How high pressure bagasse cogeneration boilers work
High pressure bagasse cogeneration boilers raise steam at about 65 to 110 bar and 485 to 540°C for back-pressure or extraction-condensing turbines that supply the sugar mill and export power to the grid. Units of 80 to 250 TPH, usually on travelling or dumping grates with pneumatic spreaders, are typical. Because exported power is sold, auxiliary power is closely watched and efficiency equipment such as large economisers and air heaters is used, bringing exit gas down to about 140 to 160°C. An ESP cleans the gas before the ID fans. Many plants run through the off-season on stored bagasse, cane trash or other biomass, and some on coal where permitted.
Where the FD fan sits
- Inlet silencer
- FD fans
- Air heater
- Undergrate air duct
- Grate zones
- Furnace
Two FD fans, each sized for about 55 to 60% of full flow, draw ambient air through silencers and push it through a large tubular air heater into the zoned undergrate plenum. Air leaves the air heater at about 220 to 280°C. Secondary air fans supply high-velocity overfire nozzles, and distributor air fans power the pneumatic spreaders, so the FD fans carry only part of the total combustion air.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 220 to 280°C, downstream of the fans |
| Pressure at the fan outlet | About 200 to 300 mmWC |
| Arrangement | Two fans in parallel, each about 55 to 60% |
| Duty variation | Season and off-season fuels, varying load |
| 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 high pressure bagasse cogeneration boilers
| Blade form | Aerofoil backward curved, for high efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled with pedestal bearings |
| Bearings and cooling | Temperature and vibration monitoring |
| Flow control | Variable speed drives with oxygen trim |
| Also specified | Inlet silencers, isolation dampers, non-return dampers on parallel fans |
What this FD fan has to cope with
Auxiliary power
Large FD fans running throttled at part load waste power that could otherwise be exported, and over a long season the loss is substantial. Aerofoil impellers with variable speed drives, selected for the normal operating point, recover most of it. On older plants, re-rating the impeller to the real measured duty is often cost-effective.
Off-season fuels need different air
Off-season biomass or coal needs a different air quantity and a different split between undergrate and secondary air than bagasse does, usually less total air because the fuel is drier. The FD fans must handle both cases without running far from best efficiency. We check both duty points at selection and confirm motor rating at the higher one.
Parallel fan sharing
Two FD fans on a common duct must share flow evenly and avoid stall at low load, and a stopped fan must not let air flow backwards through it. Matched fan curves, coordinated speed control from one controller, and non-return or isolation dampers on each fan make parallel operation and single-fan running stable.
Energy and control
As with the ID fans, FD fan power is judged against exported electricity. Variable speed drives with oxygen trim, efficient aerofoil impellers, low air heater leakage and correct air split between undergrate and secondary air give the lowest consumption. Where loads drop in the off-season, operating a single FD fan near its best point is usually more efficient than two at low output.
ID fan and FD fan in high pressure bagasse cogeneration boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After ESP, two fans in parallel | Before air heater, two fans in parallel |
| What it handles | Wet flue gas, low dust | Ambient air |
| Temperature at the fan | 140 to 160°C | Ambient, 25 to 45°C |
| Main design concern | Efficiency and reliability for export power | Efficiency and air split |
| Control | VFDs on furnace draft | VFDs with oxygen trim |
See also the Induced Draft (ID) Fan for high pressure bagasse cogeneration boilers.
Questions buyers ask
How many FD fans does a 150 TPH bagasse cogeneration boiler need?
Usually two FD fans in parallel, each about 55 to 60% of full flow. This keeps the boiler running at reduced load if one fan is out of service and keeps each fan to a practical size. Exact sizing comes from the boiler air system design.
Why are aerofoil fans used for FD duty on cogeneration boilers?
The FD fans handle clean ambient air, so aerofoil blades can be used without erosion risk. They give the highest efficiency, which matters where every kilowatt saved can be exported, and they have a stable curve suitable for parallel operation. The extra cost over a plain backward curved wheel is usually recovered quickly.
What is the FD fan temperature on a high pressure bagasse boiler?
The fan itself handles ambient air at about 25 to 45°C, because it sits ahead of the air heater. The air is heated to about 220 to 280°C in the air heater downstream of the fan. This keeps the fan smaller, cooler and simpler than one placed in hot air, and lets it work at ambient density.
Information needed to quote an FD fan for high pressure bagasse cogeneration 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 split. Shares of undergrate, secondary and distributor air for each fuel.
- Redundancy. Required capacity of one fan running alone.
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.