How activated carbon rotary kilns work
Carbonised char, most often from coconut shell, is activated in a rotary kiln at about 850 to 950°C with steam injected along the bed. The steam reacts with the carbon to open the pore structure, releasing a lean fuel gas rich in carbon monoxide and hydrogen. Burnt-off carbon is typically 40 to 60% of the char fed. The kiln off-gas passes to a post-combustion chamber where air burns the CO and hydrogen, raising the gas to 900 to 1,100°C, then to a waste heat boiler that raises the process steam. After the boiler the gas, still humid and carrying carbon fines, goes through a cyclone and bag filter or wet scrubber to the ID fan. The FD fan supplies burner and post-combustion air.
Where the ID fan sits
- Rotary kiln
- Post-combustion chamber
- Waste heat boiler
- Cyclone
- Bag filter or scrubber
- ID fan
- Stack
The fan sits at the cold end of the gas train, after the waste heat boiler and the final collector. Upstream losses include the kiln, the afterburner, the boiler passes and the collector, typically 150 to 300 mmWC in total. The fan holds the kiln feed hood at a very small suction, often only 1 to 3 mmWC, because any extra draft pulls air through the rotary seals into the kiln, where oxygen burns activated carbon instead of steam developing pores.
Gas at the ID fan
| Gas temperature, normal | 180 to 230°C after the waste heat boiler |
| Maximum or upset | 350°C or more with boiler tube fouling or a boiler bypass |
| Dust at the fan | Fine, light carbon and ash; low after a bag filter, near saturated droplets after a scrubber |
| Moisture and dew point | Steam from activation and hydrogen combustion gives 25 to 40% water vapour by volume and a water dew point of about 65 to 75°C |
| Corrosive species | Low sulphur from shell char; chlorides from some feedstocks can attack wet surfaces |
| Density at the design temperature | About 0.73 kg/m³ at 210°C, 61% 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 activated carbon rotary kilns
| Blade form | Backward curved after a bag filter; radial or backward inclined with drain holes after a wet scrubber |
| Material of construction | Mild steel for dry gas above the dew point; SS 304 or 316 wetted parts after a scrubber |
| Drive and arrangement | Direct coupled, arrangement 8, with a variable speed drive |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc for upset temperatures |
| Flow control | Variable speed drive on kiln hood pressure, with fast response |
| Also specified | Casing drain, insulated casing to stay above the dew point, and an inlet damper for start-up |
What this ID fan has to cope with
Air ingress through draft swings
If the ID fan responds slowly or the damper hunts, hood suction swings, and every surge pulls air through the kiln seals. The burn-off rises and yield falls, often without anyone tracing it back to the fan. A variable speed drive tuned for fast, small corrections and a pressure tapping that is not plugged by fines keep the draft tight.
Condensation on a humid gas
With water vapour at a third of the gas volume, any part of the casing or duct below about 75°C wets out. Carbon fines then paste onto the blades. The fan casing and ducts are insulated, the gas leaves the boiler well above the dew point, and the fan is run to purge warm before shutdown.
Carbon fires in the collector
Hot carbon fines that escape the afterburner can smoulder in a bag filter hopper and reach the fan as sparks. A spark arrestor or settling chamber before the filter, a temperature watch on the hopper and a fan casing with an access door help contain and clear any deposit fire.
Energy and control
Activation kilns usually run continuously for months, so the ID fan runs at a steady point most of the time. The saving lies in sizing that point correctly, because many fans are oversized and throttled to hold the small hood suction. A variable speed drive brings the fan down to the real duty and improves draft control at the same time. Keeping the waste heat boiler clean lowers both gas volume and resistance.
ID fan and FD fan in activated carbon rotary kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the waste heat boiler and collector | At the kiln burner and afterburner |
| What it handles | Humid combustion gas with carbon fines | Clean ambient air |
| Temperature at the fan | 180 to 230°C | Ambient |
| Main design concern | Tight hood draft and condensation | Complete burn-out of off-gas |
| Control | Kiln hood pressure | Afterburner temperature and oxygen |
See also the Forced Draft (FD) Fan for activated carbon rotary kilns.
Questions buyers ask
Why does the kiln draft matter so much in activated carbon production?
The activation reaction relies on steam, not air. Extra draft at the kiln pulls air in through the seals, and the oxygen burns carbon that should have become product. Holding a small, steady suction with the ID fan reduces air ingress and protects yield and iodine number.
What material should the ID fan be for an activated carbon kiln with a wet scrubber?
After a wet scrubber the gas is saturated and often slightly acidic, so the wetted parts are usually SS 304 or 316. If the gas is reheated above its dew point, or a bag filter is used instead, mild steel with an insulated casing is normally sufficient.
Is the ID fan of an activated carbon kiln the same as a boiler ID fan?
It does a similar job, because the waste heat boiler sits in the gas path. The difference is the very humid gas from steam activation, the light carbon fines and the need for tight hood pressure control, which a standard boiler ID fan specification does not cover.
Information needed to quote an ID fan for activated carbon rotary 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.
- Kiln operation. Kiln throughput, steam injection rate and the target hood pressure.
- Gas cleaning. Whether the fan follows a bag filter or a wet scrubber, and the gas temperature at its inlet.
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.