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 ID fan sits
- Hot gas generator
- Rotary or flash dryer
- Product cyclone
- Bag filter
- Exhaust (ID) fan
- Stack
The fan is placed after the bag filter so that it handles a small residual of ash rather than the dryer carry-over. Upstream are the dryer, which may have very high dust loading in a flash design, the cyclone and the filter. Suction at the fan is typically 200 to 350 mmWC. It holds the dryer inlet hood at a slight negative pressure to stop hot gas and ash escaping at the feed and seals.
Gas at the ID fan
| Exhaust temperature, normal | 90 to 130°C after the bag filter |
| Maximum or upset | 200°C or more if feed is lost with the generator firing |
| Dust at the fan | Fine fly ash with quartz and mullite; low after a bag filter, very abrasive if bags fail |
| Moisture and dew point | Water evaporated from ash at 15 to 30% moisture gives a dew point of about 55 to 65°C |
| Gas composition | Combustion gas from the generator diluted with tempering air; sulphur depends on the generator fuel |
| Density at the design temperature | About 0.92 kg/m³ at 110°C, 77% 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 fly ash drying and classification plants
| Blade form | Backward curved with wear protection, behind a bag filter |
| Material of construction | Mild steel casing; wear-resistant plate or hard-faced liners on blades and scroll cut-off |
| Drive and arrangement | Direct coupled, arrangement 8, at moderate tip speed |
| Bearings and cooling | Pedestal bearings with a cooling disc for upset temperatures |
| Flow control | Variable speed drive on dryer inlet suction |
| Also specified | Insulated casing, casing drain and an inspection door |
What this ID fan has to cope with
Wear from bag leaks
A torn bag passes fly ash at many grams per cubic metre, and the abrasive spheres wear the blade leading edges quickly. A broken-bag detector before the fan and wear-resistant blade tips give time to find and replace the bag before the fan is damaged.
Condensation and ash caking
Pond ash brings a lot of water, and a fan casing below the dew point at start-up or shutdown wets the ash residue. It sets like weak cement because fly ash is pozzolanic. Insulating the fan, heating up the system before feed, and purging before stopping keep deposits dry.
Dryer draft and filter pressure drop
As the bag filter loads, the ID fan works against more resistance and suction at the dryer falls, so ash and hot gas leak at the inlet. A fan with enough pressure margin and speed control on dryer inlet suction maintains draft through the cleaning cycle.
Energy and control
Ash dryers run with feed moisture that changes from day to day, and the gas volume needed to dry it changes with it. A variable speed drive on the exhaust fan, linked to dryer suction and outlet temperature, saves power when feed is drier and avoids over-drying. Reducing air leakage at the dryer seals cuts the volume the fan must handle.
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 Forced Draft (FD) Fan for fly ash drying and classification plants.
Questions buyers ask
What type of ID fan is used for a fly ash dryer?
Usually a backward curved fan with wear protection, placed after the bag filter. Where the fan must handle cyclone outlet gas without a bag filter, a radial wheel in wear-resistant plate is safer because fly ash is highly abrasive. Tip speed is kept moderate in either case to slow erosion.
Why does fly ash stick inside the dryer exhaust fan?
Fly ash is pozzolanic. When the exhaust cools below its dew point, the ash residue becomes wet and hardens. Keeping the gas and casing above the dew point and purging the system warm before stopping prevents this. An insulated casing with a drain helps further.
Can the dryer exhaust fan also serve the classifier?
It is better kept separate. The classifier runs a closed or partly closed air circuit with its own fan and collector, and its air flow sets the cut size. Linking it to the dryer fan makes both harder to control, and a change in dryer draft would shift the product grading.
Information needed to quote an ID fan for fly ash drying and classification plants
- 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.
- Feed ash. Source of the ash, its moisture range and the target product moisture.
- Dryer type. Rotary or flash dryer, evaporation rate and generator outlet temperature.
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.