How lignite fired CFBC boilers work
Indian lignite, mainly from Tamil Nadu, Gujarat and Rajasthan, is a young fuel with 35 to 55% moisture, a calorific value of about 2,200 to 3,200 kcal/kg as fired and widely varying ash and sulphur. In a CFBC boiler the crushed lignite is fed into a fast fluidised bed at 820 to 880°C, with limestone added to capture sulphur. Solids leave the furnace with the gas, are separated in a hot cyclone and returned through a loop seal. Primary air from the PA fan fluidises the bed through the grid; secondary air enters higher up. After the superheater, economiser and air heater, the gas passes an ESP and the ID fan. The large moisture content means much more flue gas per tonne of steam than a bituminous coal boiler.
Where the ID fan sits
- CFB furnace
- Hot cyclone
- Superheater and economiser
- Air heater
- ESP
- ID fan
- Chimney
The fan sits after the ESP and ahead of the chimney, usually as two fans in parallel on larger units. Upstream are the furnace, the cyclone, the back pass, a tubular air heater and the ESP, so the suction at the fan inlet is typically 250 to 400 mmWC. Locating it after the ESP limits it to fine ash and spent lime, but the gas carries 18 to 28% water vapour by volume.
Gas at the ID fan
| Gas temperature, normal | 140 to 160°C after the air heater |
| Maximum or upset | Up to 200°C with fouled air heater or at high load |
| Dust at the fan | Fine lignite ash with calcium sulphate and free lime; usually 50 to 100 mg/Nm³ after the ESP |
| Moisture and dew point | Water vapour 18 to 28% by volume, water dew point about 60 to 70°C |
| Corrosive species | SO₂ largely captured in the bed; acid dew point usually 100 to 125°C depending on lignite sulphur |
| Gas density | Lower than coal flue gas because of the high water vapour content |
| Density at the design temperature | About 0.84 kg/m³ at 150°C, 70% 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 CFBC boilers
| Blade form | Backward curved aerofoil or plate, with wear protection at the leading edges |
| Material of construction | Mild steel casing and impeller; wear-resistant plate at blade inlets |
| Drive and arrangement | Direct coupled, double inlet arrangement 6 or 8 on larger units |
| Bearings and cooling | Pedestal bearings with oil lubrication and cooling as required by size |
| Flow control | Variable speed drive or hydraulic coupling; inlet vanes on older units |
| Also specified | Inspection doors, casing drains and parallel operation with isolating dampers |
What this ID fan has to cope with
Gas volume underestimated for wet lignite
Lignite moisture varies with seam, season and stockpile exposure. A fan sized on a nominal 40% moisture is short of capacity when 50% moisture lignite arrives in the monsoon. The fan is sized on the wettest lignite expected at full load, with the volume calculated at the fan inlet temperature.
Lime and ash build-up on the blades
Free lime and calcium sulphate in the fly ash absorb moisture during cold starts and shutdowns and form hard deposits on the blades. Deposits break off unevenly and cause sudden vibration. Purging the fan with warm air before shutdown, and regular inspection, keep build-up under control.
Air heater leakage adding volume
Tubular air heaters on lignite boilers leak at the tube sheets as they age, and high-pressure combustion air flows into the gas side. The ID fan carries this air too. Measured oxygen before and after the air heater shows the leakage to include in the duty.
Parallel fan instability
Two ID fans in parallel can hunt or share load unevenly at part load if one runs near its stall region. Fans selected with a stable pressure curve and matched speed control avoid this. Each fan should also be able to carry at least 60% of full load on its own, so the boiler stays on line if one fan trips.
Energy and control
Lignite CFBC boilers in captive and utility service often run at 60 to 90% load for long periods. The ID fan is the largest auxiliary drive after the PA fan, and with lignite it is larger than on a coal boiler of the same rating. Variable speed control on furnace draft gives the largest saving. Keeping the air heater clean and leak-tight lowers both gas temperature and volume at the fan.
ID fan and FD fan in lignite fired CFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, before the chimney | Ahead of the air heater and air box |
| What it handles | Moist flue gas with fine ash and lime | Clean ambient air |
| Temperature at the fan | 140 to 160°C | Ambient, 25 to 45°C |
| Main design concern | Gas volume from wet lignite | High pressure and stable fluidisation |
| Control | Furnace draft | Bed fluidisation and furnace temperature |
See also the Forced Draft (FD) Fan for lignite fired CFBC boilers.
Questions buyers ask
Why is the ID fan bigger on a lignite boiler than on a coal boiler?
Lignite brings 35 to 55% moisture into the furnace, which leaves as water vapour in the flue gas. The flue gas volume per tonne of steam is therefore much higher, so the ID fan must move more gas and draws more power than on a coal boiler of the same output.
What causes deposits on a lignite CFBC boiler ID fan?
Fly ash from a lignite CFBC contains free lime and calcium sulphate from the limestone added for sulphur capture. When the fan cools during a shutdown, moisture lets this dust harden on the blades. Warm air purging and inspection at each outage prevent heavy deposits.
Can an old induced draft fan be upgraded for wetter lignite?
Often yes. A new impeller with a higher capacity, a change in speed, or a variable speed drive may give the extra volume within the existing casing. The motor and foundation must be checked. Send us the existing fan data and the new fuel analysis.
Information needed to quote an ID fan for lignite fired CFBC 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 for the design lignite and the worst expected lignite.
- Limestone. Limestone feed rate or Ca/S ratio used for sulphur capture.
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.