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 FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Compartment air boxes
- Nozzles
- Bubbling bed
The fan sits at ground level and pushes ambient air through the air heater into a distribution duct feeding each compartment air box. From there the air passes the nozzles and the bed. Overbed or secondary air for burning lignite volatiles in the freeboard is often supplied by a branch from the same fan or by a separate fan. Because lignite is fed over the bed, there is no pneumatic transport air fan for underbed feeding.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 150 to 200°C, downstream of the fan |
| Pressure at the fan outlet | Typically 800 to 1,100 mmWC |
| Dust | Clean air; lignite dust near the crusher and feeders |
| Flow range | Steps down as compartments are slumped, at nearly constant pressure |
| 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 AFBC boilers
| Blade form | Backward curved, high pressure, with a stable curve at low flow |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8, typically at 1,480 rpm or higher |
| Bearings and cooling | Pedestal bearings; no cooling needed |
| Flow control | Variable speed drive or inlet vanes on steam pressure, with compartment dampers |
| Also specified | Inlet silencer and acoustic lagging; branch for overbed air where used |
What this FD fan has to cope with
Instability with slumped compartments
When one or two compartments are slumped, fan flow drops but the active compartments still need full fluidising pressure. A fan with a flat or drooping curve can surge in this condition. Selecting a fan whose stable range extends to the lowest compartment flow avoids pulsation and noise.
Overbed air short at high volatile load
Lignite releases most of its volatiles above the bed. If the overbed air is a branch of the FD fan, the split changes when bed pressure changes. A separate overbed air fan, or a branch with its own control damper and flow measurement, keeps freeboard combustion complete.
Bed defluidisation on low air
Turning the fan down too far at low load lets parts of the active bed settle, which causes clinkers. A minimum air flow per active compartment, set in the control system and protected by a fan minimum speed, prevents defluidisation. Operators should know the figure for each compartment.
Energy and control
The FD fan on an AFBC boiler works at high pressure, so it draws a large share of auxiliary power. Slumping compartments lowers flow, but pressure stays high, so the saving is smaller than on a grate boiler. A variable speed drive still saves energy over vane control, and keeping bed height at the design value avoids raising fan pressure needlessly.
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 Induced Draft (ID) Fan for lignite fired AFBC boilers.
Questions buyers ask
Why does an AFBC FD fan need such high pressure?
The fluidising air must pass through the nozzles and support the bed of ash and sand, which typically needs 600 to 900 mmWC, plus the air heater and ducts. The forced draft fan pressure is therefore several times that of a grate boiler of similar output.
Why is lignite fed over the bed instead of under it?
High moisture lignite is sticky and tends to choke pneumatic underbed feed lines. Overbed feeding by spreaders or screws avoids this. It means no separate transport air fan, but more lignite volatiles burn in the freeboard, so overbed air is needed in addition to the fluidising air.
What happens to the FD fan when a bed compartment is slumped?
Its flow drops but it must still supply full pressure to the active compartments. The fan moves back along its curve towards lower flow. If that point is in the unstable region, it surges. The fan should be selected so that the lowest compartment combination is still on the stable part of its curve.
Information needed to quote an FD fan for lignite fired AFBC 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.
- Compartments. Number of compartments, air flow per compartment and the minimum number in service.
- Overbed air. Overbed or secondary air flow and whether it is supplied by the FD fan or separately.
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.