How coal slurry and middlings fired boilers work
Coal washeries produce two low grade by-products that are burnt in captive boilers: middlings, with about 35 to 45% ash and 3,000 to 4,000 kcal/kg, and slurry or filter cake, a fine coal recovered from washery water with 25 to 40% moisture. Middlings are crushed and fired like coal in AFBC or CFBC boilers. Slurry is either blended into the middlings feed or pumped as a thick paste directly into a fluidised bed, where it dries and burns. Slurry brings a lot of water and very fine ash into the furnace, which raises gas volume and dew point. The gas passes the back pass, economiser and air heater, then a multiclone and ESP before the ID fan.
Where the FD fan sits
- Inlet silencer
- FD fan
- Air heater
- Air box
- Nozzles or grid
- Fluidised bed
The fan sits at ground level and pushes ambient air through the air heater into the air box. On a CFBC it is the primary air fan with a separate secondary air fan; on an AFBC it feeds the compartment air boxes. Fan outlet pressure is typically 800 to 1,100 mmWC on an AFBC and 1,000 to 1,500 mmWC on a CFBC. Slurry paste, where used, is pumped into the bed and needs no transport air.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 150 to 230°C, downstream of the fan |
| Pressure at the fan outlet | 800 to 1,100 mmWC on AFBC; 1,000 to 1,500 mmWC on CFBC |
| Dust | Clean air; dried slurry dust near the feed area |
| 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 slurry and middlings fired boilers
| Blade form | Backward curved, high pressure, with a stable curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings; oil lubricated on larger fans |
| Flow control | Variable speed drive with protected minimum fluidising flow |
| Also specified | Inlet silencer and filter; acoustic lagging |
What this FD fan has to cope with
Agglomerates at slurry feed points
Wet slurry lumps fall into the bed, cool it locally and can bind with bed material before they dry. Where fluidisation is weak, these grow into agglomerates that block nozzles. The FD fan must hold full fluidising air near the feed points at all loads, and feed points must be spread across the bed.
Bed pressure rising with middlings ash
Middlings bring 35 to 45% ash, so the bed builds steadily. If bed drain falls behind, bed pressure rises and the fan needs more pressure. Pressure margin in the selection, and bed drain controlled on bed pressure, keep the fan within its curve. The drain is set to a target bed pressure.
Air demand changing with slurry share
Slurry needs less air per tonne than middlings, but more of the air goes to evaporate water and the bed temperature drops. Operators often over-air to keep the bed hot. Oxygen trim and correct split between fluidising and secondary air keep combustion efficient across blends.
Energy and control
The FD fan on a slurry fired fluidised bed carries a high pressure duty continuously. Keeping bed inventory at design pressure is the most effective way to limit its power. A variable speed drive in place of vane control saves further at partial load. Drier slurry also reduces the air and fan power spent simply evaporating water in the bed.
ID fan and FD fan in coal slurry and middlings fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP | Ahead of the air heater and air box |
| What it handles | Moist flue gas with fine and abrasive ash | Clean ambient air |
| Temperature at the fan | 145 to 170°C | Ambient, 25 to 45°C |
| Main design concern | Gas volume from slurry and fine deposits | Fluidisation at wet feed points |
| Control | Furnace draft | Bed fluidisation and steam pressure |
See also the Induced Draft (ID) Fan for coal slurry and middlings fired boilers.
Questions buyers ask
Can coal slurry be fired in an AFBC boiler?
Yes, usually blended with middlings or coal, or pumped as a paste into the bed. The bubbling bed's large heat capacity dries and ignites the slurry. Good fluidisation from the forced draft fan, and feed points spread across the bed, help avoid agglomeration. Paste pumps need no transport air.
Why do agglomerates form when we feed more slurry?
Wet slurry cools the bed where it lands and can stick to bed particles before drying. If fluidising air is weak at that point, lumps grow. Increasing slurry share without improving distribution or fluidisation makes this worse. Check air flow, nozzle condition and feed point spacing.
What pressure does the FD fan need on a slurry fired boiler?
On an AFBC typically 800 to 1,100 mmWC, and on a CFBC primary air duty 1,000 to 1,500 mmWC. The exact figure depends on nozzle design, bed height and air heater, with margin for a heavier bed when middlings ash is high. Grid nozzle wear is also allowed for.
Information needed to quote an FD fan for coal slurry and middlings fired 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.
- Boiler type. AFBC or CFBC, with the design bed pressure and grid pressure drop.
- Slurry feed. Number of feed points and the slurry share range.
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.