How carbon anode baking furnaces work
Carbon anodes for aluminium smelting are baked in open-top ring furnaces. Green anodes made of petroleum coke and coal tar pitch are packed in deep refractory pits, separated by hollow flue walls, and covered with packing coke. A fire of several sections moves slowly around the ring: burner ramps fire natural gas or fuel oil into the flues, pitch volatiles released from the anodes burn in the flues, and the anodes reach about 1,100 to 1,200°C. Blowing ramps push air into the cooling sections, where it is preheated by the baked anodes before reaching the fire. An exhaust ramp draws the flue gas into a ring main at 150 to 250°C and on to the fume treatment centre, then the ID fan.
Where the ID fan sits
- Flue walls under fire
- Exhaust ramp
- Ring main
- Conditioning tower
- Alumina dry scrubber and bag filter
- ID fan
- Stack
The fan sits after the fume treatment centre, where a conditioning tower cools the gas with water sprays to about 100 to 120°C and an alumina dry scrubber and bag filter capture fluorides and tar. Its suction covers the flue walls, exhaust ramp, ring main and treatment, typically 300 to 500 mmWC. Steady suction at the exhaust ramp controls the draft and fire progress through the sections.
Gas at the ID fan
| Gas temperature, normal | 100 to 130°C after the conditioning tower and filter |
| Maximum or upset | Up to 180°C if conditioning sprays fail |
| Dust at the fan | Traces of alumina, carbon and condensed tar after the filter |
| Moisture and dew point | Moisture added by conditioning; HF and SO₂ raise the acid dew point |
| Corrosive species | HF from fluorides in recycled butts, SO₂ from coke and pitch sulphur |
| Organics | PAH tar and pitch fume, mostly captured in the scrubber and filter |
| Density at the design temperature | About 0.91 kg/m³ at 115°C, 76% 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 carbon anode baking furnaces
| Blade form | Backward curved on the clean side of the filter, with smooth blades that shed tar |
| Material of construction | Mild steel with corrosion-resistant coating; SS 316 at cold spots |
| Drive and arrangement | Direct coupled, arrangement 8; often two fans in parallel |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on ring main suction |
| Also specified | Casing drain, insulation, access door and provision for washing or steaming tar |
What this ID fan has to cope with
Tar deposits on the blades
Pitch volatiles that are not fully burned in the flues, or that pass the scrubber, condense as sticky tar on the fan. Tar collects dust and alumina, builds up unevenly and unbalances the wheel. Good combustion in the fire zone, a working scrubber and periodic cleaning keep the fan in balance.
Fluoride and acid corrosion
Fluorides from recycled anode butts and sulphur in coke and pitch give HF and SO₂. Where the conditioning tower overcools the gas, acids condense on the casing. Stable conditioning temperature, insulation and stainless steel at cold spots reduce corrosion. Coating damage should be repaired at each shutdown.
Suction fluctuations and fire progress
The fire moves to the next section every day or so, and the exhaust ramp is moved with it. Suction changes affect combustion and anode quality. A fan on speed control holding ring main suction, not damper throttling, keeps draft steady. The fan curve must stay stable through the whole suction range used during a fire move.
Energy and control
Anode baking furnaces run continuously, and the ID fan is a steady, large consumer. Speed control holding ring main suction saves power and stabilises draft. Air leakage into the flue walls, ring main and treatment centre adds gas volume, and repairing leaks reduces fan power and improves fluoride capture. Fan speed trends also show leakage.
ID fan and FD fan in carbon anode baking furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the fume treatment centre | On portable ramps over the cooling sections |
| What it handles | Treated flue gas with tar and fluoride traces | Warm hall air |
| Temperature at the fan | 100 to 130°C | 35 to 55°C |
| Main design concern | Tar deposits and acid corrosion | Robust portable design and even air |
| Control | Ring main suction | Flue pressure at each ramp |
See also the Forced Draft (FD) Fan for carbon anode baking furnaces.
Questions buyers ask
What does the ID fan do on an anode baking furnace?
It draws flue gas from the exhaust ramp on the furnace, through the ring main and the fume treatment centre, and discharges it to the stack. Its suction controls the draft in the flue walls, which affects combustion and anode baking. It is sized for all fires on the ring together.
Why is there tar on my anode baking furnace ID fan?
Pitch volatiles released from the green anodes should burn in the flues. When combustion is incomplete or the scrubber does not capture all of the tar, it condenses on the fan blades. Improving combustion, checking the scrubber and cleaning the fan regularly address this. Steaming the casing can also help.
Why are two ID fans used on anode baking furnaces?
Many plants run two fans in parallel so that one can be cleaned or repaired while the other keeps the furnace under draft. Losing draft stops the fire progression and risks emissions and anode quality. The parallel fans are usually identical, each sized to carry most of the load alone, so that one can keep the fires going at reduced draft if the other trips.
Information needed to quote an ID fan for carbon anode baking furnaces
- 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.
- Furnace. Number of sections, fires and pits, and the anode production rate.
- Fume treatment. Conditioning tower outlet temperature and the scrubber and filter design.
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.