How Indian coal travelling grate boilers work
Travelling grate boilers firing Indian coal cover roughly 10 to 60 TPH, typically in paper, chemical and process plants. Coal is thrown onto the moving grate by pneumatic or mechanical spreaders, so fines burn in suspension while coarser pieces fall to the grate and burn as a thin bed carried slowly to the ash discharge. Indian coal at 30 to 45% ash and 3,500 to 4,500 kcal/kg gives a large ash flow, and spreader firing carries a substantial share of it into the gas, with unburnt carbon that is often reinjected from the boiler hoppers. The gas passes the boiler bank, economiser and tubular air heater to a multiclone and ESP or bag filter, then the ID fan. Undergrate air comes from the FD fan through several zones, with overfire air from a separate fan.
Where the FD fan sits
- Inlet screen and silencer
- FD fan
- Air heater
- Undergrate duct
- Zone dampers
- Travelling grate
The FD fan sits near the air heater and pushes ambient air through it, then into an undergrate duct feeding four to six zones. A separate overfire air fan supplies jets above the grate for the suspension-burning fines, and a high pressure fan supplies reinjection and spreader air. The FD fan pressure covers the air heater, duct, zone dampers, grate and bed, typically 100 to 180 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 150 to 200°C, downstream of the fan |
| Pressure at the fan outlet | Typically 100 to 180 mmWC |
| Dust | Coal dust in the boiler house |
| Zones | Four to six undergrate zones |
| 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 Indian coal travelling grate boilers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on steam pressure with oxygen trim |
| Also specified | Inlet screen and silencer, and zone damper actuators |
What this FD fan has to cope with
Thin, uneven spreader bed
Spreaders distribute coal unevenly across the grate width and length, especially with wet or poorly sized coal. Thin areas let air through freely and thick areas starve. The fan must hold pressure across the zones so dampers can correct the distribution, and a stable curve avoids hunting as the bed changes.
Grate overheating on low-ash spots
Even high-ash Indian coal sometimes arrives with low-ash lots or blended imported coal. A thin ash layer exposes the grate bars to radiant heat, and undergrate air is the only cooling. The fan must supply enough air to cool the grate even when the bed is thin.
Air heater leakage
As air heater tubes erode or corrode, the FD fan supplies air that bypasses the grate and goes straight to the gas side. The fan appears short of capacity, the operator opens the zone dampers further, and the ID fan becomes overloaded. Measuring oxygen before and after the air heater identifies leakage early, before either fan is blamed or replaced.
Energy and control
The FD fan on a travelling grate boiler responds to steam demand and should hold the excess air close to what the coal needs. A variable speed drive with oxygen trim reduces excess air and cuts power on both FD and ID fans. Controlling coal size and moisture improves spreader distribution, which allows lower excess air without unburnt carbon.
ID fan and FD fan in Indian coal travelling grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter, before the chimney | Before the air heater, feeding the grate zones |
| What it handles | Flue gas with spreader carry-over ash | Clean ambient air |
| Temperature at the fan | 150 to 180°C | Ambient |
| Main design concern | Ash wear during collector failures | Even zone pressure for a spreader bed |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Induced Draft (ID) Fan for Indian coal travelling grate boilers.
Questions buyers ask
What is the difference between FD and overfire air fans on a travelling grate boiler?
The FD fan supplies undergrate air through the grate to burn the coal bed. The overfire air fan supplies high velocity jets above the grate to burn the fines and volatiles released by spreader firing, and to mix the furnace gases. On high ash Indian coal both are needed for good burnout and low unburnt carbon in the fly ash.
How much undergrate air pressure does an Indian coal travelling grate boiler need?
Typically 100 to 180 mmWC at the FD fan outlet, depending on the air heater, the ducts and zone dampers, the grate bar type and the bed thickness. Spreader-fired beds are thin, so most of the pressure is in the air heater and dampers. We confirm the figure from the boiler design and from measured zone pressures on site.
Does the FD fan on a travelling grate boiler handle hot air?
No. On an Indian coal travelling grate boiler the FD fan pushes ambient air into the tubular air heater, which heats it to 150 to 200°C downstream of the fan. The FD fan is therefore selected at ambient density and needs no high-temperature construction, which keeps it smaller and simpler than the ID fan on the same boiler.
Information needed to quote an FD fan for Indian coal 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. Undergrate, overfire and reinjection air quantities.
- Grate zones. Number of zones and design zone pressures.
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.