How steel plant lime calcining plants work
Steel plants need large tonnages of reactive, low-sulphur lime and calcined dolomite for BOF, EAF and ladle furnace slag making, and for sinter. Most integrated plants calcine their own in captive lime plants: rotary kilns with vertical preheaters and coolers, parallel flow regenerative shaft kilns or annular shaft kilns. A distinctive feature is fuel: many captive kilns burn steel works gases such as blast furnace gas, coke oven gas or mixed gas, sometimes with natural gas or petcoke. Lean gases such as blast furnace gas produce much larger flue gas volumes per tonne of lime than coal or natural gas. Limestone sized 10 to 60 mm is calcined at about 1,000 to 1,100°C, and kiln dust, rich in lime fines, is collected in a bag filter and often sent to the sinter plant.
Where the ID fan sits
- Kiln
- Vertical preheater
- Gas cooler or dilution air
- Bag filter
- ID fan
- Stack
On a rotary kiln with a vertical preheater, the gas leaves the preheater at 250 to 350°C and is cooled by dilution air or a cooler to 180 to 250°C before the bag filter. The ID fan follows the filter. It controls kiln inlet draft and must overcome the preheater, the cooler and the filter, typically 400 to 700 mmWC. On shaft kilns the waste gas fan is smaller and works at lower temperature, typically 100 to 200°C.
Gas at the ID fan
| Gas temperature, normal | 180 to 250°C after the bag filter on rotary kilns |
| Maximum or upset | 300 to 350°C if dilution fails or the preheater is disturbed |
| Dust at the fan | Below 30 to 50 mg/Nm³ after the bag filter |
| Dust character upstream | Fine lime and limestone dust, hygroscopic, highly alkaline |
| Gas volume | Large with BF gas firing because of the high nitrogen and CO₂ content of the fuel |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 steel plant lime calcining plants
| Blade form | Backward curved aerofoil or plate, efficient for large clean volumes |
| Material of construction | Mild steel; wear liners on leading edges where filter bags may fail |
| Drive and arrangement | Direct drive, arrangement 8, double inlet on large kilns |
| Bearings and cooling | Pedestal bearings with heat slinger |
| Flow control | Variable speed drive on kiln inlet draft |
| Also specified | Vibration monitoring and inspection doors |
What this ID fan has to cope with
Volume underestimated on lean gas
A fan sized from a coal or natural gas fired kiln will be too small when blast furnace gas or mixed gas is used, because these fuels carry much inert gas. Sizing from the actual fuel analysis and air ratio, including any change of fuel blend, avoids a kiln that cannot reach output.
Lime dust build-up
Lime dust picks up moisture and CO₂ and forms hard deposits on blades and casing, especially after kiln stops when the fan cools and condensation forms. The deposits break off unevenly and unbalance the rotor. Smooth blade surfaces, sufficient gas temperature, a warm-up before restart and cleaning access limit this.
Fuel gas variability
Steel works gas calorific value varies with blast furnace and coke oven operation. When it falls, the kiln burns more gas and the flue gas volume rises. The fan needs margin and speed control to follow these swings.
Energy and control
Captive lime plants run continuously and their ID fans are large, so power is a real cost. Speed control on kiln draft and minimising false air at the preheater and seals reduce volume. Using steel works gas instead of purchased fuel improves plant economy even though the fan handles more volume.
ID fan and FD fan in steel plant lime calcining plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the preheater and bag filter | Feeding the cooler and burner |
| What it handles | Kiln gas with lime dust | Ambient air |
| Temperature at the fan | 180 to 250°C | Ambient |
| Main design concern | Large volume from lean gas firing | Air ratio with variable fuel gas |
| Control | Kiln inlet draft | Fuel flow and lime discharge |
See also the Forced Draft (FD) Fan for steel plant lime calcining plants.
Questions buyers ask
Why does a lime kiln fired on blast furnace gas need a bigger ID fan?
Blast furnace gas has a low calorific value and is mostly nitrogen and CO₂, so much more gas must be burnt per tonne of lime. The flue gas volume is several times higher, and the ID fan, bag filter and ducting must be sized for it.
What temperature does a steel plant lime kiln ID fan handle?
On rotary kilns with preheaters, the gas reaches the fan at about 180 to 250°C after the cooler and bag filter. On shaft kilns the waste gas is cooler, typically 100 to 200°C, and the fan is smaller.
Where does the lime kiln dust go in a steel plant?
Lime kiln dust collected in the bag filter is usually sent to the sinter plant or used as a flux. The dust handled by the fan is only what passes the filter, but its alkalinity and tendency to build up still matter.
Information needed to quote an ID fan for steel plant lime calcining plants
- 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.
- Fuel. Fuel or gas blend, calorific value and its normal variation.
- Kiln type. Rotary with preheater, PFR or annular shaft, and the output in tonnes per day.
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.