How dolomite calcining kilns work
Dolomite, a calcium magnesium carbonate, is calcined to light-burnt dolomite at about 1,000 to 1,150°C for steel slag conditioning, ferroalloy and magnesium metal production, or dead-burnt to doloma at 1,700°C and above for refractories. Light-burnt dolomite is made in rotary kilns, often with a preheater, and in shaft kilns. The magnesium carbonate part decomposes at a lower temperature than the calcium part, so the dust carries a mix of half-calcined material and fine MgO. Coal, petcoke or gas is fired through a burner at the discharge end, with primary air from a fan and secondary air drawn through a rotary or contact cooler. The exhaust leaves a preheater at 250 to 350°C, or a long kiln at 600 to 850°C, and is cooled before a bag filter and the ID fan.
Where the FD fan sits
- Inlet filter
- Primary air FD fan
- Burner air channels
- Burner
- Kiln
The fan stands on the firing floor near the burner. It feeds the burner air channels, and on indirect firing systems it also supplies the air that conveys pulverised coal from the fuel bin. Primary air is typically 8 to 15% of the combustion air, at 800 to 2,000 mmWC. Secondary air comes hot from the cooler, drawn in by the ID fan, so the primary air fan has no part in cooling the product.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Plant air with dolomite and coal dust; inlet filter needed |
| Pressure at the fan outlet | Typically 800 to 2,000 mmWC |
| Flow | 8 to 15% of combustion air |
| Variation | Adjusted to shape the flame for the product |
| 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 dolomite calcining kilns
| Blade form | Backward curved or radial at high tip speed |
| Material of construction | High-tensile steel impeller |
| Drive and arrangement | Direct coupled, with standby on continuous kilns |
| Bearings and cooling | Standard bearings with vibration monitoring |
| Flow control | Variable speed drive; channel dampers at the burner |
| Also specified | Inlet silencer and emergency air for burner cooling |
What this FD fan has to cope with
Flame shape and calcination
Light-burnt dolomite overburns easily, losing reactivity. A long, lazy flame from low primary air pressure gives overburnt and underburnt material together. Enough pressure for a compact flame, and stable control, keep the product consistent.
Coal conveying air
Where the fan also conveys pulverised coal, too little air lets coal settle in the pipe and pulse into the flame. The fan is sized for the minimum conveying velocity at all firing rates.
Burner protection
The burner pipe inside the kiln depends on primary air for cooling. A standby or emergency fan avoids damage during a fan trip or power failure.
Energy and control
The primary air fan is small in power but runs continuously. Too much cold primary air displaces hot secondary air from the cooler and raises fuel use, so the energy gain from a correct primary air setting comes mainly in fuel. Speed control instead of throttling saves fan power as well.
ID fan and FD fan in dolomite calcining kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | On the firing floor, feeding the burner |
| What it handles | Kiln gas, CO₂ and cooler air | Ambient air |
| Temperature at the fan | 170 to 230°C | Ambient |
| Main design concern | Fine dust and filter loading | Flame shaping pressure |
| Control | Kiln hood pressure | Burner air pressure |
See also the Induced Draft (ID) Fan for dolomite calcining kilns.
Questions buyers ask
What is the FD fan on a dolomite kiln?
On a dolomite calcining kiln the forced draft fan is normally the primary air fan for the burner. Secondary air reaches the kiln hot from the cooler, drawn in by the ID fan or pushed by a separate cooler fan.
How much primary air does a coal-fired dolomite kiln use?
Typically 8 to 15% of the total combustion air, at 800 to 2,000 mmWC. Modern burners use less primary air at higher velocity, which saves fuel.
Why does the primary air fan affect dolomite quality?
It controls the flame shape. A poorly shaped flame overburns some material and underburns the rest, so reactivity varies. Stable air pressure keeps calcination even.
Information needed to quote an FD fan for dolomite calcining 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.
- Burner and fuel. Burner type, fuel and whether the fan conveys pulverised coal.
- Air requirement. Primary air flow and pressure needed at the burner.
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.