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 FD fan sits
- Fuel storage hall extract
- Inlet filter
- FD fan
- Air heater
- Undergrate plenum or air distributor
- Grate or fluidised bed
The FD fan takes air from ducts at the roof of the litter storage hall, through a coarse filter, and pushes it through the air heater to the grate or bed. Drawing the air from the hall keeps the building at negative pressure and burns the odour. On a grate the fan works against 150 to 250 mmWC; on a bubbling bed, 700 to 1,000 mmWC. A secondary air fan, often also drawing from the hall, feeds the overfire nozzles.
Air at the FD fan
| Air temperature at the fan | Ambient to about 45°C, warmer when litter in the hall is composting |
| Air after the air heater | 150 to 200°C, downstream of the fan |
| Dust | Fine litter dust, feathers and bedding fragments |
| Humidity and gases | Humid air with ammonia from the stored litter |
| Pressure at the fan outlet | 150 to 250 mmWC for a grate; 700 to 1,000 mmWC for a fluidised bed |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 poultry litter fired boilers
| Blade form | Backward curved, with smooth blades that do not hold dust |
| Material of construction | Mild steel with an epoxy or similar protective coating; no copper alloys in contact with the air |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Standard bearings, sealed against dust |
| Flow control | Variable speed drive on air flow with oxygen trim |
| Also specified | Coarse inlet filter with differential pressure alarm, access door and casing drain |
What this FD fan has to cope with
Dust and feathers on the impeller
Air from the litter hall carries dust, feathers and bedding fragments that build up on the impeller and filter. Without a filter the wheel unbalances within weeks. A coarse filter sized for low velocity, with an alarm on its pressure drop, and an access door for cleaning the wheel keep the fan in balance.
Ammonia and humid air
Stored litter releases ammonia and moisture. Ammonia attacks copper and brass fittings, and humid air condenses in cold ducts. A coated steel fan without copper alloy parts, sealed bearings, and a drain at the low point of the casing handle the condition. Insulating the duct from the hall where it runs outdoors limits condensation in cooler months.
Hall suction and combustion demand
The fan must hold the hall at negative pressure and supply combustion air at the same time. When the boiler turns down, the hall extract falls too. A bypass or a separate hall extract fan to the furnace covers low load without upsetting the excess air.
Energy and control
Litter plants run steadily at base load, so the FD fan runs near one point. A variable speed drive with oxygen trim keeps excess air near design and avoids throttling losses. Because the fan also ventilates the fuel hall, minimum speed is set by the hall suction needed, and the selection must give good efficiency at that point as well as at full load.
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 Induced Draft (ID) Fan for poultry litter fired boilers.
Questions buyers ask
Why does a poultry litter boiler draw combustion air from the fuel store?
Stored litter releases ammonia and odour. Drawing the forced draft fan air from the storage hall keeps the building under suction so odour does not escape, and the ammonia is burnt in the furnace. It also removes dust that would otherwise settle in the hall.
What material is used for the FD fan on a litter fired boiler?
Coated mild steel is usual, with no copper or brass parts, since ammonia attacks copper alloys. The air drawn from the litter hall is humid and dusty, so the casing has a drain and an access door for cleaning, and the bearings are sealed against dust. Stainless steel is rarely needed.
Does the FD fan on a poultry litter boiler need a filter?
Yes. Air from the litter hall carries dust, feathers and bedding fragments. A coarse filter with a pressure drop alarm protects the impeller and the air heater from build-up. The filter's pressure drop at its dirty condition must be included in the FD fan pressure, or airflow falls as it loads.
Information needed to quote an FD fan for poultry litter fired boilers
- 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 source. Whether combustion air is drawn from the fuel hall, and the hall volume and suction required.
- Firing system. Grate or fluidised bed, and the share of secondary air.
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.