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 ID fan sits
- Calcining kiln
- Cyclone
- Bag filter
- Exhaust (ID) fan
- NOx scrubber, SCR or oxidiser
- Stack
The fan sits after the bag filter, which recovers valuable and toxic catalyst fines, and before NOx treatment. The gas usually reaches it at 150 to 250°C after duct heat losses or a cooler. Keeping the fan before the wet scrubber lets it run dry, but some plants put it after the scrubber, where it handles saturated acid gas. It holds the kiln at a small suction so fumes do not escape at the feed and discharge seals.
Gas at the ID fan
| Air temperature, normal | 150 to 250°C after the bag filter |
| Maximum or upset | Up to 350 to 400°C during binder burn-off peaks |
| Dust at the fan | Low; fine catalyst carrying nickel, cobalt, molybdenum or similar metals |
| Moisture and dew point | High water vapour from drying; nitric acid dew point well above water dew point |
| Corrosive species | NOx, ammonia and ammonium nitrate, organic acids from binder burn-out |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 62% 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 catalyst calcining kilns
| Blade form | Backward curved, on clean gas after the filter |
| Material of construction | SS 304L or 316L impeller and casing to resist nitric acid condensate |
| Drive and arrangement | Direct coupled or belt drive, arrangement 8 or 9 |
| Bearings and cooling | Shaft cooling disc for the higher binder burn-off temperatures |
| Flow control | Variable speed drive on kiln pressure |
| Also specified | Insulated casing, gas-tight shaft seal and casing drain |
What this ID fan has to cope with
Nitric acid condensation
NOx and water condense together as dilute nitric acid on any surface below the dew point, especially at start-up and in cold casing corners. Mild steel corrodes rapidly; low carbon stainless and insulation hold the fan above the dew point.
Ammonium nitrate deposits
Where ammonia and NOx both leave the kiln they can form ammonium nitrate on cooler surfaces. The deposit is corrosive and, in thick layers, a fire and decomposition risk. Keeping the fan hot and washing deposits off at shutdown are essential.
Toxic metal dust
Catalyst fines with nickel or cobalt are a health hazard. A leaking fan casing or seal spreads them around the plant. Gas-tight joints and a sealed shaft keep them in the system.
Energy and control
Catalyst calciners often run in campaigns of one product grade, with sweep air set by recipe. The exhaust fan rarely needs its full duty. A variable speed drive controlled on kiln pressure follows recipe changes and avoids drawing extra air, which would dilute NOx treatment and raise the fuel or reagent it needs.
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 Forced Draft (FD) Fan for catalyst calcining kilns.
Questions buyers ask
What does the exhaust fan do on a catalyst calciner?
It draws the sweep air and the gases released during calcination out of the kiln, through the dust filter, and delivers them to NOx and fume treatment, while holding the kiln at a slight suction.
Why is stainless steel used for catalyst kiln ID fans?
The gas carries NOx and water vapour, which condense as nitric acid on cool surfaces. Low carbon stainless steel resists this acid, while mild steel corrodes quickly. Insulation keeps the casing above the dew point at low load.
Does a catalyst calcining kiln need both an ID fan and an FD fan?
Usually yes. The FD side supplies combustion air to the shell burners or a controlled process air flow to the product, and the ID fan removes the process gas. On electric kilns only the process air and exhaust fans are needed.
Information needed to quote an ID fan for catalyst calcining 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.
- Decomposition gases. Expected NOx, ammonia and organic content, and whether these peak during particular stages of the recipe.
- Downstream treatment. Type of NOx or fume treatment after the fan and its pressure drop.
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.