How poultry litter fired boilers work
Poultry litter is the mix of droppings and bedding, usually rice husk, sawdust or wood shavings, removed from broiler sheds between flocks. It is fired in dedicated power and steam plants on a moving grate or a bubbling fluidised bed. As fired it carries 20 to 40% moisture, 10 to 25% ash and a calorific value of about 2,500 to 3,500 kcal/kg. It is far richer in nitrogen, chlorine, sulphur, phosphorus, potassium and calcium than wood or husk, and it smells of ammonia. The ash is a saleable fertiliser. The flue gas passes the superheater, economiser and air heater, then dry lime or sorbent injection and a bag filter to remove HCl, SO₂ and dust, before the ID fan.
Where the ID fan sits
- Grate or fluidised bed
- Superheater
- Economiser
- Air heater
- Sorbent injection
- Bag filter
- ID fan
- Chimney
The fan follows the bag filter, after the lime or sodium bicarbonate injection that removes most of the acid gases. Dust at the fan is low, but it is fine, salty and absorbs moisture. Suction at the fan is typically 250 to 350 mmWC, because the bag filter with its sorbent cake adds a high and rising resistance. The gas leaves the air heater at a temperature set to stay above the acid dew point, and the fan must not cool it further.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater and bag filter |
| Maximum or upset | Up to 220°C with fouled surfaces |
| Dust at the fan | Low after the bag filter; fine salts and sorbent that absorb moisture |
| Moisture and dew point | Water vapour 15 to 25%; HCl and SO₃ give an acid dew point that can exceed 120°C |
| Corrosive species | HCl and SO₂, reduced by sorbent but not removed; ammonia slip where SNCR is used for NOx |
| Density at the design temperature | About 0.81 kg/m³ at 165°C, 68% 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 poultry litter fired boilers
| Blade form | Backward curved, for efficiency on clean, corrosive gas |
| Material of construction | Mild steel or corrosion-resistant steel, fully insulated; SS 316 for drains, inlet box and shaft seal |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Gas-tight shaft seal, casing drain, insulation and cladding |
What this ID fan has to cope with
Acid condensation at low load
With an acid dew point that can exceed 120°C, any part of the fan cooled by ambient air, such as the inlet box, the shaft seal or an uninsulated casing patch, collects acid condensate. Full insulation and cladding, an SS 316 seal and drain, and a minimum gas temperature at low load keep metal above the dew point.
Hygroscopic ash at shutdown
Litter ash is rich in potassium and calcium chlorides that absorb moisture from the air once the fan stops. Wet deposits become corrosive paste. Purging the fan with warm air before shutdown, or washing and drying it on longer stops, prevents corrosion pits that later grow into cracks at the blade roots.
Rising bag filter resistance
Sorbent injection builds a thick filter cake, and the filter resistance rises between cleaning pulses and as the bags age. The fan must have pressure in hand for the end-of-life bag condition. A drive that adds speed as resistance rises keeps furnace draft steady without running the fan fast all the time.
Energy and control
Litter-fired plants run at steady base load, since fuel arrives continuously from sheds and cannot be stored for long. The ID fan is a large auxiliary, and much of its pressure is spent on the bag filter. Keeping the bag filter cleaning cycle and sorbent dosing well tuned reduces the resistance the fan has to overcome, and a variable speed drive turns that into a power saving.
ID fan and FD fan in poultry litter fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Drawing from the fuel hall, before the air heater |
| What it handles | Flue gas with HCl, SO₂ and hygroscopic ash | Hall air with ammonia and litter dust |
| Temperature at the fan | 150 to 180°C | Ambient to about 45°C |
| Main design concern | Acid corrosion | Dust build-up and ammonia |
| Control | Furnace draft | Air flow, oxygen and hall suction |
See also the Forced Draft (FD) Fan for poultry litter fired boilers.
Questions buyers ask
Why does the ID fan corrode on a poultry litter fired boiler?
Litter contains chlorine and sulphur that form HCl and SO₃ in the flue gas, giving an acid dew point well above that of wood firing. Where the induced draft fan casing or seals run cool, acid condenses. Insulation, stainless drains and seals, and a minimum gas temperature prevent it.
Is a stainless steel ID fan needed for poultry litter?
Not for the whole fan if the gas stays above its acid dew point in service. The impeller and casing can be mild or corrosion-resistant steel, fully insulated. Stainless steel is used for the drain, inlet box and shaft seal, where condensation happens first. Keeping the gas temperature up at low load matters more than the material choice.
What dust control is used before the ID fan on a litter fired boiler?
Usually dry sorbent injection with lime or sodium bicarbonate followed by a bag filter, which removes dust and most of the acid gas. The ID fan then handles low dust but must overcome the bag filter's high resistance, which rises as the sorbent cake builds and as the bags age, so the fan is sized on the end-of-life bag condition.
Information needed to quote an ID fan for poultry litter fired boilers
- 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.
- Acid gas data. HCl and SO₂ at the boiler outlet and after the bag filter, if measured.
- Gas cleaning. Sorbent type, bag filter pressure drop clean and at bag end of life.
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.