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 ID fan sits
- Burner and burning zone
- Chain or dryer section
- Feed end housing
- ESP or venturi scrubber
- ID fan
- Stack
The fan sits after the collector. With an ESP it handles dry gas at 180 to 260°C with residual lime dust; with a venturi scrubber it handles saturated gas at 65 to 80°C carrying alkaline droplets. It works against the kiln, feed end, collector and ducting, typically 200 to 400 mmWC with an ESP and much more with a venturi. The ID fan sets the total gas the kiln can pass, so when a mill wants more lime its capacity is usually the first limit reached.
Gas at the ID fan
| Gas temperature, normal | 180 to 260°C after an ESP; 65 to 80°C after a scrubber |
| Maximum or upset | 300 to 350°C with low mud feed or a broken chain section |
| Dust at the fan | Lime and lime mud fines with sodium salts; 50 to 150 mg/Nm³ after an ESP |
| Moisture and dew point | High moisture from the mud, typically 25 to 35% by volume; dew point near 65 to 75°C |
| Corrosive species | SO₂ and reduced sulphur from NCG burning; sodium sulphate and carbonate fume |
| 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 pulp mill lime kilns
| Blade form | Backward curved with wear protection after an ESP; radial tipped where the fan follows a scrubber with heavy carry-over |
| Material of construction | Mild steel or boiler quality steel for ESP service; SS 316 or duplex stainless after a wet scrubber |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc for the hot dry case |
| Flow control | Variable speed drive on hood draft |
| Also specified | Wash nozzles on scrubber duty, insulated casing on ESP duty, and an access door |
What this ID fan has to cope with
Sodium fume build-up
Sodium compounds vaporised in the burning zone condense as fine fume and stick to the impeller, especially where gas temperature is near their softening range or the blade surface is cooler. Deposits unbalance the wheel. Smooth blades, regular inspection and in-situ balancing during kiln stops are the practical answer.
Fan as the capacity limit
Mills raise lime output by feeding drier mud and firing harder, which increases gas volume. A fan already near its maximum speed caps production. A new impeller with higher flow on the existing casing, checked against motor and tip speed limits, is a common upgrade.
Corrosion at the wet end
After a venturi, saturated gas with SO₂ and alkaline droplets attacks mild steel. When NCGs are burned, reduced sulphur makes this worse. Stainless construction and a drained casing are needed; an ESP-duty fan must also be kept above dew point. Wash nozzles keep alkaline deposits under control.
Energy and control
Lime kilns run continuously, but output follows the pulp mill and the causticising balance, so the ID fan often runs between 60 and 90% of design. A variable speed drive on hood draft saves energy against damper control, and good excess air control saves more fuel than fan power, because every percentage point of oxygen at the feed end adds gas the fan must move and lowers lime output.
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 Forced Draft (FD) Fan for pulp mill lime kilns.
Questions buyers ask
What limits the capacity of a pulp mill lime kiln?
Often the ID fan. It sets the total gas the kiln can pass, and firing harder or feeding wetter mud raises gas volume. Checking fan speed, damper position and motor current at full output shows whether the fan is the limit. A new impeller can often raise capacity.
Why does the ID fan on my lime kiln vibrate?
The usual cause is uneven build-up of lime dust and sodium fume on the impeller, which then breaks away in patches. Cleaning and balancing during kiln stops fixes it, and a smoother blade design reduces build-up. Check also for erosion at the blade tips, which removes metal unevenly, and for a cooled casing that encourages fume to condense.
Can an ESP-duty ID fan run after a scrubber on a lime kiln?
Not as it is. After a scrubber the gas is saturated and corrosive, so a mild steel fan will corrode. A wet-duty fan in stainless steel with a drain and wash connections is needed. The impeller is also changed to a blade form that tolerates wet deposits and liquid carry-over.
Information needed to quote an ID fan for pulp mill lime 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.
- Collector. Whether the kiln has an ESP or venturi scrubber, and its pressure drop.
- Capacity target. Present and target lime output, and whether NCGs are burned.
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.