How bagasse travelling grate boilers work
Travelling grate bagasse boilers, typically 40 to 150 TPH, spread bagasse pneumatically across a slowly moving grate that carries ash continuously to the front of the furnace. Most of the fine bagasse burns in suspension; the coarser fibre lands on the grate and burns out as it travels. Continuous ash discharge gives steady furnace conditions without the cycles of a dumping grate. Bagasse at about 50% moisture produces a large, wet flue gas flow, which passes the superheater, boiler bank, economiser and a large air heater before an ESP. Two ID fans in parallel are common, with FD, secondary air and distributor air fans on the air side.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate duct
- Zone dampers
- Travelling grate
The FD fan, or two fans in parallel on larger units, pushes ambient air through the tubular air heater into a duct that feeds several compartments under the grate. Each compartment damper sets air along the grate length. A separate secondary air fan supplies overfire nozzles, and a distributor air fan powers the pneumatic bagasse spreaders. Undergrate air is typically 40 to 60% of the total, with the rest above the grate.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 200 to 250°C, downstream of the fan |
| Pressure at the fan outlet | About 150 to 250 mmWC |
| Share of total air | About 40 to 60% as undergrate air |
| Dust | Bagasse pith around the intake |
| 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 bagasse travelling grate boilers
| Blade form | Backward curved or aerofoil, for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled with pedestal bearings |
| Bearings and cooling | Standard bearings, vibration monitoring on larger fans |
| Flow control | Variable speed drive on steam pressure with oxygen trim |
| Also specified | Inlet screen and silencer; outlet isolation damper where two fans run in parallel |
What this FD fan has to cope with
Uneven air across a wide grate
Wide travelling grates need even air along and across their length. If plenum pressure sags at part load, the edge zones and the rear of the grate starve, and bagasse piles there and burns poorly. A fan with a steadily rising pressure curve, controlled to plenum pressure rather than by a single damper, keeps air distribution even.
Air heater erosion and leakage
Sand in the flue gas erodes the inlet ends of air heater tubes, and leaking tubes pass FD air directly to the gas side. FD flow and pressure must include this leakage, which grows over the season, otherwise the grate runs short of air in the last months before the overhaul. The same leakage adds to ID fan load.
Coordination with secondary air
If the FD fan supplies too much of the total air, overfire turbulence weakens and unburnt fibre carries over to the air heater and ESP, where it can catch fire. The FD and secondary air fans are sized and controlled together, so that both the total air and the split between them can be held.
Energy and control
The FD fan on a travelling grate bagasse boiler runs at high load for months. A variable speed drive with oxygen trim keeps excess air low as bagasse moisture changes, which saves power on the FD fan and, more importantly, reduces wet gas volume through the ID fans. Clean inlet screens and tight air heater tubes add further savings.
ID fan and FD fan in bagasse travelling grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After ESP, two fans in parallel | Before the air heater and grate zones |
| What it handles | Large wet flue gas flow, low dust | Ambient air |
| Temperature at the fan | 140 to 170°C | Ambient, 25 to 45°C |
| Main design concern | Efficiency and parallel operation | Even air along a wide grate |
| Control | Furnace draft, VFDs on both fans | Steam pressure with oxygen trim |
See also the Induced Draft (ID) Fan for bagasse travelling grate boilers.
Questions buyers ask
How much air does the FD fan supply on a bagasse travelling grate boiler?
Typically 40 to 60% of the total combustion air is supplied under the grate by the FD fan. The rest is overfire or secondary air and distributor air, from separate fans at higher pressure. The split depends on the grate design, bagasse fineness and how much of the fuel burns in suspension before reaching the grate.
Does the FD fan on a bagasse boiler need two fans?
Larger boilers, typically above about 60 to 80 TPH, often use two FD fans in parallel so the boiler can continue at reduced load if one is out of service. Smaller boilers commonly use a single fan with a robust motor and a well-maintained bearing arrangement. The choice depends on how much a boiler trip costs the mill.
Why does undergrate air pressure drop at part load?
With a fixed speed fan and damper control, plenum pressure falls as dampers close and the fan moves along its curve. Air then favours the zones nearest the duct inlet. Controlling FD fan speed to hold plenum pressure keeps air even along the grate at all loads, and the zone dampers then only set the split.
Information needed to quote an FD fan for bagasse travelling grate 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 in the design.
- Grate zones. Number of undergrate compartments and design plenum pressure.
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.