How pulverised coal burner hot air generators work
A pulverised coal burner hot air generator burns finely ground coal in a refractory-lined combustion chamber and mixes the flame gas with dilution air to give hot gas at the temperature a dryer needs, usually 300 to 800°C. Coal is ground in a small vertical or ball mill and carried to the burner by primary air; swirl secondary air from a combustion air fan completes combustion. The hot gas goes to a rotary dryer, flash dryer, spray dryer or the grinding mill of a cement, slag, minerals or ceramics plant. After drying, the gas carries the product moisture and fine dust through a cyclone and bag filter, and the main exhaust fan, which acts as the ID fan, draws the whole system and discharges to stack.
Where the ID fan sits
- PC burner and chamber
- Dilution air mixing
- Dryer or mill
- Cyclone
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter, the last item before the stack, so the whole circuit from the combustion chamber through the dryer runs under suction. Its suction is the sum of the hot gas duct, the dryer, the cyclone and the bag filter, typically 350 to 600 mmWC. Placing it after the filter keeps the product dust away from the impeller, but means the fan sees gas cooled by evaporation, often only 20 to 40°C above its dew point.
Gas at the ID fan
| Gas temperature, normal | 90 to 130°C after the bag filter |
| Maximum or upset | Up to 200°C when feed to the dryer stops and the hot gas is not cut back |
| Dust at the fan | Fine product dust and coal ash, typically 20 to 50 mg/Nm³ after the filter; much higher if bags are damaged |
| Moisture and dew point | Water vapour of 15 to 30% by volume from the product; dew point often 55 to 70°C |
| Corrosive species | SO₂ from coal sulphur; acid condensate forms at the cold casing during start-up and shutdown |
| Density at the design temperature | About 0.93 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 pulverised coal burner hot air generators
| Blade form | Backward curved with plain, easily cleaned blades, or radial where build-up is severe |
| Material of construction | Mild steel with a corrosion allowance; SS 304 or SS 316 for inlet box and impeller where the gas runs close to dew point |
| Drive and arrangement | Direct coupled, arrangement 8, impeller speed chosen for the 350 to 600 mmWC system suction |
| Bearings and cooling | Pedestal bearings with a cooling disc; plain grease lubrication at these temperatures |
| Flow control | Variable speed drive controlling suction at the hot air generator outlet |
| Also specified | Insulated casing, casing drain, cleaning doors and a bypass or fresh air bleed for start-up |
What this ID fan has to cope with
Wet build-up on the impeller
When the gas approaches its dew point, fine product dust that passes the filter sticks to wet blades and builds unevenly, causing vibration. Insulating the fan and ducts, warming the system on hot gas before feed starts, and a blade shape without dead pockets all reduce it.
Acid condensation at start-up
During each cold start the casing is below the acid dew point of the coal-fired gas, and dilute sulphuric acid forms on it. Stainless steel for the wetted parts that run coldest, or a warm-up sequence on bypass, protects the fan.
Coal fires and explosions
Pulverised coal that escapes unburnt can ignite in the dryer or filter, and a flame-out followed by re-ignition can pressurise the circuit. The fan must not be the weak point: casing and ducts are rated for the explosion venting scheme of the plant, and the fan keeps running to purge before re-ignition.
Bag failure and sudden dust load
A burst bag sends product dust and coal ash straight to the fan, eroding the blades and upsetting balance within days, and the plant may keep running unaware until vibration trips the fan. A broken bag detector ahead of the fan, wear liners at the blade tips and a vibration alarm limit the damage and give time to change bags.
Energy and control
The dryer load varies with feed moisture and throughput, so the exhaust fan seldom needs full flow. A variable speed drive on outlet suction lets the fan follow the dryer instead of throttling at a damper. Reducing air ingress at dryer seals also matters: every leak is extra gas the fan must move and cooler gas closer to dew point.
ID fan and FD fan in pulverised coal burner hot air generators
| ID fan | FD fan | |
|---|---|---|
| Position | After the dryer and bag filter | At the burner front |
| What it handles | Moist, dusty dryer exhaust gas | Clean ambient combustion air |
| Temperature at the fan | 90 to 130°C, near dew point | Ambient, 25 to 45°C |
| Main design concern | Condensation and wet build-up | Flame stability across turndown |
| Control | Suction at the generator outlet | Ratio to coal feed |
See also the Forced Draft (FD) Fan for pulverised coal burner hot air generators.
Questions buyers ask
Where is the ID fan on a pulverised coal hot air generator?
It is normally at the end of the circuit, after the dryer, cyclone and bag filter. From there it pulls the hot gas from the generator through the dryer, so the whole system runs under suction and fine dust does not leak out. Placing it after the filter also keeps most of the dust off the impeller.
Why does the exhaust fan after my dryer vibrate after a few weeks?
Most often because moist product dust has built up on the blades unevenly. The gas is close to its dew point and fine dust that passes the filter sticks. Cleaning restores balance; insulating the system and keeping the gas warmer stop recurrence. Vibration monitoring catches the problem early.
What material should the dryer exhaust fan be on a coal fired hot air generator?
Mild steel is usual when the gas stays well above its dew point. Where the gas is close to dew point, or the coal is high in sulphur, stainless steel for the impeller and inlet box gives much longer life. With mild steel, the casing must be well insulated.
Information needed to quote an ID fan for pulverised coal burner hot air generators
- 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.
- Product and moisture. Product dried, feed and final moisture, and throughput, so the water vapour in the gas can be calculated.
- Filter and dust. Type of final collector and the outlet dust level expected at the fan.
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.