How lignite fired AFBC boilers work
In a lignite fired AFBC boiler the crushed lignite is fed over the bed by spreaders or screw feeders into a bubbling bed of ash and sand held at 800 to 880°C. Lignite with 35 to 50% moisture is too sticky for pneumatic underbed feed lines, so overbed feeding is the norm. Being highly volatile, much of the lignite burns in the freeboard above the bed, so overbed air is important. In-bed tubes and water walls take up the heat; the gas then passes the superheater, convection bank, economiser and air heater, then a multiclone and ESP or bag filter before the ID fan. Limestone may be added for sulphur capture on high sulphur lignite. Turndown is often achieved by slumping bed compartments.
Where the ID fan sits
- Bubbling bed and freeboard
- Superheater
- Convection bank
- Economiser
- Air heater
- Multiclone and ESP
- ID fan
- Chimney
The fan sits after the ESP or bag filter and ahead of the chimney. On mid-size captive units of 20 to 100 TPH a single fan is common; larger units use two. The suction at the fan inlet covers the freeboard, the back pass, the economiser, the air heater and both collectors, typically 200 to 320 mmWC on a clean boiler. Fouling of the convection bank can add significantly to this between soot blowing or shutdowns.
Gas at the ID fan
| Gas temperature, normal | 145 to 170°C after the air heater |
| Maximum or upset | Up to 210°C with fouled convection surfaces |
| Dust at the fan | Fine lignite ash with sodium and calcium compounds; 50 to 100 mg/Nm³ after an ESP or bag filter |
| Moisture and dew point | Water vapour 16 to 26% by volume; water dew point about 55 to 65°C |
| Corrosive species | Varies with lignite sulphur; acid dew point 100 to 130°C, lower when limestone is added |
| 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 AFBC boilers
| Blade form | Backward curved, with wear protection at the blade inlet |
| Material of construction | Mild steel casing and impeller; wear-resistant plate where only a multiclone precedes the fan |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings outside the gas stream with a cooling disc |
| Flow control | Variable speed drive on furnace draft with a wide speed range |
| Also specified | Inspection doors, casing drain and access for washing deposits |
What this ID fan has to cope with
Back pass fouling raising resistance
Lignite ash rich in sodium and calcium forms sticky deposits on the superheater and convection bank. As they build, draft loss across the back pass rises and the ID fan works harder to hold the furnace. A fan selected with pressure margin for the fouled condition, and regular soot blowing, avoid load restrictions.
Deposits on the impeller
The same alkali-rich fines that foul the boiler can stick to the fan blades, especially after a wet shutdown. Uneven deposits shed and cause vibration. Regular inspection and washing at shutdown, with access doors placed to reach the blades, keep the fan balanced. Smooth blade surfaces also help.
Wide turndown from slumped compartments
When compartments are slumped at low load, gas volume falls sharply but furnace draft must still be held. A damper-controlled fan becomes unstable near its minimum. A variable speed drive with a wide speed range follows the step changes in gas volume without hunting. An inlet damper remains for isolation.
Wet lignite overloading the fan
In the monsoon, lignite moisture can rise five to ten points, and gas volume rises with it. A fan sized on dry-season lignite becomes the limit on boiler output. The duty should be calculated on the wettest expected lignite. A stockpile moisture record over a monsoon gives the realistic figure.
Energy and control
Captive lignite AFBC boilers in textile, chemical and cement plants often run at partial load with one or more compartments slumped. Without speed control the ID fan wastes power through its damper for much of the year. A variable speed drive on furnace draft saves energy across that range, and keeping the convection bank clean with soot blowing lowers the pressure the fan must develop.
ID fan and FD fan in lignite fired AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Ahead of the air heater and air boxes |
| What it handles | Moist flue gas with alkali-rich ash | Clean ambient air |
| Temperature at the fan | 145 to 170°C | Ambient, 25 to 45°C |
| Main design concern | Gas volume and back pass fouling | High pressure and stability when compartments slump |
| Control | Furnace draft | Steam pressure and bed fluidisation |
See also the Forced Draft (FD) Fan for lignite fired AFBC boilers.
Questions buyers ask
Why does the ID fan run at full damper on our lignite AFBC boiler?
Usually because the back pass has fouled with sticky lignite ash or the lignite is wetter than design, or both. Fouling adds resistance and wetter lignite adds gas volume. Check draft loss across the convection bank and fuel moisture before concluding that the fan is undersized.
Is a lignite AFBC boiler ID fan different from a coal AFBC fan?
Yes. A lignite AFBC produces more flue gas per tonne of steam because of the fuel moisture, and its ash tends to foul rather than erode. The induced draft fan is therefore larger for the same steam output and needs margin for fouling rather than heavy wear protection.
Can a variable speed drive be added to an existing lignite boiler ID fan?
In most cases yes. The motor, cables and fan must be checked for operation at reduced speed, including bearing lubrication and any critical speed in the new range. The saving is usually significant on boilers that operate at partial load with slumped bed compartments. A harmonic check on the supply is also advisable.
Information needed to quote an ID fan for lignite fired AFBC 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, sodium and sulphur for the design and the wettest expected lignite.
- Turndown. Number of bed compartments and the minimum load with compartments slumped.
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.