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 ID fan sits
- Furnace and grate
- Superheaters
- Boiler bank
- Economiser
- Air heater
- ESP
- ID fans
- Chimney
Two ID fans, each typically 55 to 60% of full flow, follow a multi-field ESP and discharge to a tall chimney. Suction at the fan inlet is usually 250 to 350 mmWC, made up of the furnace, superheater, bank, a large economiser, a tubular air heater and the ESP. With low dust after the ESP and a clean, steady gas, the fans can use high efficiency blading. Each fan is isolated by dampers so that one can be repaired at reduced load.
Gas at the ID fan
| Gas temperature, normal | 140 to 160°C after the air heater |
| Maximum or upset | Up to 200°C with fouled heat surfaces or ESP bypass |
| Dust at the fan | Low after ESP, typically 30 to 50 mg/Nm³ |
| Moisture and dew point | About 20 to 25% water vapour on bagasse; lower on off-season fuels |
| Fuel range | Bagasse in season; cane trash, other biomass or coal off-season |
| Density at the design temperature | About 0.84 kg/m³ at 150°C, 70% 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 high pressure bagasse cogeneration boilers
| Blade form | Aerofoil backward curved, for highest efficiency on clean gas, with leading edge protection |
| Material of construction | Mild steel; liners at the leading edge |
| Drive and arrangement | Direct coupled, double inlet double width, pedestal bearings |
| Bearings and cooling | Sleeve or anti-friction bearings with temperature and vibration monitoring |
| Flow control | Variable speed drives on both fans, coordinated from furnace draft |
| Also specified | Inlet and outlet isolation dampers, inlet boxes designed for even flow, performance test points |
What this ID fan has to cope with
Auxiliary power eating export revenue
An ID fan running a few points below its best efficiency, or throttled by dampers, wastes power every hour of a long season. Aerofoil impellers selected so the normal operating point sits near peak efficiency, and variable speed drives, directly increase exportable power. This is often the main case for a retrofit.
Different gas volumes in and out of season
Bagasse gives a large wet gas volume; drier off-season fuels give less gas at different temperature. A fan selected only for bagasse runs far from its best point in the off-season. We check efficiency at each fuel case and select so both fall in a reasonable range.
Parallel operation at low load
At low off-season load, two fans in parallel can each be pushed toward the stall region and start to hunt, with flow swinging between them. Control logic that drops to one fan below a set load, with that fan selected to carry the load alone near its best efficiency, avoids instability and saves power at the same time.
Trips cost power sales
An ID fan trip takes down the boiler, the turbine and the power export, and on a grid-connected plant that can also bring penalties. Bearing temperature and vibration monitoring, inlet boxes designed for even flow, and planned in-situ balancing at each overhaul reduce unplanned stops and catch developing faults early.
Energy and control
Cogeneration plants judge the ID fans by kilowatts per tonne of steam and by export lost to auxiliaries. Variable speed drives on both fans, aerofoil impellers matched to the real operating point, low air heater leakage and well-controlled excess air together decide that figure. Where an older plant has oversized fans running on dampers, a new impeller or a drive retrofit usually shows a short payback through extra export.
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 Forced Draft (FD) Fan for high pressure bagasse cogeneration boilers.
Questions buyers ask
How much power does the ID fan use on a bagasse cogeneration boiler?
It is typically one of the two largest auxiliary loads on the boiler, with the FD fan, and its share of gross generation is significant. The exact figure depends on gas volume, suction and fan efficiency. We can estimate it from boiler data or measured fan current and pressure.
Is an aerofoil ID fan suitable for a high pressure bagasse boiler?
Yes, behind a well-performing ESP. The dust at the fan is low enough, and aerofoil blades give the highest efficiency available, which matters directly for export power. Leading edge protection is added because field sand reaches the fan whenever ESP performance drops, and a hollow aerofoil blade must not be allowed to wear through.
Should we run one or two ID fans in the off-season?
Where the off-season load is low, running one fan near its best efficiency is usually better than running two at low output close to stall. The fans, dampers and control logic must be designed for single-fan operation at that load, and the change-over procedure must keep the furnace draft steady.
Information needed to quote an ID fan for high pressure bagasse cogeneration 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.
- Fuel cases. Gas volume and temperature for bagasse and each off-season fuel.
- Power cost. Value of exported power per kWh, used to judge efficiency options.
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.