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 FD fan sits
- Inlet filter
- FD fan (cooler or combustion air)
- Lime cooler
- Secondary air to kiln
- Burner
On a rotary kiln with a vertical cooler, the cooler air fan blows ambient air up through the hot lime. The air reaches 500 to 700°C and enters the kiln as secondary combustion air. A primary air fan feeds the burner. On shaft kilns, combustion and cooling air blowers supply air to the lances and the cooling zone. The fans handle ambient air.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the cooler | 500 to 700°C, downstream of the fan |
| Dust | Ambient lime plant dust; filtered intake recommended |
| Pressure at the fan outlet | Set by the cooler bed and burner, typically 400 to 900 mmWC |
| Air ratio | Must follow the fuel gas calorific value |
| 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 steel plant lime calcining plants
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive linked to fuel flow and lime discharge |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Air ratio with changing fuel gas
When the calorific value of BF or mixed gas falls, the air demand per cubic metre of fuel changes. Fixed air settings then give either excess air or incomplete combustion, and lime reactivity suffers. Air control on fuel analysis or flue oxygen keeps calcination steady.
Cooler air and lime temperature
Too little cooler air leaves hot lime at discharge, damaging belt conveyors and storage bins; too much air lowers secondary air temperature, raises fuel use and adds volume to the ID fan. The cooler air fan needs fine flow control, usually speed control linked to lime output and discharge temperature.
Dusty intake air
Lime plant air is dusty and alkaline, especially near crushing, screening and lime loading. It clogs inlet filters and coats fan surfaces, and the falling delivery shows up as hotter lime and unstable combustion. A clean air intake, a filter with a pressure alarm and regular maintenance keep the fan delivery steady.
Energy and control
The cooler air fan recovers heat from the lime into the combustion air, so getting its flow right saves fuel directly. Speed control linked to lime output and kiln firing avoids over-cooling. On steel works gas, accurate air ratio control also cuts the flue gas volume, saving power on the larger ID fan.
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 Induced Draft (ID) Fan for steel plant lime calcining plants.
Questions buyers ask
What is the FD fan on a rotary lime kiln?
Usually the cooler air fan, which blows ambient air through the lime cooler. The hot air leaves the cooler as secondary combustion air for the kiln burner, recovering heat from the lime. A separate primary air fan feeds the burner directly, and on gas firing the fuel gas may need its own booster.
How does blast furnace gas change the combustion air requirement?
Blast furnace gas needs less air per cubic metre than natural gas because of its low calorific value, but much more gas is burnt. The total air and the flue gas volume both rise, and the air ratio must follow gas quality.
Does a PFR lime kiln have FD fans?
Yes, though they are often positive displacement blowers rather than centrifugal fans. Parallel flow regenerative kilns use combustion air and cooling air blowers, and a waste gas fan after the filter. The blowers handle ambient air and must deliver steady volume against the changing resistance of the limestone bed.
Information needed to quote an FD fan for steel plant lime calcining plants
- 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, lime output and the air needed for cooling.
- Burner. Burner type and primary air requirement.
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.