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 FD fan sits
- Inlet silencer
- FD (primary air) fans
- Air heater
- Windbox
- Nozzle grid
- Dense ash bed
Primary air fans push ambient air through the air heater to the windbox and grid, at 1,300 to 1,800 mmWC on typical units. Secondary air fans deliver air at lower pressure to the furnace walls. Because the coal supplies abundant bed material, bed inventory and bottom ash drain rate control the bed pressure, and therefore the pressure the primary air fans must provide.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 200 to 260°C, downstream of the fan |
| Pressure at the fan outlet | Primary air 1,300 to 1,800 mmWC; secondary air 600 to 900 mmWC |
| Dust | Clean air; silenced and screened inlets |
| Flow range | Primary air held above minimum fluidising flow |
| 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 high ash Indian coal CFBC boilers
| Blade form | Backward curved, high pressure |
| Material of construction | Mild steel casing; high strength steel impeller |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Oil-lubricated pedestal bearings with monitoring |
| Flow control | Variable speed drive or inlet vanes with minimum flow protection |
| Also specified | Inlet silencer, acoustic lagging, non-return damper for parallel fans |
What this FD fan has to cope with
Bed inventory and pressure
High ash coal fills the loop quickly with bed material. If bottom ash drains or ash coolers fall behind, bed pressure rises, and the primary air fans move toward the top of their curve. A fan with pressure margin, and bed pressure trends displayed to the operator, allow drains to be corrected before fluidisation suffers or the fans trip on overload.
Coarse ash and stones
Stones and coarse shale in Indian coal and rejects do not burn and accumulate near the grid, where they resist fluidisation and can damage nozzles. Bottom ash drains remove them, but the primary air fans need pressure margin to keep the bed moving while this happens, rather than running at their limit with no reserve.
Air heater leakage
Primary air at 1,300 mmWC and more leaks into the gas side through tubular air heater joints and tubes eroded by the heavy ash. Leakage increases both primary air and ID fan loads, and robs the bed of air. Allowing for realistic leakage in fan sizing and repairing or plugging tubes at shutdowns keeps the fans within their range.
Energy and control
Primary air fans on high ash coal CFBC boilers run at very high pressure, so every mmWC saved reduces power. Keeping bed inventory at design, maintaining grid nozzles and reducing air heater leakage all cut fan power. Variable speed drives save most on the secondary air fans, which carry most of the turndown.
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 Induced Draft (ID) Fan for high ash Indian coal CFBC boilers.
Questions buyers ask
Why does the primary air fan pressure rise on a high ash coal CFBC boiler?
Usually because bed inventory is rising faster than the bottom ash drains remove it, or because coarse stones and shale are building up near the grid. Adjusting drain rates and checking the ash coolers usually restores the pressure. A slow, steady rise over months more often points to grid nozzle wear or blockage.
How is the CFBC primary air fan different on Indian coal than on biomass?
With high ash Indian coal, the bed is rich in coal ash, denser, and constantly topped up by the fuel, so pressure is often higher and inventory control matters most. With biomass, the bed is mostly added sand, and agglomeration from alkali in the ash is the main concern. The fan duty differs accordingly.
What pressure do CFBC primary air fans need on high ash coal?
Typically 1,300 to 1,800 mmWC at the fan outlet, covering the air heater, ducts, windbox, grid nozzles and a dense bed of coal ash. The exact figure is set by the boiler design and the bed inventory the plant runs at. We confirm it from boiler data and measured windbox pressure before selecting or rerating a fan.
Information needed to quote an FD fan for high ash Indian coal 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.
- Bed inventory. Design bed pressure and bottom ash drain arrangement.
- Air system. Primary, secondary and loop seal air flows and pressures.
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.