How pulp mill lime kilns work
In a kraft pulp mill the lime kiln recovers quicklime from the lime mud produced in recausticising. Washed mud at typically 70 to 80% dry solids is fed to the cold end of a long rotary kiln, where it dries, heats and calcines to calcium oxide at about 1,000 to 1,100°C in the burning zone. The kiln is fired with furnace oil, gas or petcoke, and often also burns non-condensable gases from the digester and evaporators. Combustion gas flows counter to the mud and leaves the feed end at about 200 to 300°C with lime dust and sodium fume. It passes an ESP or a venturi scrubber, then the ID fan and stack. Secondary air is drawn through the product coolers, preheated by the hot lime.
Where the FD fan sits
- Inlet filter
- FD fan
- Primary air duct
- Burner
- Burning zone
The primary air fan sits on the firing floor beside the burner. It delivers air through the burner annulus at high velocity to shape the flame and mix fuel. On petcoke firing the same or a separate fan conveys the pulverised fuel. The fan does not supply most of the combustion air; secondary air is pulled through the product coolers or a hood by the ID fan, preheated to several hundred degrees by the hot lime.
Air at the FD fan
| Air temperature at the fan | Ambient, 30 to 50°C at the firing floor |
| Radiant heat | Firing floor near the kiln hood is hot; fan protected from radiation |
| Dust | Lime dust around the hood; inlet filtered |
| Pressure at the fan outlet | Typically 500 to 1,000 mmWC for a multi-channel burner |
| Share of air | Usually 10 to 20% of total combustion air |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 pulp mill lime kilns
| Blade form | Backward curved or radial, selected for high pressure at modest flow |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings, shielded from hood radiation |
| Flow control | Variable speed drive with burner channel dampers |
| Also specified | Inlet filter and silencer, and a cooling air connection to the burner tip when the kiln is off |
What this FD fan has to cope with
Flame shape and ring formation
Too little primary air gives a long, lazy flame that moves heat toward the feed end and promotes mud rings; too much shortens and overheats the flame. A fan with enough pressure for the burner's momentum and fine control of flow lets the operator set the flame correctly.
Burner protection on trip
If the primary air fan stops while the kiln is hot, the burner tip overheats in radiation from the kiln. The fan, or a dedicated cooling air supply, keeps a minimum flow through the burner whenever the kiln is hot. This is often a small dedicated cooling fan.
Heat and lime dust at the fan
The firing floor is hot and dusty. Fans drawing air there run warm and collect lime. Taking intake air from a cooler, cleaner location and shielding the fan improves bearing life and keeps the impeller clean. Motor cooling also improves, since the motor no longer draws hot air off the firing floor.
Energy and control
Primary air is a small share of the air, but it is cold and enters at high pressure, so its cost is both fan power and fuel. Lime kiln operators keep primary air as low as flame quality allows to maximise the hot secondary air from the coolers. A variable speed drive lets the burner be tuned for each fuel without wasting pressure across a damper.
ID fan and FD fan in pulp mill lime kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or scrubber, before the stack | On the firing floor, feeding the burner |
| What it handles | Kiln gas with lime dust and sodium fume | Ambient air |
| Temperature at the fan | 180 to 260°C dry, or 65 to 80°C wet | Ambient |
| Main design concern | Deposits, corrosion and capacity | Flame shape and burner protection |
| Control | Hood draft | Burner tuning |
See also the Induced Draft (ID) Fan for pulp mill lime kilns.
Questions buyers ask
What is the FD fan on a lime kiln?
On a pulp mill lime kiln it is usually the primary air fan, which supplies air through the burner to shape the flame. Most combustion air enters as secondary air through the coolers, drawn in by the ID fan. Its pressure and flow set the flame shape and length.
How much primary air does a lime kiln burner need?
Typically 10 to 20% of the total combustion air, at 500 to 1,000 mmWC for a multi-channel burner. The figure depends on the burner design and fuel; petcoke usually needs more than oil or gas. Burner suppliers give the required pressure, and the fan is sized with margin above it.
Why should primary air be kept low on a lime kiln?
Primary air enters cold, while secondary air is preheated by the hot lime in the coolers. More primary air means less hot secondary air, which raises fuel use. The forced draft fan is set for good flame shape at the lowest practical flow. Less cold air also helps output.
Information needed to quote an FD fan for pulp mill lime 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.
- Burner. Burner make or type, fuel and required primary air pressure.
- Fuel. Fuels fired, and whether the fan also conveys pulverised petcoke.
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.