How coal rotary dryers work
Coal rotary dryers reduce the moisture of washed coal, lignite or wet imported coal from about 15 to 35% to the level needed by the kiln, boiler or briquetting plant. Hot gas from a coal-fired furnace, often burning a share of the product, or from a waste heat source, enters a co-current drum at a limited temperature, typically 350 to 600°C, to avoid igniting the coal. Many plants recirculate part of the exhaust to lower the oxygen content of the drying gas, holding it typically below about 10 to 12% for lignite and high-volatile coals. The exhaust leaves at about 70 to 95°C, humid and laden with fine coal dust, and passes cyclones and a bag filter with explosion protection, or a wet scrubber, before the ID fan and the stack.
Where the ID fan sits
- Coal-fired furnace
- Rotary drum
- Discharge hood
- Cyclones
- Bag filter with explosion vents
- ID fan
- Recirculation or stack
The fan follows the bag filter or scrubber, and its discharge often splits into a recirculation duct back to the furnace mixing chamber and a vent to the stack. Its suction covers the furnace, drum, cyclones and filter, typically 250 to 400 mmWC. The fan sits after the collector so that it handles little coal dust; any dust that does reach it is a fire and explosion hazard.
Gas at the ID fan
| Gas temperature, normal | 70 to 95°C at the fan inlet |
| Maximum or upset | Up to 150°C or more if feed is lost or a fire starts in the drum |
| Dust at the fan | Fine coal dust, explosive; low after the bag filter |
| Moisture and dew point | High moisture from coal, dew point typically 55 to 70°C |
| Oxygen and gases | Oxygen held low by recirculation; SO₂ from furnace firing; CO if coal begins to oxidise |
| Density at the design temperature | About 0.99 kg/m³ at 85°C, 83% 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 rotary dryers
| Blade form | Backward curved plate, open shape that does not trap dust |
| Material of construction | Mild steel with corrosion allowance; non-sparking inlet ring or rub strip |
| Drive and arrangement | Coupled drive with a variable speed drive; motor rated for the area classification |
| Bearings and cooling | Bearings outside the gas, temperature monitored |
| Flow control | Variable speed drive on drum suction, with recirculation damper on oxygen |
| Also specified | Earthing, casing drain, insulated casing, vibration and bearing temperature monitoring |
What this ID fan has to cope with
Dust explosion risk
Coal dust that passes a leaking bag or deposits in the duct can ignite from friction or hot spots. The fan is built to avoid rubbing contact, with non-sparking rub strips and earthing, and the dryer has explosion vents, oxygen monitoring and spark detection upstream.
Condensation and acid corrosion
The humid exhaust carries SO₂ from the furnace, and at shutdown or low load it condenses as weak acid in the casing and on the impeller backplate. An insulated casing, a drain at the lowest point and a corrosion allowance on plate thickness extend life.
Coal deposits and self-heating
Damp coal fines stick in corners of the casing, the inlet box and the duct. Deposits that dry out can self-heat and smoulder, becoming an ignition source. Smooth internals, inspection doors and a cleaning schedule at every stop prevent build-up.
Energy and control
Feed moisture changes with the weather and the coal source, sometimes by ten percentage points within a week. A variable speed drive on the ID fan follows the gas volume, and recirculating exhaust saves furnace fuel while keeping oxygen low. Tight drum seals reduce air in-leakage, which adds both oxygen and volume.
ID fan and FD fan in coal rotary dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After bag filter, feeding recirculation and stack | At the coal-fired furnace |
| What it handles | Humid gas with explosive coal fines | Ambient combustion air |
| Temperature at the fan | 70 to 95°C | Ambient |
| Main design concern | Explosion safety and condensation | Low excess air |
| Control | Drum suction and oxygen | Firing rate and oxygen |
See also the Forced Draft (FD) Fan for coal rotary dryers.
Questions buyers ask
How is explosion risk managed on a coal dryer ID fan?
By limiting oxygen in the drying gas with exhaust recirculation, removing dust before the fan with a bag filter fitted with explosion vents, and building the fan to avoid sparks: earthing, non-sparking rub strips and no rubbing contact. Motor and instruments match the hazardous area classification.
Why recirculate exhaust in a coal rotary dryer?
Recirculated exhaust is moist and low in oxygen. Mixing it with the furnace gas lowers the oxygen content of the drying gas, so coal dust in the drum and collector is less likely to ignite, and it recovers heat that would otherwise go up the stack.
What temperature does a coal dryer exhaust fan run at?
Usually about 70 to 95°C, close to the dew point of the humid gas, which is often 55 to 70°C. The casing is insulated and drained, and the plant avoids running the outlet any cooler than it must, so condensation does not form in the fan.
Information needed to quote an ID fan for coal rotary dryers
- 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.
- Coal. Coal type, volatile matter, feed and product moisture, and dust explosion data if available.
- Recirculation. Recirculated gas share, target oxygen level and the hazardous area classification.
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.