How magnesite dead burning kilns work
Magnesite, magnesium carbonate, is dead-burnt to dense sintered magnesia for refractories at about 1,700 to 2,000°C. Many plants use two stages: the ore is first calcined to caustic magnesia at 800 to 1,000°C, then ground, briquetted and dead-burnt in a high temperature shaft kiln or rotary kiln. Single-stage rotary kilns burn raw magnesite directly. Furnace oil, petcoke, coal or producer gas is fired with hot secondary air from a grate or rotary cooler. Because the burning temperature is very high, the kiln exit gas is hot, often 700 to 1,000°C on a long rotary kiln without a preheater. The gas carries fine MgO dust and is cooled by heat recovery, air dilution or water spray before a bag filter and the ID fan.
Where the FD fan sits
- Inlet filter
- Cooler FD fans
- Undergrate compartments
- Sintered magnesia bed
- Kiln hood
- Kiln
The fans are arranged along the grate cooler, each feeding one or more undergrate compartments. Air passes up through the bed of sintered magnesia, discharged from the kiln at about 1,400 to 1,600°C. The hot air from the first compartments goes to the kiln as secondary air; the air from the later compartments is vented. Pressures are highest under the hot end, typically 500 to 900 mmWC, and lower towards the cold end. A separate primary air fan supplies the burner.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the bed | Up to about 1,000°C in the first compartments, downstream of the fan |
| Dust | Magnesia dust around the cooler; filtered intakes advised |
| Pressure at the fan outlet | Typically 500 to 900 mmWC at the hot end, less at the cold end |
| Variation | Bed depth and grain size change with kiln output |
| 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 magnesite dead burning kilns
| Blade form | Backward curved, with a steep curve for changing bed resistance |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, one fan per compartment or group |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive or inlet vanes on each fan, on compartment air flow |
| Also specified | Inlet filters, flow measurement per fan, non-return dampers |
What this FD fan has to cope with
Red river on the grate
Fine material segregates and forms a dense, hot stream along the cooler that the air cannot penetrate. The hot-end fans need pressure margin and individual control so more air can be forced into the dense zone.
Grate overheating
If a hot-end fan trips, the grate plates under it overheat in minutes. A standby or interlocked fan arrangement and alarms on compartment pressure protect the grate.
Hood pressure balance
The cooler fans push air into the hood while the kiln ID fan and cooler vent fan draw it away. Without coordinated control the hood swings and blows dust or draws cold air. Air flow control on the cooler fans and pressure control on the ID fan keep the balance.
Energy and control
Cooler fans together use significant power, and they run with the kiln. Speed control per fan, set on air flow, avoids the losses of throttled dampers. The air from the hot compartments is recovered heat, so getting the split right between secondary air and vent air reduces fuel use, which is high on dead burning kilns.
ID fan and FD fan in magnesite dead burning kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After gas cooling and the bag filter | Under the grate cooler compartments |
| What it handles | Cooled kiln gas, often with dilution air | Ambient cooling air |
| Temperature at the fan | 200 to 250°C, upsets to 450°C | Ambient |
| Main design concern | Cooling failure and hot upsets | Pressure through hot, dense beds |
| Control | Kiln hood pressure | Compartment air flow |
See also the Induced Draft (ID) Fan for magnesite dead burning kilns.
Questions buyers ask
Which fans are the FD fans on a magnesite kiln?
The cooler fans supply most of the combustion air as hot secondary air, so they are the main forced draft fans on a magnesite dead burning kiln. A smaller primary air fan supplies the burner.
What pressure do grate cooler fans need?
Typically 500 to 900 mmWC under the hot end, where the bed is dense and hot, and less towards the cold end. Each fan is selected for its own compartment.
Why does each cooler compartment have its own fan?
The bed resistance and cooling need differ along the cooler. Separate fans or controlled groups let the air be set for each zone, giving better heat recovery and protecting the grate.
Information needed to quote an FD fan for magnesite dead burning 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.
- Cooler. Cooler type, number of compartments, grate area and product rate.
- Air per compartment. Air flow and pressure for each compartment or group.
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.