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 ID fan sits
- Fluidised bed furnace
- Cyclone (CFBC)
- Back pass
- Air heater
- Multiclone
- ESP
- ID fan
- Chimney
The fan sits after the ESP. Suction at the inlet is typically 220 to 400 mmWC depending on whether the boiler is an AFBC or CFBC. With slurry in the feed, the gas carries more water vapour and the fly ash includes very fine particles from the slurry, which the ESP must capture. Some fine ash still reaches the fan, and in a moist gas it tends to stick rather than erode.
Gas at the ID fan
| Gas temperature, normal | 145 to 170°C after the air heater |
| Maximum or upset | Up to 210°C with fouled back pass |
| Dust at the fan | Very fine slurry ash and abrasive middlings ash; 50 to 150 mg/Nm³ after the ESP |
| Moisture and dew point | Water vapour 12 to 22% by volume with slurry; water dew point about 50 to 62°C |
| Corrosive species | Sulphur from pyrite in middlings; acid dew point about 110 to 135°C |
| Density at the design temperature | About 0.83 kg/m³ at 155°C, 69% 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 coal slurry and middlings fired boilers
| Blade form | Backward curved, smooth blades, with wear liners at the leading edge |
| Material of construction | Mild steel casing and impeller; wear-resistant plate at blade inlets |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing insulation, drain, inspection doors and washing access |
What this ID fan has to cope with
Gas volume rising with slurry share
Slurry with 30 to 40% water adds a large volume of vapour per tonne of fuel. When the washery sends more slurry, the ID fan must move more gas for the same steam. The fan is sized on the highest slurry share and moisture expected, with margin for air ingress around paste feed points.
Fine ash sticking to the blades
Slurry ash is extremely fine, and in moist gas near start-up or at low load it cakes on blade surfaces and inside the casing. Uneven deposits cause vibration. Keeping gas temperature at the fan well above the dew point, smooth blade finish and cleaning at outages control this.
Abrasion from middlings ash
Middlings ash contains shale and quartz and is harder than ordinary coal ash. Where the ESP loses efficiency, it wears blade leading edges and the casing near the cut-off. Liners at these points, and a dust alarm after the ESP, give protection. Replacement liners are kept at site.
Energy and control
Captive boilers at washeries run on whatever by-product mix the washery produces, so gas volume changes from shift to shift. A variable speed ID fan on furnace draft follows these changes efficiently. Reducing slurry moisture before firing, where filter presses or drying allow it, cuts gas volume at the fan and improves boiler efficiency at the same time.
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 Forced Draft (FD) Fan for coal slurry and middlings fired boilers.
Questions buyers ask
How does coal slurry firing affect the ID fan?
Slurry carries 25 to 40% water, which becomes water vapour in the flue gas. Gas volume and dew point rise, so the ID fan must move more gas and the gas at the fan must be kept warmer to avoid sticky deposits. The fan should be sized on the highest slurry share.
Why does fine ash stick inside our induced draft fan?
Very fine slurry ash in moist flue gas cakes on surfaces when gas temperature falls towards the dew point, for example at start-up, low load or shutdown. Keeping the fan inlet temperature well above dew point and purging with warm air before stopping reduce deposits.
Can middlings be fired in the same boiler as slurry?
Yes, and many washery plants blend them. Middlings provide a steadier fuel with higher calorific value, while slurry adds moisture and fines. The fans must be selected for the range of blends, from mostly middlings to the highest slurry share the plant intends to burn.
Information needed to quote an ID fan for coal slurry and middlings fired 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.
- Fuel blend. Middlings and slurry analysis, with the slurry moisture and share range.
- Feed system. Whether slurry is fed as paste, blended with middlings or dried first.
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.