How odour thermal oxidisers for rendering plants work
Rendering plants cook animal by-products to recover fat and meal, and the cookers release large volumes of vapour carrying ammonia, amines, aldehydes, mercaptans and hydrogen sulphide. The cooker vapour is first condensed, and the non-condensable gases, small in volume but intensely odorous, are sent to a thermal oxidiser held at about 750 to 850°C for 0.5 to 1 second. Many plants also collect building air from the raw material and press areas and use it as combustion air to the oxidiser. The oxidiser is fired on gas, oil or biogas, and the hot gas passes a heat exchanger or a waste heat boiler that raises process steam. An ID fan draws the gas through the heat recovery to the stack; an FD fan supplies the odorous air as combustion air.
Where the FD fan sits
- Raw material and press hall extraction
- Grease filter
- FD fan
- Burner and air registers
- Thermal oxidiser
The fan collects air from enclosed raw material bins, conveyors and press areas and pushes it into the oxidiser burner and air registers. The inlet side is a network of ducts and hoods under suction, so odours stay contained; the discharge supplies the burner pressure, usually 150 to 300 mmWC. Where building air exceeds the combustion air need, the surplus goes to a biofilter or scrubber.
Air at the FD fan
| Air temperature at the fan | Ambient to about 45°C, humid |
| Odorous and corrosive species | Ammonia, amines and H₂S at low concentration |
| Grease and moisture | Fat aerosol and condensate in the ducts, especially near cookers |
| Dust | Meal dust from conveyors and presses |
| Pressure at the fan outlet | Typically 150 to 300 mmWC to the burner |
| 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 odour thermal oxidisers for rendering plants
| Blade form | Backward curved, with blades that shed grease and dust |
| Material of construction | SS 304 or SS 316, or FRP casing with stainless impeller, for the wet and corrosive building air |
| Drive and arrangement | Direct drive with the motor outside the gas stream |
| Bearings and cooling | Standard bearings; sealed shaft entry |
| Flow control | Variable speed drive on burner air demand, with a bypass to the biofilter for surplus air |
| Also specified | Casing drain, washing nozzles and an inspection door |
What this FD fan has to cope with
Corrosion from wet foul air
Building air in a rendering plant is warm and saturated, and carries ammonia and H₂S. Mild steel fans in this duty rust through in a few years. Stainless or FRP construction, a drain at the lowest point and sloped ductwork back to a sump avoid standing water in the casing.
Grease build-up on the impeller
Fat aerosol from cookers and presses settles on the blades and casing, adding unbalanced mass and a fire load inside the fan. A grease filter on the inlet, wash nozzles in the casing with a drain to the effluent system, and a cleaning routine tied to vibration readings keep the fan running smoothly between shutdowns.
Combustion air that is also odour air
The fan must give the burner the air it needs at every firing rate while also extracting enough from the building to keep it under suction. If the two demands differ, a diverter to a biofilter or scrubber takes the surplus so that neither duty is compromised.
Energy and control
Using building air as combustion air destroys the odour at no extra fuel cost, but only up to the burner's air demand. Matching extraction volume to what the oxidiser can burn, and closing hoods on idle equipment, keeps the fan from moving air that the oxidiser must then heat. Speed control on burner demand, rather than a throttled damper, saves power through the batch cycle.
ID fan and FD fan in odour thermal oxidisers for rendering plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the waste heat boiler or exchanger | Ahead of the oxidiser burner |
| What it handles | Oxidised flue gas with SO₂ and high moisture | Humid, odorous building air |
| Temperature at the fan | 180 to 250°C | Ambient to about 45°C |
| Main design concern | Acid dew point corrosion | Wet corrosion and grease |
| Control | Oxidiser chamber pressure | Burner air demand and building extraction |
See also the Induced Draft (ID) Fan for odour thermal oxidisers for rendering plants.
Questions buyers ask
Can building air be used as combustion air in a rendering plant oxidiser?
Yes. Using building air as combustion air is common in rendering plants and destroys the odour in that air without extra fuel. The combustion air fan then handles humid, odorous air carrying ammonia and H₂S, so it needs corrosion-resistant construction, a casing drain and a grease filter on the inlet to keep deposits off the impeller.
What material should the FD fan on an odour oxidiser be made of?
When the fan draws foul building air, stainless steel, usually SS 304 or SS 316, or an FRP casing with a stainless impeller is usual. Mild steel corrodes quickly in the warm, wet, ammonia-bearing air of a rendering plant. If the fan draws only clean ambient air, mild steel is adequate for the combustion air duty.
How much air does the combustion air fan on a thermal oxidiser supply?
It supplies the combustion air for the burner plus any extra air needed to hold the chamber oxygen level for complete oxidation, typically several percent excess oxygen. In rendering plants it is often sized to take as much of the building extraction as the oxidiser can accept, with the remainder sent to a biofilter or scrubber.
Information needed to quote an FD fan for odour thermal oxidisers for rendering plants
- 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.
- Building extraction. Areas extracted, air volume and whether it is to be used as combustion air.
- Burner data. Burner type, fuel and the combustion air pressure needed at full firing.
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.