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 ID fan sits
- Cooker vapour condenser
- Non-condensable gas line
- Thermal oxidiser
- Waste heat boiler or exchanger
- ID fan
- Stack
The fan sits after the heat recovery stage, where the gas has been cooled from about 800°C to 180 to 250°C. It holds the oxidiser at a slight suction, typically −2 to −5 mmWC, so that odorous gas cannot leak from burner ports or inspection doors. Its suction covers the chamber, the boiler or exchanger and any economiser, usually 100 to 200 mmWC. A fan ahead of the oxidiser would leak foul gas from every joint, which is why the hot side is under induced draft.
Gas at the ID fan
| Gas temperature, normal | 180 to 250°C after the waste heat boiler |
| Maximum or upset | Up to 350°C with boiler tubes fouled or a bypass open |
| Dust at the fan | Very low; fat aerosols are destroyed in the chamber |
| Moisture and dew point | High water vapour from cooker gas and combustion, often 15 to 25% by volume |
| Corrosive species | SO₂ and some SO₃ from oxidised H₂S and mercaptans; NOx from ammonia |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 odour thermal oxidisers for rendering plants
| Blade form | Backward curved, on clean gas with no dust |
| Material of construction | Mild steel with the gas kept above dew point; SS 316 impeller and casing lining where the plant cycles often |
| Drive and arrangement | Direct drive, arrangement 8, insulated casing |
| Bearings and cooling | Shaft cooling disc; standard bearings at this temperature |
| Flow control | Variable speed drive on chamber pressure |
| Also specified | Casing drain, insulated inlet duct and a warm purge before shutdown |
What this ID fan has to cope with
Sulphuric acid corrosion
Rendering gas carries sulphur from protein, so the flue gas contains SO₂ and some SO₃, and the high moisture raises the acid dew point. If the heat recovery cools the gas too far, or the fan stands cold between cooking batches, acid forms on the casing and impeller. Keeping the outlet temperature above dew point and using stainless parts where the plant cycles daily controls the damage.
Odour escape on low draft
If the fan cannot hold suction when cooker vapour surges at batch discharge, foul gas leaks from the chamber and boiler casing, the very complaint the plant installed the oxidiser to solve. The fan is sized for the peak non-condensable flow plus building air, with speed control fast enough to follow the batch cycle.
Deposits after a burner flame-out
If the oxidiser temperature drops after a burner flame-out, fat aerosols and partly burnt organics pass through and coat the boiler tubes and the fan. A low-temperature interlock that diverts the odour gas to a standby scrubber, and an inspection door on the fan casing, prevent sticky build-up from unbalancing the impeller.
Energy and control
Rendering is batch work: cookers are filled, heated and discharged, so the odour gas flow swings within each cycle while the building air is fairly steady. Speed control on chamber pressure lets the ID fan follow these swings instead of throttling. The bigger energy lever is the waste heat boiler, which turns oxidiser fuel into process steam; the fan must be selected for the boiler's real fouled resistance to keep that recovery working.
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 Forced Draft (FD) Fan for odour thermal oxidisers for rendering plants.
Questions buyers ask
Why does a rendering plant thermal oxidiser need an ID fan?
The ID fan keeps the oxidiser and the heat recovery equipment under suction, so odorous gas cannot leak out of burner ports, doors and casing joints. It also overcomes the resistance of the waste heat boiler or exchanger. Natural draft from the stack is rarely enough once heat recovery is added to a rendering plant oxidiser.
What causes corrosion in the ID fan after an odour oxidiser?
Sulphur compounds in the rendering gas, mainly H₂S and mercaptans, burn to SO₂ and some SO₃. With high moisture in the flue gas the acid dew point is raised, and sulphuric acid condenses wherever the fan casing or impeller falls below it, especially during shutdowns between cooking batches or when heat recovery cools the gas too far.
What temperature should the ID fan on an odour oxidiser be designed for?
With a waste heat boiler the gas usually reaches the ID fan at 180 to 250°C. The fan should also be rated for the upset case, with fouled boiler tubes or a bypass open, which is often 300 to 350°C. The impeller stress and bearing cooling are checked at that upset temperature, not only the normal one.
Information needed to quote an ID fan for odour thermal oxidisers for rendering plants
- 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.
- Odour gas sources. Flow of cooker non-condensables and building air, and how they vary through the batch cycle.
- Heat recovery. Type of boiler or exchanger, its pressure drop and the outlet gas temperature.
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.