How activated carbon regeneration furnaces work
Spent granular activated carbon from water treatment, sugar decolourising, solvent recovery or gold recovery circuits is dewatered and fed to a rotary kiln or multiple hearth furnace. The carbon is dried, the adsorbed organics are desorbed and pyrolysed, and the char left in the pores is gasified with steam at about 750 to 900°C. Many smaller units, especially in gold plants, are indirectly fired, with the burner heating the outside of the tube. The off-gas carries steam, organic vapours and fine carbon to a thermal oxidiser at 850 to 1,100°C, then a quench, a wet scrubber where acid gases are present, and the ID fan. The FD fan provides combustion air for the furnace and oxidiser burners.
Where the ID fan sits
- Regeneration kiln or hearth
- Thermal oxidiser
- Quench
- Wet scrubber
- Mist eliminator
- ID fan
- Stack
The fan sits after the scrubber and mist eliminator, at the cold end of the system. It must overcome the furnace, the oxidiser, the quench, the packed or venturi scrubber and the droplet separator, typically 250 to 450 mmWC when a venturi is used. Placing it at the wet end means it handles cool, saturated gas of small volume rather than hot oxidiser gas, at the cost of a wet, sometimes acidic duty.
Gas at the ID fan
| Gas temperature, normal | 60 to 80°C, saturated after the scrubber |
| Maximum or upset | Up to 200°C or more if quench water fails, until the trip acts |
| Dust at the fan | Little; fine carbon and salt in droplet carry-over |
| Moisture and dew point | Saturated gas with free droplets; condensate collects in the casing |
| Corrosive species | HCl and SO₂ traces where the spent carbon held chlorinated or sulphur compounds; chlorides from scrubber recycle |
| Density at the design temperature | About 1.03 kg/m³ at 70°C, 86% 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 regeneration furnaces
| Blade form | Backward inclined or radial, plate blades that shed droplets |
| Material of construction | SS 316 or 316L wetted parts for chloride duty; FRP for strongly acidic gas on smaller fans |
| Drive and arrangement | Belt driven or direct coupled, arrangement 9 or 8 |
| Bearings and cooling | Bearings outside the gas stream with a shaft seal against wet gas |
| Flow control | Variable speed drive on furnace or oxidiser draft |
| Also specified | Casing drain at the lowest point, a high-temperature trip, and a flushing connection for salt deposits |
What this ID fan has to cope with
Chloride corrosion of wetted parts
Spent carbon from municipal water or chemical plants often carries chlorinated organics. After oxidation the chlorine appears as HCl, which the scrubber removes only partly. The fan casing and wheel, constantly wet, pit and crack in ordinary steel. SS 316L or FRP, with welds pickled and the drain kept clear, gives a long life.
Quench failure
If the quench water supply fails, gas at oxidiser temperature reaches an FRP or lightly built stainless fan within seconds. A temperature trip on the scrubber inlet that stops the burner and opens an emergency vent upstream protects the fan, and the fan material is chosen with this event in mind.
Salt and carbon deposits on the blades
Droplets that pass the mist eliminator carry dissolved salts and carbon fines, which dry on the blades and build up unevenly. A water flushing nozzle in the casing, used on a schedule, and a well-maintained mist eliminator keep the wheel clean and in balance between shutdowns.
Energy and control
Regeneration furnaces often run in campaigns, processing stockpiles of spent carbon at varying feed rates. A variable speed drive on draft lets the fan follow the furnace through start-up, partial feed and hot standby, which saves much of the power a throttled fan uses. Lower pressure drop across a well-maintained scrubber and mist eliminator also reduces fan energy directly.
ID fan and FD fan in activated carbon regeneration furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the quench, scrubber and mist eliminator | At the furnace and oxidiser burners |
| What it handles | Saturated, sometimes acidic gas | Clean ambient air |
| Temperature at the fan | 60 to 80°C | Ambient |
| Main design concern | Chloride corrosion and quench failure | Air to match varying organic load |
| Control | Furnace or oxidiser draft | Burner ratio and oxidiser oxygen |
See also the Forced Draft (FD) Fan for activated carbon regeneration furnaces.
Questions buyers ask
What material is used for the ID fan of a carbon regeneration kiln?
After a wet scrubber the gas is saturated and may contain HCl, so wetted parts are usually SS 316L. FRP is used on smaller fans with strongly acidic gas. A dry system with a bag filter and gas above the dew point can use mild steel.
Does a gold plant carbon regeneration kiln need an ID fan?
Many small indirectly fired kilns vent the tube off-gas and the burner flue separately, sometimes by natural draft. An ID fan is needed when the off-gas is treated in an oxidiser or scrubber, or when steady draft is required for the carbon quality. The fan is then sized on the treated gas, not on the kiln alone.
How do I protect the ID fan if the quench water fails?
Fit a temperature trip at the scrubber inlet that stops the burners and opens an emergency vent ahead of the quench. The fan is then exposed to hot gas only for a few seconds, and its materials, especially for an FRP fan, are checked for that short event.
Information needed to quote an ID fan for activated carbon regeneration 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.
- Spent carbon source. What the carbon has adsorbed, and whether chlorinated or sulphur organics are present.
- Scrubber chemistry. Scrubbing liquor, its pH and chloride level, and the gas condition after the mist eliminator.
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.