How high ash Indian coal CFBC boilers work
CFBC boilers are the standard choice for high ash Indian coal, washery rejects and lignite in captive power and process steam above about 50 TPH. Fuel with 35 to 55% ash and 2,500 to 4,000 kcal/kg is crushed to below about 6 to 8 mm and fed into a fast bed at 840 to 900°C. Hot cyclones return the solids, and the high ash content keeps the loop well supplied with bed material, so the problem is draining coarse ash rather than adding sand. Fly ash leaving the cyclones is fine, abrasive and in large quantity, often 50 to 100 g/Nm³ at the ESP inlet. The gas passes the back-pass, air heater and a multi-field ESP to two ID fans.
Where the ID fan sits
- Riser furnace
- Hot cyclones
- Back-pass superheaters and economiser
- Tubular air heater
- Multi-field ESP
- ID fans
- Chimney
Two ID fans in parallel sit after the ESP. The ESP on high ash coal is large, often five or six fields, because even 99.8% collection leaves meaningful dust at the outlet when the inlet load is 50 to 100 g/Nm³. Suction at the fan inlet is typically 400 to 550 mmWC. The fans are usually large single or double inlet centrifugal machines.
Gas at the ID fan
| Gas temperature, normal | 135 to 160°C after the air heater |
| Maximum or upset | Up to 200°C with eroded air heater tubes or bypass |
| Dust at the fan | Fine quartz-rich fly ash; 50 to 150 mg/Nm³ after the ESP, much more during field trips |
| Moisture and dew point | Water dew point 45 to 55°C; higher with lignite |
| Corrosive species | Low sulphur coal; with lignite, limestone controls SO₂ |
| Density at the design temperature | About 0.84 kg/m³ at 150°C, 70% 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 high ash Indian coal CFBC boilers
| Blade form | Backward curved with replaceable wear liners on the leading edges |
| Material of construction | Mild steel casing with liners at the cut-off; high strength steel impeller with wear-resistant liners |
| Drive and arrangement | Direct coupled, arrangement 8 or double inlet arrangement 6 |
| Bearings and cooling | Oil-lubricated pedestal bearings with cooling and condition monitoring |
| Flow control | Variable speed drive on each fan, matched for parallel operation |
| Also specified | Thickness survey points on blades, liner change access, and a spare rotor policy |
What this ID fan has to cope with
Fine ash erosion at high speed
Large CFBC ID fans run at higher tip speeds than small boiler fans, and erosion rises steeply with velocity. Fine quartz ash that slips the ESP polishes the blade inlet and the hub zone until liners need changing. Liner design that allows replacement within an annual shutdown keeps the fans in service.
ESP capacity on rejects
When the plant burns more washery rejects than the boiler was designed for, the ash load and gas volume rise together. The ESP is then overloaded, its outlet dust increases, and the ID fans see both more volume and more dust at the same time. Fan selection with margin for the worst intended fuel, and an ESP check before the change, protect against this.
Parallel fan matching
Two ID fans on a common duct must share flow evenly. Uneven wear changes one fan's curve over time, and at low load the weaker fan can drift toward stall while the other carries most of the gas. Periodic performance checks on both fans, matched speed control and wear liners replaced on both rotors together keep the pair stable.
Energy and control
ID fans on high ash coal CFBC boilers carry large gas volumes because of the high ash and excess air, and they are among the largest auxiliary loads. Variable speed drives save significantly against inlet vanes at part load. Reducing air heater leakage, which grows as tubes erode with the ash, reduces both gas volume and dust at the fans.
ID fan and FD fan in high ash Indian coal CFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multi-field ESP, before the chimney | Before the air heater, feeding the windbox |
| What it handles | Large volume of flue gas with fine quartz ash | Clean ambient air |
| Temperature at the fan | 135 to 160°C | Ambient |
| Main design concern | Erosion and parallel stability | Very high pressure for a dense ash bed |
| Control | Furnace pressure | Fluidising air and bed pressure |
See also the Forced Draft (FD) Fan for high ash Indian coal CFBC boilers.
Questions buyers ask
Why is the ID fan on an Indian coal CFBC boiler so large?
High ash coal produces more flue gas per unit of useful heat, and CFBC boilers have high suction from the cyclones, back-pass, air heater and a large multi-field ESP. The combination of large volume and high pressure makes the ID fans on an Indian coal CFBC boiler among the largest and most power-hungry machines in the plant.
How often do CFBC ID fan liners need changing on high ash coal?
It depends on ESP performance, tip speed and the coal or rejects fired. With a healthy ESP, liners on the leading edges typically last several years. With frequent field trips the interval shortens sharply. Measuring blade and liner thickness at fixed points at every shutdown gives a reliable change schedule based on the plant's own wear rate.
Can we upgrade the ID fans when switching to more washery rejects?
Often yes, by fitting new impellers designed for the higher volume, by running at higher speed with a variable speed drive, or both. Before that, we check the existing casing, shaft, bearings, motor and foundation against the new duty, and confirm that the higher speed does not push erosion beyond what the liners can handle.
Information needed to quote an ID fan for high ash Indian coal CFBC 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 range. Coal, rejects and lignite blends with ash content and calorific value.
- ESP and fan history. ESP outlet dust, field trip history and present liner life.
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.