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 FD fan sits
- Inlet silencer
- FD (primary air) fan
- Air heater
- Air box
- Distribution grid
- Fluidised bed
The primary air fan sits at ground level and pushes ambient air through the air heater into the air box beneath the furnace. From there it passes through the grid nozzles and the dense bed. The secondary air fan, usually separate, supplies air at lower pressure through ports higher in the furnace. Loop seal blowers are separate again. The PA fan must overcome the air heater, ducts, grid and the full weight of the bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 180 to 250°C, downstream of the fan |
| Pressure at the fan outlet | Typically 1,000 to 1,500 mmWC |
| Share of total air | Usually 40 to 60% as primary air, rest as secondary |
| Dust | Clean air; lignite dust around the crusher house |
| 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 CFBC boilers
| Blade form | Backward curved, high pressure, with a steep and stable characteristic |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8, at 1,480 rpm or higher for the pressure needed |
| Bearings and cooling | Pedestal bearings, oil lubricated on larger fans |
| Flow control | Variable speed drive or inlet vanes on bed fluidisation and furnace temperature |
| Also specified | Inlet silencer; non-return damper where two fans run in parallel |
What this FD fan has to cope with
Loss of fluidisation
If PA flow falls below the minimum fluidising velocity at low load, parts of the bed settle and agglomerate. The fan and its control must hold a minimum air flow regardless of load. A stable fan curve and a minimum speed or vane setting prevent defluidisation.
Bed pressure changes with ash content
Lignite ash and spent limestone change the bed inventory and pressure drop. When the bed grows, the PA fan needs more pressure for the same air flow. The fan is selected with pressure margin for the maximum bed inventory, not the clean grid. Regular bed ash drain to hold the design bed pressure keeps the fan working where it was selected.
Noise
High pressure PA fans are loud, and their intakes are often near walkways. An inlet silencer and acoustic lagging on the casing are normally part of the specification. The fan speed and blade passing frequency are also chosen to avoid tonal noise that a silencer cannot easily absorb. Noise limits at the boundary of the plant are confirmed before selection.
Energy and control
The PA fan on a CFBC boiler draws high pressure all the time, so it is usually the largest single auxiliary load. Its flow cannot be cut much at low load because the bed must stay fluidised, so savings come from running the secondary air down instead and from controlling bed inventory so the PA fan does not work against an overfilled bed. A variable speed drive still saves energy over vane or damper control.
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 Induced Draft (ID) Fan for lignite fired CFBC boilers.
Questions buyers ask
Is the FD fan the same as the PA fan on a CFBC boiler?
On most CFBC boilers the primary air fan is the main forced draft fan, supplying fluidising air through the grid. A separate secondary air fan provides air higher in the furnace. Some plants call both FD fans. The PA fan works at much higher pressure than the SA fan.
Why does the primary air fan on a lignite CFBC need such high pressure?
The air must pass through the grid nozzles and support the weight of the dense bed of ash, limestone and lignite. Grid and bed together typically need 800 to 1,200 mmWC, and the air heater and ducts add more. The fan is selected with margin over this for a heavy bed.
What happens if the forced draft fan trips on a CFBC boiler?
The bed slumps immediately and the boiler trips on loss of fluidisation. Fuel feed stops through the interlocks. The fan's reliability, its bearings and its motor protection therefore matter as much as its efficiency. Two fans in parallel, each able to keep the bed fluidised at reduced load, are common on larger units for this reason.
Information needed to quote an FD fan for lignite fired CFBC 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.
- Grid and bed. Design grid pressure drop and bed pressure drop at full load and maximum inventory.
- Air split. Primary and secondary air flows at full and minimum load.
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.