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 FD fan sits
- Inlet screen
- FD fan
- Air header
- Kiln burner
- Afterburner air nozzles
The fan supplies a header that feeds the kiln start-up and support burner and the air nozzles of the post-combustion chamber. Once the kiln is up to temperature, the activation reaction supplies most of the heat, and the kiln burner turns down or stops. Most of the fan's flow then goes to the afterburner, where it burns the CO and hydrogen released by activation. The fan works against the burner, nozzles and dampers, typically 200 to 400 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Char and carbon dust around the kiln and screening area |
| Pressure at the fan outlet | Typically 200 to 400 mmWC |
| Air split | Shifts from kiln burner at start-up to afterburner in normal running |
| 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 activated carbon rotary kilns
| Blade form | Backward curved for a steady, efficient supply on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings; no cooling needed |
| Flow control | Variable speed drive on header pressure, with branch dampers set by afterburner temperature and oxygen |
| Also specified | Inlet filter against carbon dust, and an outlet non-return damper if a standby fan is installed |
What this FD fan has to cope with
Incomplete burn-out in the afterburner
Kiln off-gas varies with feed, steam rate and char quality. If the air supply does not follow it, CO passes the afterburner, and can burn in the waste heat boiler or appear at the stack. Branch control on afterburner temperature and outlet oxygen, from a fan with pressure margin, keeps combustion complete.
Carbon dust on the impeller
Carbon dust is fine and conductive, and settles on fan blades and motor windings. Around the kiln discharge and screening area it hangs in the air. An inlet duct taken from a clean area with a filter avoids impeller imbalance and motor faults, and a totally enclosed motor suits the location.
Loss of combustion air
If the FD fan trips, unburnt gas reaches the boiler and the ID fan. The burner management system trips the fuel and opens the emergency stack, and a standby fan with automatic changeover is worth considering on larger lines, because restarting a hot activation kiln after a long air failure costs both time and product.
Energy and control
Air demand at the afterburner depends on how much gas the kiln releases, which changes with steam rate and feed. A fixed-speed fan with throttled branches wastes power. A variable speed drive on header pressure, with branch dampers trimming each user, matches supply to demand and keeps excess air, and therefore the ID fan load, low.
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 Induced Draft (ID) Fan for activated carbon rotary kilns.
Questions buyers ask
Why is combustion air not blown into an activated carbon kiln?
Air in the kiln would burn the carbon. Activation uses steam, and the reaction products, CO and hydrogen, are burnt after the kiln in a post-combustion chamber. The combustion air fan therefore serves mainly the afterburner and a start-up or support burner, and its control must keep that air out of the kiln itself.
What size of FD fan is needed for an activated carbon afterburner?
It is calculated from the volume and composition of the kiln off-gas, which depend on throughput, burn-off and steam rate, plus excess air for complete combustion. Send the kiln data and we will work out the air volume and pressure, with turndown for the start-up period when the kiln releases little gas.
Can the afterburner air fan run on a VFD?
Yes. A variable speed drive on header pressure with branch dampers on each air line is a common and efficient arrangement, and it gives the combustion control a stable base to work from. The drive also lets the fan run slowly during heat-up, when the kiln releases little gas and the afterburner needs little air.
Information needed to quote an FD fan for activated carbon rotary 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.
- Off-gas. Expected off-gas flow and CO and hydrogen content at design kiln throughput.
- Air users. Number of burners and afterburner air nozzles, and the air split between them.
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.