How catalyst calcining kilns work
Refinery, petrochemical and chemical catalysts are calcined after impregnation or extrusion to decompose metal nitrates, carbonates or ammonium salts and burn out binders and pore formers, at typically 400 to 700°C. Small and medium plants use indirectly heated rotary kilns, with gas burners or electric heaters outside the shell, or belt and tunnel calciners; larger plants sometimes use direct-fired kilns. A controlled flow of air sweeps the product to carry away the released gases. The sweep gas leaves at 200 to 400°C carrying water vapour, NOx from nitrate decomposition, ammonia, organic fumes and fine catalyst dust containing nickel, cobalt, molybdenum or other metals. It passes a cyclone or bag filter and then a NOx scrubber, SCR or thermal oxidiser before the stack. Combustion flue gas from an indirect-fired shell has its own simple stack or draft fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Air heater (if fitted)
- Shell burners or kiln air inlet
- Calcining kiln
On indirect kilns a combustion air fan feeds the burners in the heating jacket, and a separate process air fan delivers sweep air to the kiln discharge end, sometimes through a heater to avoid chilling the product. On direct-fired kilns one fan supplies burner air. Outlet pressure is modest, usually 150 to 500 mmWC, but the sweep air must be metered, because it sets the oxygen available for burn-off.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after any heater | Up to 200 to 400°C downstream of the fan, where process air is preheated |
| Dust | Filtered air; product purity requires clean air |
| Pressure at the fan outlet | Typically 150 to 500 mmWC |
| Flow control | Process air metered to the recipe |
| 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 catalyst calcining kilns
| Blade form | Backward curved, with a stable curve for accurate flow |
| Material of construction | Mild steel; stainless steel for process air where product contamination matters |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive with flow measurement |
| Also specified | Fine inlet filter for process air; flow meter in the delivery duct |
What this FD fan has to cope with
Too much air and runaway burn-off
Excess sweep air during binder or pore former burn-off can cause local overheating that damages the catalyst surface. Metered process air, set to the recipe, keeps burn-off controlled.
Contamination of the product
Rust or dust from an unfiltered mild steel air system can reach the catalyst. Filtered air and stainless construction downstream of the filter avoid contamination of sensitive grades.
Flow drift as filters load
As the inlet filter loads, a fan on fixed speed delivers less air. Speed control on measured flow keeps sweep air constant between filter changes. This matters most on long campaigns between filter changes.
Energy and control
Catalyst calcining runs to recipe in campaigns, with periods of low output or idling between grades. The fans are small, but they run long hours. Speed control on measured flow saves power at low settings and keeps air, and the heat it carries away, to what each recipe needs. A fan left at full speed simply cools the kiln.
ID fan and FD fan in catalyst calcining kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before NOx treatment | Before the burners or kiln air inlet |
| What it handles | Hot sweep air with NOx, ammonia and metal fines | Clean filtered air |
| Temperature at the fan | 150 to 250°C | Ambient |
| Main design concern | Nitric acid condensation and deposits | Accurate, clean air flow |
| Control | Kiln pressure | Recipe air flow |
See also the Induced Draft (ID) Fan for catalyst calcining kilns.
Questions buyers ask
Is the FD fan on a catalyst kiln a combustion air fan?
On kilns heated by gas burners, yes. On electric or externally heated kilns the forced draft fan instead supplies the controlled process air that sweeps the product, and burner air comes from a separate fan where needed.
Why does catalyst calcining need controlled air flow?
The air supplies oxygen for burning binders and carries away decomposition gases. Too little leaves residues; too much can overheat the catalyst during burn-off. Metered air keeps the product in specification.
Should process air for a catalyst kiln be filtered?
Yes. Dust and rust can contaminate the catalyst. A fine inlet filter and stainless construction after it are common for sensitive grades. The filter should be sized so that its loss does not limit the fan at the end of a change interval.
Information needed to quote an FD fan for catalyst calcining 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.
- Air duties. Whether the fan supplies burner air, process sweep air or both, and the flow range for each recipe.
- Air heating. Any heater after the fan, and the process air temperature required at the kiln.
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.