How coal washery rejects CFBC boilers work
Coal washeries separate clean coal from shale and stone, leaving rejects with typically 50 to 65% ash and a calorific value of about 1,500 to 2,500 kcal/kg. Captive power plants at or near washeries burn these rejects in CFBC boilers, which tolerate low grade fuel because the hot circulating solids sustain combustion. Rejects are crushed to below about 6 to 8 mm and fed to the lower furnace. For every tonne of steam a large mass of ash must be handled: much of it is drained as bed ash and the rest leaves as fly ash through the cyclone, back pass and air heater to an ESP. The ash is mostly quartz, clay and shale, which makes it highly abrasive.
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 to the air box beneath the grid. Secondary air is supplied by a separate fan at lower pressure. Fan outlet pressure is typically 1,100 to 1,600 mmWC, at the higher end for CFBC, because a high ash bed is heavy and its inventory is harder to hold steady. Bed ash coolers or drains remove the excess ash continuously.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 180 to 240°C, downstream of the fan |
| Pressure at the fan outlet | Typically 1,100 to 1,600 mmWC |
| Share of total air | Usually 45 to 60% as primary air |
| Dust | Clean air; fine reject dust near 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 coal washery rejects CFBC boilers
| Blade form | Backward curved, high pressure, with a steep stable curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8, at high speed |
| Bearings and cooling | Pedestal bearings, oil lubricated on larger fans |
| Flow control | Variable speed drive or inlet vanes with protected minimum fluidising flow |
| Also specified | Inlet silencer and filter; non-return dampers for parallel fans |
What this FD fan has to cope with
Bed pressure swings with fuel ash
When the washery sends rejects with more ash, the bed grows faster than the ash drain removes it, and bed pressure rises. The PA fan needs enough pressure margin to hold air flow until the drain catches up. Without margin, fluidisation weakens and the bed can sinter or defluidise in the lower furnace.
Grid nozzle wear and back-sifting
Abrasive bed material wears grid nozzles, and when PA flow falls, bed ash sifts back into the air box. This changes the grid pressure drop and can block air paths. Holding a minimum PA flow at all times and inspecting nozzles at each outage protect both the fan and the grid.
Crusher house dust at the intake
Reject crushing generates large quantities of fine dust. A PA fan intake near the crusher house draws this in, coating the impeller and fouling the air heater. Intake location and an inlet filter, with regular checks of filter pressure drop, prevent it. Silencer baffles also stay clean.
Energy and control
The PA fan on a rejects CFBC is often the biggest single auxiliary drive in the plant. Because its flow is set by fluidisation, not just by load, the best saving comes from controlling bed inventory so that the fan does not work against excess bed pressure. A variable speed drive in place of vane control adds a further saving at partial load.
ID fan and FD fan in coal washery rejects CFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP | Ahead of the air heater and grid |
| What it handles | Flue gas with heavy quartz-rich fly ash | Clean ambient air |
| Temperature at the fan | 140 to 160°C | Ambient, 25 to 45°C |
| Main design concern | Erosion when the ESP is weak | Pressure margin for a heavy, variable bed |
| Control | Furnace draft | Bed fluidisation |
See also the Induced Draft (ID) Fan for coal washery rejects CFBC boilers.
Questions buyers ask
Why does the primary air fan on a washery rejects CFBC need more pressure than on a coal CFBC?
With 50 to 65% ash in the fuel, the bed is heavier and its inventory changes more as fuel quality varies. The fan must hold fluidising air against the highest bed pressure expected between ash drains, so its design pressure is usually at the high end of the CFBC range.
Is the FD fan the same as the PA fan on a CFBC?
On most CFBC boilers, yes. The primary air fan acts as the forced draft fan for the bed. A secondary air fan supplies the upper furnace and loop seal blowers fluidise the cyclone return leg. Each has a separate duty and pressure, and they are selected together.
What happens to fluidisation when washery rejects quality changes?
Higher ash raises bed inventory and pressure, lower ash reduces it. If the PA fan cannot hold air flow when the bed grows, fluidisation weakens. Good bed ash drain control, combined with a fan that has pressure margin, keeps the bed fluidised through the usual swings in rejects quality.
Information needed to quote an FD fan for coal washery rejects 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.
- Fuel range. Ash and calorific value of the best and worst rejects expected.
- Bed. Design bed pressure drop and the bed ash drain arrangement.
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.