch axial fans are more compact and hold efficiency across a wide load range by changing blade angle. Centrifugal ID fans with VFDs are also used and tolerate fly ash better. The choice turns on space, ash load at the fan, maintenance capability and the expected part-load profile.
Is a booster fan needed when FGD is added to a supercritical unit?
Not always. New units are commonly designed with ID fans that carry the full head through the absorber. On existing units the ID fans usually lack that margin, so either a booster fan is added or the ID fans are upgraded with new impellers or blades and larger motors. A system resistance study at several loads shows which route is practical.
What causes ID fan vibration on a pulverised coal power unit?
The usual causes are uneven ash deposits or erosion on the rotor, bearing wear, loose foundations and aerodynamic instability such as stall or poor inlet box flow. Trend data from the DCS helps separate mechanical from aerodynamic causes. In-situ balancing to ISO 21940-11 corrects unbalance, but stall needs a change in operating point or control.
Information needed to quote an ID fan for supercritical pulverised coal power units
- 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.
- FGD status. Whether an absorber is installed or planned, and whether a booster fan exists or will be removed.
- Load profile. Hours per year at full load, at 55 to 70% load and in single-fan operation.
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.