How lignite fired pulverised fuel boilers work
Pulverised lignite firing is used mainly in large utility and captive power units. Because the lignite arrives with 40 to 55% moisture, it is dried and ground together in beater wheel or fan mills, using hot flue gas drawn back from the top of the furnace, often mixed with hot air, as the drying medium. The mill throws the dried lignite and the moist drying gas straight into the burners. Combustion air comes from the FD fan through a regenerative or tubular air heater. The flue gas, carrying 20 to 30% water vapour and most of the fuel ash as fine fly ash, passes the superheater, reheater, economiser and air heater, then an ESP and the ID fans. Gas volume per MW is markedly higher than on a bituminous coal unit.
Where the FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Hot air duct
- Burner windbox and mills
- Furnace
The fans, usually two in parallel, draw ambient air through silencers and push it through the air heater into the hot air ducts. Most of the air goes to the burner windboxes; part goes to the mills where it is mixed with recirculated flue gas for drying. The fan outlet pressure is typically 250 to 400 mmWC, set by the air heater, ducts and windbox. On the largest units axial fans are common; centrifugal fans suit industrial and medium sizes.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 250 to 330°C, downstream of the fan |
| Pressure at the fan outlet | Typically 250 to 400 mmWC |
| Dust | Clean air; lignite dust near the bunkers if the intake is poorly placed |
| Leakage | Air lost across the air heater must be added to the delivered flow |
| 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 lignite fired pulverised fuel boilers
| Blade form | Backward curved aerofoil, double inlet, for high efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 6 or 7 |
| Bearings and cooling | Pedestal bearings, oil lubricated on large fans |
| Flow control | Variable speed drive or inlet vanes, on total air flow from the combustion control |
| Also specified | Inlet silencer, non-return and isolating dampers for parallel running |
What this FD fan has to cope with
Air heater leakage stealing delivered air
Part of the FD fan output leaks straight across the air heater into the gas, so the furnace receives less than the fan delivers. As seals wear, the fan speeds up to hold oxygen and the ID fan must also work harder. The FD duty includes the expected leakage at worn-seal condition, not only the clean design value.
Parallel fan imbalance
Two FD fans feeding a common air heater can share flow unevenly, especially when starting the second fan or at low load. A fan with a steadily rising characteristic, a non-return damper and a load-balancing control loop keeps both fans stable. The same applies when one fan is taken out at low load.
Air demand changes with mill drying
When lignite is wetter, the mills take more hot air and recirculated gas for drying, and the split between mill air and burner air changes. The FD fan must have margin to supply the wet lignite case without starving the burners of secondary air. The mill and burner air flows are measured separately.
Energy and control
Large lignite units run for long periods at reduced load, and FD fans with inlet vane control lose efficiency well below their design point. Variable speed drives give a steady saving, and reducing air heater leakage helps both the FD and ID fans at once. Keeping excess oxygen at the economiser near its target, typically 3 to 4% on a pulverised fuel unit, avoids moving air the lignite does not need.
ID fan and FD fan in lignite fired pulverised fuel boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, at the chimney | Ahead of the air heater |
| What it handles | Moist flue gas with fine fly ash | Clean ambient air |
| Temperature at the fan | 145 to 170°C | Ambient, 25 to 45°C |
| Main design concern | Very large gas volume and leakage | Air heater leakage and parallel stability |
| Control | Furnace draft | Total air flow and excess oxygen |
See also the Induced Draft (ID) Fan for lignite fired pulverised fuel boilers.
Questions buyers ask
What does the FD fan do on a lignite pulverised fuel boiler?
The FD fan supplies combustion air through the air heater to the burner windboxes, and on many designs hot air to the mills for drying. It handles clean ambient air at the fan, and the air is heated downstream. Its flow sets the excess air for the whole furnace.
Why does the forced draft fan current rise over time?
The most common cause is growing air heater leakage as seals wear. More air bypasses the furnace, so the fan delivers more to keep the oxygen target. Fouled air heater passages and clogged intake filters also add resistance. Checking the oxygen rise across the air heater shows whether leakage is the cause.
Are FD fans on lignite units centrifugal or axial?
Both are used. Large utility units frequently use axial fans with variable pitch blades. Industrial and medium units, and many older units, use double inlet backward curved centrifugal fans, which are robust, simple to maintain and well suited to retrofit of the impeller or the whole rotor.
Information needed to quote an FD fan for lignite fired pulverised fuel 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. Air to burners and air to mills at full load, for design and wet lignite.
- Parallel running. Number of fans, whether one fan must carry partial load alone, and the control signals used.
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.