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 FD fan sits
- Inlet filter
- FD fan
- Air header
- Furnace burner
- Thermal oxidiser burner and air ports
The fan sits near the burner skid and feeds the furnace burner, which heats the kiln shell or hearths, and the thermal oxidiser burner and its secondary air ports. On indirectly fired kilns the furnace burner air never contacts the carbon; on directly fired hearths it does, and must be closely controlled to avoid burning product. The fan works against the burner and air train resistance, typically 150 to 350 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clean, apart from carbon dust near the feed and product handling |
| Pressure at the fan outlet | Typically 150 to 350 mmWC |
| Load variation | Organic load in the off-gas changes with the type and loading of spent carbon |
| 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 regeneration furnaces
| Blade form | Backward curved, on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or motor mounted |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or inlet damper, with ratio valves at each burner |
| Also specified | Inlet filter, low air pressure switch for the burner safety system |
What this FD fan has to cope with
Varying organic load in the oxidiser
Carbon from a sugar refinery releases a different volume and type of organics than carbon from a solvent recovery plant. The oxidiser needs more or less air as the load changes. A fan with pressure margin and a burner control that trims air on oxidiser outlet oxygen keeps destruction efficient.
Air leakage into directly fired hearths
On multiple hearth furnaces the burner air enters the carbon space. Excess air burns regenerated carbon and raises losses. Accurate ratio control at each burner, fed from a stable air header, keeps oxygen in the hearths low. Separate air for each hearth burner, with its own valve, gives the operator finer control.
Fan sized for cold start only
Some fans are sized for the heat-up period with no organics and then run throttled. Checking the duty at normal running, with the oxidiser at design load, gives a better selection and better control. The heat-up duty is then met by running the fan faster for that short period on a variable speed drive.
Energy and control
Combustion air for a regeneration furnace is a modest load, but the fan often runs throttled for long periods during low feed or hot standby. A variable speed drive on header pressure reduces this. Minimising excess air at the oxidiser cuts fuel, which is usually the larger cost, and also reduces the gas the ID fan and scrubber must handle.
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 Induced Draft (ID) Fan for activated carbon regeneration furnaces.
Questions buyers ask
What does the FD fan supply on a carbon regeneration furnace?
It supplies combustion air to the furnace burner and to the thermal oxidiser burner and air ports. On indirectly fired kilns this air heats the tube from outside; on directly fired hearths some of it enters the carbon space and must be carefully limited to protect the regenerated product.
How much combustion air does a thermal oxidiser need for carbon regeneration off-gas?
It depends on the off-gas volume, its organic and CO content, the auxiliary fuel and the excess oxygen needed at the outlet. These vary with the spent carbon, so the fan is sized for the heaviest expected organic load with turndown for the lightest load and for hot standby periods between campaigns.
Should the combustion air fan have a standby?
On continuous plants treating hazardous organics, a standby fan or a fan with a spare motor is often specified, because loss of air stops the oxidiser and forces the furnace to shut down safely. Restarting then means reheating the furnace and oxidiser, which costs fuel and time.
Information needed to quote an FD fan for activated carbon regeneration furnaces
- 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.
- Firing arrangement. Whether the furnace is directly or indirectly fired, and the burners served by the fan.
- Oxidiser data. Oxidiser temperature, residence time and the design organic load in the off-gas.
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.