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 ID fan sits
- Rotary kiln
- Preheater or kiln inlet
- Gas cooler or dilution air
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter. Before the filter the gas is cooled by dilution air, water spray or an air-to-air cooler to suit the bags. The fan works against the cooler and kiln, the preheater, the gas cooler and the filter, typically 350 to 550 mmWC, and its speed sets the hood draft that draws secondary air from the cooler. On kilns with a rotary cooler, all the cooling air is drawn in by this fan.
Gas at the ID fan
| Gas temperature, normal | 170 to 230°C after the bag filter |
| Maximum or upset | Up to 280°C if gas cooling fails |
| Dust at the fan | Low after the filter; very fine dolomite and MgO dust if bags fail |
| Gas composition | High CO₂ from calcination of both carbonates |
| Moisture and dew point | Low to moderate; water spray cooling raises it |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 dolomite calcining kilns
| Blade form | Backward curved or aerofoil, on clean gas after the filter |
| Material of construction | Mild steel or boiler quality plate, stress-checked at the upset temperature |
| Drive and arrangement | Direct coupled, variable speed drive |
| Bearings and cooling | Shaft cooling disc above about 200°C; pedestal bearings |
| Flow control | Variable speed drive on kiln hood pressure |
| Also specified | Casing drain, inspection door and dust-tight shaft seal |
What this ID fan has to cope with
Fine dust and filter loading
Dolomite breaks into fine, light dust as it calcines, and a much larger share leaves with the gas than on a lime kiln. The bag filter pressure drop rises faster, and the fan must have pressure margin for a loaded filter, not just a clean one.
Water spray cooling and sticky dust
Where the gas is cooled with water sprays, poor atomisation wets the dust, which then hydrates and sticks in ducts and on the fan. Good spray control, a dry outlet temperature and insulation keep the dust dry up to the fan.
Rotary cooler air drawn by the fan
On kilns with a rotary cooler the ID fan draws all the cooling air as well as the kiln gas. A fan sized on kiln gas alone leaves the cooler short of air and the product hot. The duty includes the full cooler air quantity.
Energy and control
Dolomite kilns supplying steel plants follow steel production, so output varies. A variable speed drive on the ID fan, controlled on hood pressure, saves heavily at part load. Reducing false air at the kiln seals and preheater lowers gas volume, fan power and fuel use together. Keeping the filter pressure drop in check with timely cleaning avoids running the fan at a higher speed than the kiln needs.
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 Forced Draft (FD) Fan for dolomite calcining kilns.
Questions buyers ask
What is the difference between a dolomite kiln and a lime kiln ID fan?
The duty is similar, but the dolomite calcining kiln produces finer, lighter dust and more of it leaves with the gas. The filter loads faster and gas cooling is more sensitive to wetting, so the fan needs more pressure margin.
What temperature does a dolomite kiln ID fan handle?
Usually 170 to 230°C after the bag filter, with the gas cooled before the filter by dilution air, sprays or an air cooler. Long kilns without preheaters need more cooling to reach this.
Does the ID fan supply the cooling air on a dolomite kiln?
On kilns with a rotary cooler, yes. The air is drawn through the cooler by the kiln suction. Kilns with a vertical contact cooler use a separate cooler fan instead.
Information needed to quote an ID fan for dolomite calcining 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.
- Product. Light-burnt or dead-burnt dolomite, output and kiln type.
- Cooler. Cooler type and whether its air is drawn by the ID fan.
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.