How agro-Residue pulp mill boilers work
Agro-residue pulp and paper mills make pulp from wheat straw, rice straw, bagasse or sarkanda grass instead of wood. Their steam and power boilers, usually AFBC or travelling grate units of 10 to 60 TPH, burn the mill's own residues: bagasse pith from depithing, straw dust and rejects, rice husk bought in, and coal for support. These fuels differ sharply. Wet pith can be 50% moisture, straw ash is rich in potassium and chloride and melts at low temperature, and husk ash is mostly silica. Flue gas passes the boiler bank, economiser and air heater at 150 to 180°C, then an ESP or bag filter before the ID fan. Chemical recovery from silica-rich agro black liquor is a separate and harder problem, often handled by a separate boiler.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Air box
- Bed nozzles or grate
- Furnace
The FD fan sits at ground level near the air heater and pushes ambient air through it into the air box. On an AFBC boiler the air passes the distributor nozzles and fluidises the bed, so outlet pressure is high, typically 600 to 900 mmWC. On a travelling grate it supplies undergrate air at lower pressure. A separate secondary air or pneumatic fuel distribution fan is common, because pith and straw burn partly in suspension.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 130 to 180°C, downstream of the fan |
| Dust | Pith fibre and straw dust in the boiler house; inlet screened |
| Pressure at the fan outlet | 600 to 900 mmWC on AFBC; 100 to 200 mmWC on a travelling grate |
| Flow range | Varies with fuel moisture and mix |
| 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 agro-Residue pulp mill boilers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on steam pressure and oxygen, with inlet damper for start-up |
| Also specified | Inlet screen with an intake away from fuel handling, and an inlet silencer |
What this FD fan has to cope with
Pith and straw fibre in the fan
Boiler houses in agro mills are thick with fibre from fuel handling. An unscreened fan collects fibre on its blades, loses flow and goes out of balance. A screened intake drawn from outside the fuel yard prevents it. The screen is cleaned on a fixed schedule, since a choked screen reduces air.
Bed fluidisation with mixed fuels
On AFBC boilers air must keep the bed fluidised while the fuel mix shifts. Light pith tends to burn above the bed, and coal in it. The fan must hold enough pressure at low flow to keep the bed fluid, so a stable, rising curve matters more than peak efficiency.
Air heater leakage
Tubular air heaters on these boilers suffer cold-end corrosion from chlorides, and leaking tubes pass FD air straight to the ID fan. The FD fan is sized with the measured leakage, and leaks are reported, since they also load the ID fan. Leaks rise over time.
Energy and control
Steam demand from the paper machines is steady, but air demand moves with fuel moisture and mix. A variable speed drive on the FD fan tied to oxygen trim keeps excess air under control and avoids the high dry gas loss that comes with throttled fixed-speed fans running at too much air. On AFBC units, minimum fluidising flow sets a floor below which the fan must not go.
ID fan and FD fan in agro-Residue pulp mill boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter, before the chimney | At the air heater inlet, feeding the air box |
| What it handles | Flue gas with mixed agro ash | Ambient air with fibre risk |
| Temperature at the fan | 150 to 180°C | Ambient |
| Main design concern | Wet fuel gas volume and alkali deposits | Fluidising pressure and clean intake |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Induced Draft (ID) Fan for agro-Residue pulp mill boilers.
Questions buyers ask
What pressure does the FD fan on an AFBC boiler firing pith need?
Typically 600 to 900 mmWC to cover the air heater, air box, distributor nozzles and bed. The exact figure depends on bed depth and nozzle design. A travelling grate boiler needs much lower pressure. Distributor nozzle pressure drop should be measured on the running boiler, as nozzles erode or block over time.
Why does my FD fan lose air in a bagasse pith fired boiler?
Most often fibre has built up on the impeller or inlet screen, or the air heater is leaking. Cleaning the fan and screen, and checking air heater leakage, usually restores flow. Taking the intake from outside the fuel handling area prevents it recurring. A choked air heater also adds resistance.
Do agro mill boilers need a separate secondary air fan?
Usually yes. Pith, straw and husk burn partly in suspension, so overfire or secondary air is needed above the bed or grate. It is supplied by a separate fan at higher pressure than the forced draft fan on a grate boiler. It also distributes fuel.
Information needed to quote an FD fan for agro-Residue pulp mill 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.
- Firing system. AFBC or travelling grate, and the bed or grate area.
- Air split. Share of primary and secondary air at full load.
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.