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 ID fan sits
- CFB furnace
- Hot cyclone
- Back pass
- Air heater
- ESP
- ID fan
- Chimney
The fan, or two in parallel on larger units, sits after the ESP and before the chimney. ESP inlet dust on a rejects CFBC can reach 60 to 100 g/Nm³, several times that of a CFBC on good coal, so even a small loss of ESP efficiency puts a lot of ash into the fan. Suction at the fan inlet is typically 280 to 420 mmWC, including the cyclone, back pass, air heater and a multi-field ESP.
Gas at the ID fan
| Gas temperature, normal | 140 to 160°C after the air heater |
| Maximum or upset | Up to 200°C with fouled air heater |
| Dust at the fan | Quartz and shale fines; 50 to 150 mg/Nm³ normally, several g/Nm³ with an ESP field out |
| Moisture and dew point | Rejects moisture of 8 to 15%; water dew point about 45 to 55°C |
| Corrosive species | Sulphur from pyrite in rejects, often 0.5 to 1.5%; acid dew point controlled by limestone addition |
| 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 coal washery rejects CFBC boilers
| Blade form | Backward curved plate with bolted wear liners; radial tipped where the ESP is undersized or frequently tripped |
| Material of construction | Mild steel casing with liners at the cut-off and inlet cone; impeller in wear-resistant plate |
| Drive and arrangement | Direct coupled, arrangement 8, at a moderate tip speed |
| Bearings and cooling | Pedestal bearings with cooling disc, oil lubricated on larger fans |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Vibration monitoring, inspection doors, spare impeller or liner set held at site |
What this ID fan has to cope with
Erosion when an ESP field trips
With such high inlet dust, a single ESP field out of service can raise the dust at the fan to several grams per cubic metre. Quartz ash then wears the blades rapidly. Wear-resistant plate, replaceable liners at the leading edge and cut-off, and an operating rule to reduce load when a field trips keep the impeller in service.
Rising vibration from uneven wear
Erosion removes metal unevenly from blade to blade, and vibration rises steadily over months. Online vibration monitoring, with trend alarms, shows when in-situ balancing or a liner change is needed. Without it, bearing damage often comes first and ends the run unplanned. Balancing is done to ISO 21940-11.
Air heater leakage and wear
The same abrasive ash wears tubular air heater tubes, which then leak. Air passing into the gas side adds to the ID fan volume and lowers gas temperature. Fan duty should include expected leakage, and tube condition should be checked at each shutdown so that the fan is not quietly pushed to its limit.
Energy and control
Washery rejects plants usually run as base load captive power stations, but fuel quality changes from day to day as washery output varies. When ash rises, more air and gas move per unit of steam, and the ID fan works harder. A variable speed drive on furnace draft follows these swings efficiently. Restoring a worn impeller profile with new liners also recovers efficiency lost as blades erode.
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 Forced Draft (FD) Fan for coal washery rejects CFBC boilers.
Questions buyers ask
How long does an ID fan impeller last on a washery rejects boiler?
It depends almost entirely on ESP performance. Behind a well maintained ESP, a wear-resistant impeller can run for several years. If ESP fields trip often, the same impeller may need liners within months. Monitoring ESP field status and fan vibration together gives early warning. Dust measurement at the fan inlet confirms it.
Why is fly ash so abrasive on a coal washery rejects CFBC?
Washery rejects are mostly shale, sandstone and clay with free quartz. When burnt, their ash keeps that hard, angular mineral matter. With 50 to 65% ash in the fuel, the mass of this material moving through the boiler is very large, so erosion is the dominant issue for the induced draft fan.
Can the existing ID fan casing be kept when replacing a worn impeller?
Usually yes. A new impeller in wear-resistant plate, designed to fit the existing casing, shaft and bearings, restores performance and is quicker than replacing the whole fan. The casing liners should be renewed at the same time and the rotor balanced to ISO 21940-11. Liners can then be replaced at site.
Information needed to quote an ID fan for coal washery rejects 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.
- ESP performance. ESP inlet and outlet dust and the frequency of field trips.
- Wear history. Life of the present impeller and photographs of worn areas.
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.