How fly ash drying and classification plants work
Fly ash taken from ash ponds, wet conditioned silos or mixed with water at the power station is too moist for classification or blending, so it is dried before an air classifier splits it into fine and coarse fractions. A rotary or flash dryer is used, fired by a hot gas generator burning coal, biomass, oil or gas, with inlet gas typically 400 to 650°C. Fly ash is fine, glassy alumino-silicate with spherical particles and hard unburnt carbon, and it is easily carried by the gas. The dryer exhaust passes through a cyclone and a bag filter before the ID fan. The FD fan supplies combustion air to the hot gas generator. The classifier has its own circulating fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Hot gas generator
- Mixing chamber
- Dryer inlet
The fan sits at the hot gas generator and supplies combustion air under the grate or to the burner. On many generators a second branch or a separate fan supplies dilution air into the mixing chamber to temper the gas. The fan works against the grate or burner, the generator and the mixing chamber, which typically take 100 to 250 mmWC, while the ID fan provides the suction through the dryer.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Fly ash dust in the yard; inlet screening needed |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Fuel | Coal, biomass, oil or gas in the generator, which sets the air quantity |
| 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 fly ash drying and classification plants
| Blade form | Backward curved, for ambient air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive tied to generator firing and dryer inlet temperature |
| Also specified | Inlet screen and filter against ash dust |
What this FD fan has to cope with
Ash dust drawn into the fan
Fly ash plants are dusty, and fine ash drawn into the combustion air fan coats the blades and wears the bearings. An inlet duct from a clean area and a simple filter avoid both, and the filter is far cheaper to change than a set of bearings.
Inlet temperature swings
If combustion air does not follow the firing rate, the generator outlet temperature hunts, and the dryer inlet temperature with it. The ash is then over-dried or left wet. A fan with stable pressure and a drive linked to the firing control keep the generator steady.
Over-sized generator air
Generators are often sized for the wettest pond ash. During drier periods the fan runs throttled, and excess air raises the gas volume at the ID fan. Variable speed and trimming excess air on generator outlet oxygen fix both, and reduce fuel at the same time.
Energy and control
Generator firing follows the evaporation demand of the dryer, which changes with ash moisture. A fixed-speed combustion air fan with a damper wastes power at reduced load. A variable speed drive linked to the firing control saves power, and keeping excess air low reduces the dilution needed and the ID fan load.
ID fan and FD fan in fly ash drying and classification plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and bag filter | At the hot gas generator |
| What it handles | Humid dryer exhaust with fly ash fines | Clean ambient air |
| Temperature at the fan | 90 to 130°C | Ambient |
| Main design concern | Ash wear and caking below dew point | Steady dryer inlet temperature |
| Control | Dryer inlet suction | Generator firing rate |
See also the Induced Draft (ID) Fan for fly ash drying and classification plants.
Questions buyers ask
Is the FD fan on a fly ash dryer the same as the combustion air fan?
Yes. In a fly ash drying plant the FD fan supplies the hot gas generator with combustion air, and sometimes dilution air. It handles ambient air and does not see the ash-laden dryer exhaust. It is therefore a simple mild steel fan, much smaller and cheaper than the dryer exhaust fan.
How is the dryer inlet temperature controlled on a fly ash dryer?
By the firing rate of the generator and the amount of dilution air mixed in before the dryer. The combustion air fan and dilution damper are controlled together on dryer inlet temperature, while the ID fan holds the draft at the dryer inlet hood. Dryer outlet temperature then trims the set point.
What pressure does a hot gas generator combustion air fan need?
Usually 100 to 250 mmWC, depending on the grate or burner and the generator design. A grate-fired coal or biomass generator needs more pressure than a gas burner, because the air passes through the fuel bed. The fan is selected on the actual generator data, with margin for a thicker bed.
Information needed to quote an FD fan for fly ash drying and classification plants
- 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.
- Generator. Fuel, firing capacity and the outlet temperature of the hot gas generator.
- Dilution. Whether dilution air is supplied by the same fan or a separate one.
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.