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 ID fan sits
- Furnace and burners
- Superheater and reheater
- Economiser
- Air heater
- ESP
- ID fan
- Chimney
The fans, usually two in parallel, stand after the ESP at the base of the chimney. Upstream are the furnace, the convection pass, the economiser, a rotary or tubular air heater and a multi-field ESP. Inlet suction is typically 300 to 450 mmWC. A regenerative air heater leaks several percent of combustion air into the gas side, which the ID fans must also carry. On the largest utility units axial fans are common; centrifugal fans serve industrial and medium units and retrofits.
Gas at the ID fan
| Gas temperature, normal | 145 to 170°C after the air heater |
| Maximum or upset | Up to 200°C with air heater fouling or a mill outage |
| Dust at the fan | Fine lignite fly ash, usually 50 to 100 mg/Nm³ after the ESP |
| Moisture and dew point | Water vapour 20 to 30% by volume; water dew point about 60 to 70°C |
| Corrosive species | Lignite sulphur from under 1% to 3% or more; acid dew point up to about 130°C on high sulphur lignite |
| Air heater leakage | Typically 6 to 12% on rotary air heaters, rising with seal wear |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 lignite fired pulverised fuel boilers
| Blade form | Backward curved aerofoil, double inlet, for high efficiency at large volume |
| Material of construction | Mild steel; wear-resistant plate at blade inlets and the inlet cone |
| Drive and arrangement | Direct coupled, arrangement 6 or 7, with the motor outside the gas path |
| Bearings and cooling | Sleeve or antifriction bearings with forced oil lubrication on large fans |
| Flow control | Variable speed drive or hydraulic coupling; inlet vane control on older units |
| Also specified | Isolating dampers for parallel running, vibration monitoring and casing drains |
What this ID fan has to cope with
Air heater seal wear raising fan load
As the radial and axial seals of a rotary air heater wear, more combustion air leaks into the gas side. Over a few years the ID fan current climbs and the fan approaches its limit at full load. Duty calculations should include worn-seal leakage, and the fan margin should cover it so that the unit does not become ID fan limited between overhauls.
Moisture swings from lignite quality
Lignite moisture changes with mine face and season. A few points more moisture raises gas volume in proportion. Units sized close to the design lignite often lose load in the wet season. The volume margin is set on the wettest lignite expected, not on the average.
Parallel operation and stall
Two large ID fans in parallel can drift apart in load when one fan's vanes or speed lag. If one moves into its stall region the pair hunts. Fans with a stable characteristic, matched controls and a common load-balancing signal avoid this. Starting the second fan against a running fan needs a defined procedure.
Fly ash erosion behind a weak ESP field
Lignite fly ash is less abrasive than high ash Indian coal ash, but when an ESP field trips the dust at the fan rises sharply. Wear-resistant plate at the blade inlet and inlet cone, where velocity is highest, gives protection until the field is restored.
Energy and control
Lignite pulverised fuel units carry grid or captive base load but are increasingly asked to run at 55 to 80% for long periods. The ID fans are among the largest auxiliary loads in the station, and inlet vane control wastes much of their power at reduced load. Variable speed drives, and attention to air heater leakage and ESP pressure drop, usually give the best return of any auxiliary energy measure on these units.
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 Forced Draft (FD) Fan for lignite fired pulverised fuel boilers.
Questions buyers ask
Why do lignite power plants need such large ID fans?
The moisture in lignite, often 40 to 55%, is evaporated in the mills and leaves with the flue gas. That water vapour adds a large volume to the combustion products, so the ID fans of a lignite unit move far more gas than those of a coal unit with the same electrical output.
Can a centrifugal ID fan replace an axial fan on a lignite boiler?
On medium units it can, if there is space for the larger casing and the foundation can be adapted. On the largest utility units axial fans are usually retained because of their size and efficiency at high volume. Each case needs a site survey and a duty check against the present operating data.
How does air heater leakage affect the induced draft fan?
Every cubic metre of combustion air that leaks across the air heater must be pulled out again by the ID fan, and it cools the gas as it mixes. The fan therefore moves more volume at a slightly lower temperature. Leakage of 10% or more is common on worn rotary heaters.
Information needed to quote an ID fan for lignite fired pulverised fuel boilers
- 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.
- Lignite range. Moisture, ash and sulphur of the design and worst lignite, with the mill drying gas arrangement.
- Air heater. Type of air heater and measured leakage from oxygen readings at its inlet and outlet.
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.