How sponge iron rotary kilns work
A coal-based sponge iron plant reduces iron ore lumps or pellets in a long inclined rotary kiln using non-coking coal, at about 1,000 to 1,050°C. Coal is fed with the ore and also injected from the discharge end. Air for the process enters through shell air tubes along the kiln length and a central air pipe at the discharge end. Kiln off-gas leaves the feed end at about 850 to 1,000°C, carrying char fines, ore fines and unburnt CO. It passes a dust settling chamber and an after-burning chamber where the CO is burnt, then either a waste heat boiler or a gas conditioning tower, an ESP and the ID fan to the stack. The reduced product is cooled in a rotary cooler with external water spray.
Where the FD fan sits
- Ambient air
- Shell air (FD) fans on kiln
- Air tubes through shell
- Kiln freeboard
- Reduction zone
The shell air fans are mounted on the kiln shell at intervals and rotate with it, each feeding an air tube that injects air into the freeboard above the bed. A separate central air fan at the discharge end supplies air through a central pipe. The pressure is modest, typically 100 to 300 mmWC, but each fan must be robust enough to rotate continuously with the kiln and sit close to a hot shell for months at a time.
Air at the FD fan
| Air temperature at the fan | Ambient, 30 to 50°C, but close to a hot kiln shell |
| Shell temperature nearby | Shell surface often 200 to 350°C |
| Dust | Ambient plant dust; inlet screens recommended |
| Pressure at the fan outlet | Typically 100 to 300 mmWC |
| Mounting | Rotating with the kiln, with power through slip rings |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 sponge iron rotary kilns
| Blade form | Backward curved or radial, compact and robust |
| Material of construction | Mild steel, with a well-supported casing for the rotating mounting |
| Drive and arrangement | Direct coupled motor on a common base fixed to the shell |
| Bearings and cooling | Bearings selected for the continuous rotating mount; heat shields between shell and motor |
| Flow control | Damper or variable speed per fan, adjusted to set the kiln temperature profile |
| Also specified | Inlet screens, a precisely balanced rotor for low vibration on the shell, and a weatherproof motor |
What this FD fan has to cope with
Wrong temperature profile
Too much air at one tube burns coal and product locally, raising the bed temperature and starting accretions on the lining. Too little leaves volatiles unburnt and the zone cold. Each fan must be adjustable over a useful range so the operator can shape the profile along the kiln as ore and coal quality change.
Heat from the kiln shell
Motors and bearings sit close to a hot shell and in its radiant heat, with no cool air around them. Heat shields, motors with a high temperature insulation class, adequate stand-off from the shell and grease suited to the temperature prevent premature motor and bearing failures.
Vibration on a rotating base
An unbalanced fan rotating with the kiln transmits load into its mounting and the shell, and fasteners loosen over time. Precision balancing to ISO 21940-11, a stiff fixing frame and periodic checks of the mounting bolts are essential for fans that cannot be reached easily while the kiln runs.
Energy and control
Shell air fans are small individually, but a kiln may carry many of them, running for the whole campaign. Matching each fan to its tube flow, rather than running all at full output and trimming by damper, saves power and gives finer control of the temperature profile. The right air distribution also reduces coal consumption and accretion, which matter far more than the fan power.
ID fan and FD fan in sponge iron rotary kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, before the chimney | On the kiln shell and at the discharge end |
| What it handles | Kiln off-gas with fine char and ore dust | Ambient air |
| Temperature at the fan | 150 to 200°C, higher in upsets | Ambient, near a hot shell |
| Main design concern | Wear and temperature excursions | Air distribution and rotating mounting |
| Control | Kiln inlet draft | Air per tube for the temperature profile |
See also the Induced Draft (ID) Fan for sponge iron rotary kilns.
Questions buyers ask
What are shell air fans on a sponge iron kiln?
Shell air fans are small FD fans mounted on the sponge iron kiln shell. They rotate with the kiln and blow ambient air through tubes into the freeboard, where it burns volatiles and CO above the bed and maintains the temperature profile needed for reduction. Each one is adjusted separately by the kiln operator.
How many shell air fans does a rotary kiln need?
It depends on kiln length and design. A typical coal-based kiln has several, spaced along the preheating and reduction zones. The number and capacity are set by the kiln designer from the air needed at each point along the kiln, plus the central air supplied from the discharge end.
Can a standard fan be mounted on a rotating kiln?
Only if it is designed for that duty. The fan must be precisely balanced, its mounting stiff, and the motor and bearings protected from shell heat. Power comes through slip rings, so the fan, motor and cabling must also tolerate continuous rotation and the dust around the kiln.
Information needed to quote an FD fan for sponge iron rotary kilns
- 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.
- Kiln air scheme. Number of shell air tubes, air flow per tube and the central air requirement.
- Mounting. Shell temperature at the fan location and the space available on the shell.
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.