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 ID fan sits
- Rotary or shaft kiln
- Dust chamber
- Gas cooler or dilution air
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter, which needs the gas cooled to about 200 to 250°C from a kiln exit of 700°C and more. The cooling may be a waste heat recovery unit, an air-to-air cooler, dilution air or water spray, and the fan must carry the extra volume of any dilution air. The fan works against the kiln, dust chamber, cooler and filter, typically 300 to 500 mmWC. Where no filter is fitted, as on some older kilns with only a cyclone, the fan sees hot dusty gas and needs heat-resisting and wear-resistant construction.
Gas at the ID fan
| Gas temperature, normal | 200 to 250°C after cooling and the bag filter |
| Maximum or upset | 350 to 450°C if cooling or dilution fails |
| Dust at the fan | Low after a filter; fine, hard MgO dust after a cyclone only |
| Gas volume | High where dilution air is used for cooling |
| Corrosive species | SO₂ from oil, coal or petcoke |
| Density at the design temperature | About 0.71 kg/m³ at 230°C, 59% 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 magnesite dead burning kilns
| Blade form | Backward curved after a bag filter; radial-tipped with wear liners after a cyclone only |
| Material of construction | Boiler quality plate stress-checked at 450°C; heat-resisting steel impeller where upsets are frequent |
| Drive and arrangement | Direct coupled, arrangement 8, casing on sliding supports |
| Bearings and cooling | Shaft cooling disc and heat shield; water-cooled bearings where gas can exceed about 400°C |
| Flow control | Variable speed drive on kiln hood pressure |
| Also specified | Turning gear or slow running for hot stops; temperature trip on the filter inlet |
What this ID fan has to cope with
Gas cooling failure
The kiln gas at 700 to 1,000°C is far above what the filter and fan can take. If the cooler fails, a dilution damper sticks or spray water is lost, hot gas reaches the fan within seconds. The fan rotor is designed for the upset temperature, and a fast-acting emergency dilution damper protects both filter and fan.
Dilution air volume
Cooling by air dilution can double the gas volume at the fan. A fan sized on kiln gas alone cannot hold the draft. The duty includes the dilution air at its maximum, and indirect cooling is worth considering to reduce fan size.
Abrasive MgO dust
Sintered MgO dust is hard. Where the fan follows a cyclone only, the blade leading edges and casing wear quickly. Wear-resistant plate, lower tip speed or adding a bag filter before the fan extend life.
Energy and control
Dead burning kilns run continuously at very high fuel use per tonne, and the ID fan is a large drive. Speed control on hood pressure saves power at reduced output. Indirect gas cooling or heat recovery instead of dilution air reduces the gas volume the fan must move, and limiting air ingress at the kiln seals lowers both fan power and fuel use.
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 Forced Draft (FD) Fan for magnesite dead burning kilns.
Questions buyers ask
What temperature does a magnesite kiln ID fan handle?
After gas cooling and a bag filter, usually 200 to 250°C. The fan must also withstand 350 to 450°C during cooling failures, so its rotor and bearings are designed for the upset temperature rather than the normal one.
Why is dilution air used on a magnesite dead burning kiln?
The kiln exit gas is too hot for the bag filter. Dilution air is the simplest way to cool it, but it increases the gas volume and the ID fan size. Heat recovery or an air-to-air cooler keeps the fan smaller.
Can the fan run without a bag filter on a magnesite kiln?
It can, after a cyclone, but the fan then handles hot, abrasive MgO dust and wears quickly. Wear liners and a radial-tipped impeller are needed, and dust emissions will usually require a filter anyway.
Information needed to quote an ID fan for magnesite dead burning kilns
- 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.
- Gas cooling. Method of cooling before the filter, dilution air quantity and the temperature at the fan.
- Kiln type. Single or two stage process, kiln type and output.
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.