How coal washery rejects AFBC boilers work
Some captive plants burn coal washery rejects, with 45 to 60% ash and 1,800 to 2,800 kcal/kg, in bubbling bed AFBC boilers, usually blended with middlings or better coal to keep the bed hot. The crushed rejects are fed over or under the bed into a bubbling bed at 800 to 900°C, where in-bed tubes absorb much of the heat. A large share of the ash stays in the bed and is drained continuously, while fines are elutriated and leave with the gas through the superheater, convection bank, economiser and air heater. A multiclone and an ESP clean the gas before the ID fan. Unburnt carbon in the fly ash is higher than on a CFBC, and fly ash is sometimes returned to the bed.
Where the ID fan sits
- Bubbling bed and freeboard
- Superheater and bank
- Economiser
- Air heater
- Multiclone
- ESP
- ID fan
- Chimney
The fan sits after the ESP, which is preceded by a multiclone that takes the coarser carry-over and often feeds a fly ash recirculation line. Suction at the fan inlet is typically 220 to 350 mmWC on units of 20 to 100 TPH. Because a share of the ash goes to the bed drain, the ESP inlet loading is usually lower than on a CFBC burning the same rejects, but it is still high by industrial boiler standards.
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 | Quartz-rich ash and char; 50 to 150 mg/Nm³ after the ESP, much more if a field is out |
| Moisture and dew point | Water dew point about 45 to 55°C |
| Corrosive species | Moderate sulphur from pyrite; acid dew point about 110 to 130°C |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 AFBC boilers
| Blade form | Radial tipped or backward curved with liners, depending on ESP reliability |
| Material of construction | Impeller in wear-resistant plate; mild steel casing with liners at cut-off and inlet |
| Drive and arrangement | Direct coupled, arrangement 8, at moderate tip speed |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Vibration monitoring, inspection doors and a stock of replacement liners |
What this ID fan has to cope with
Abrasion from char and quartz together
Unburnt carbon particles from a bubbling bed are porous and light, but they carry embedded mineral matter, and the ash itself is quartz-rich. Together they wear the blade leading edge and the casing near the cut-off. Wear-resistant plate, liners at those points and moderate tip speed give a reasonable interval between repairs.
Recirculation upsets increasing dust
Where fly ash from the multiclone is recirculated to the bed, a blocked or overloaded recirculation line pushes more dust on to the ESP. If the ESP is already heavily loaded, some of it reaches the fan. A dust monitor after the ESP and an alarm on recirculation flow give early warning.
Fouling and erosion of the back pass
Rejects ash both erodes and deposits on the convection bank and economiser, depending on local velocity and temperature. Resistance rises and the ID fan works harder to hold draft. Pressure margin in the fan selection and regular soot blowing prevent load limits. Draft readings across each section show where.
Energy and control
Washery rejects AFBC boilers usually serve captive loads with variable demand and frequently change compartments in and out of service. A variable speed ID fan follows these step changes efficiently and avoids long running at small damper openings. Keeping the fan's wear liners in good condition also matters for energy, because an eroded impeller loses efficiency gradually without any obvious failure.
ID fan and FD fan in coal washery rejects AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multiclone and ESP | Ahead of the air heater and air boxes |
| What it handles | Flue gas with abrasive ash and char | Clean ambient air |
| Temperature at the fan | 145 to 170°C | Ambient, 25 to 45°C |
| Main design concern | Erosion behind the ESP | Pressure margin for a fast-growing bed |
| Control | Furnace draft | Steam pressure and bed fluidisation |
See also the Forced Draft (FD) Fan for coal washery rejects AFBC boilers.
Questions buyers ask
Is an AFBC or CFBC better for washery rejects?
Both are used. A CFBC usually achieves better burnout and handles lower grade rejects, but sends more ash to the ESP. An AFBC keeps more ash in the bed and has simpler cyclone-free construction, but may need blending with better fuel. For the ID fan, the AFBC usually means somewhat lower dust load.
Why is a radial tipped ID fan used on some washery rejects boilers?
A radial tipped wheel is less efficient but more tolerant of abrasive dust and easier to repair at site. Where the ESP is older or often loses fields, it gives a more predictable life. Where the ESP is reliable, a backward curved induced draft fan with liners saves energy.
What causes unburnt carbon in the fly ash of a rejects AFBC?
Fine particles of rejects are carried out of the bubbling bed before they burn completely. Low calorific value fuel and limited freeboard residence time add to this. Fly ash recirculation and overbed air help burnout, and lower carbon in the fly ash also means less light char reaching the fan.
Information needed to quote an ID fan for coal washery rejects AFBC 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.
- Collector. Multiclone and ESP arrangement and the measured dust after the ESP.
- Fuel blend. Share of rejects, middlings and coal in the blend, with their ash content.
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.