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 FD fan sits
- Inlet filter
- FD fan
- Air flow damper
- Burner windbox
- Swirl register
- Combustion chamber
The fan is placed near the burner front and delivers ambient air to the windbox or swirl register. The pulverised coal is carried separately by a primary air fan or the mill vent fan, and dilution air enters the mixing chamber further downstream, often drawn in by the system suction. The FD fan therefore supplies only combustion air, against the resistance of the damper, windbox and swirl vanes, typically 250 to 450 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the burner | Ambient on most units; some take air from a jacket around the combustion chamber at 80 to 150°C, downstream of the fan |
| Dust | Clean air if the inlet is away from the coal mill and dryer |
| Burner pressure | Swirl registers need steady pressure; low pressure lengthens the flame |
| Pressure at the fan outlet | Typically 250 to 450 mmWC |
| 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 pulverised coal burner hot air generators
| Blade form | Backward curved, giving a stable rising pressure curve for burner control |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 depending on size |
| Bearings and cooling | Standard grease-lubricated bearings; no cooling needed |
| Flow control | Variable speed drive or inlet vane control, ratioed to coal feed rate |
| Also specified | Inlet filter and silencer, and a low-pressure switch in the flame safeguard |
What this FD fan has to cope with
Flame instability at low firing
At low coal rates a fan running on a near-closed damper gives poor air distribution in the register and the flame lifts off or pulses. Speed control keeps the damper open and the register velocity steady down to the minimum firing rate.
Coal dust drawn into the inlet
Where the fan inlet is near the mill or the coal hopper, fine coal is drawn in and blown back to the burner unmeasured, upsetting the air-fuel ratio. An inlet duct from a clean area, with a filter, solves this.
Wrong split between combustion and dilution air
If the combustion air is short, unburnt coal reaches the dryer and the bag filter, creating a fire risk. The FD fan is sized on the full stoichiometric need plus excess air at maximum firing, not on what the dilution air can make up.
Energy and control
Hot air generators on dryers are run to a hot gas temperature set point, so the coal rate and combustion air move with the dryer load and the feed moisture. Ratio control of the FD fan to coal feed, through a variable speed drive, saves power and keeps excess air low. Lower excess air raises flame temperature and reduces the volume the exhaust fan must handle.
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 Induced Draft (ID) Fan for pulverised coal burner hot air generators.
Questions buyers ask
What is the difference between primary air and combustion air on a pulverised coal burner?
Primary air carries the ground coal from the mill to the burner and is only part of what is needed for combustion. Secondary or combustion air, from the FD fan, supplies the rest through the swirl register and controls flame shape. The ratio between them is set by the burner design.
Does the FD fan supply dilution air on a hot air generator?
Usually not. Dilution air is often drawn into the mixing chamber by the suction of the exhaust fan, or supplied by a separate fan. The FD fan supplies the combustion air at the higher pressure the burner needs. Using the FD fan for dilution would waste power at burner pressure.
How is the combustion air fan controlled on a coal fired hot air generator?
It is ratioed to the coal feeder speed, with a trim from oxygen in the hot gas where an analyser is fitted. A variable speed drive is the usual method; inlet vanes are an alternative on older units. The fan must also hold a minimum flow for flame stability at low fire.
Information needed to quote an FD fan for pulverised coal burner hot air generators
- 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.
- Burner data. Burner make or type, maximum and minimum coal rate, and required air pressure at the register.
- Air sources. Whether primary, combustion and dilution air come from separate fans, and their quantities.
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.