How imported coal travelling grate boilers work
Travelling grate boilers firing imported coal, typically 6 to 40 TPH in food, beverage and process plants, burn Indonesian, South African or Australian coal on a moving grate. The coal is sized, usually 6 to 32 mm, and fed by a hopper and gate or by spreaders. Indonesian grades are high in volatiles and moisture, low in ash and often low in ash fusion temperature. The low ash leaves a thin protective layer on the grate, and the volatiles release quickly, so overfire air carries a large share of combustion. Flue gas passes the boiler bank, economiser and air heater, then a multiclone and bag filter or ESP, before the ID fan and chimney.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate duct
- Zone dampers
- Travelling grate
The fan pushes ambient air through the air heater to the undergrate zones, typically at 100 to 160 mmWC. An overfire air fan supplies jets above the grate. With imported coal, the overfire share is often 20 to 30% of total air, larger than with Indian coal, and the undergrate air must be distributed so that the grate is never starved of cooling air at the rear zones.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 140 to 190°C, downstream of the fan |
| Pressure at the fan outlet | Typically 100 to 160 mmWC |
| Dust | Coal dust in the boiler house |
| Air split | Undergrate air plus 20 to 30% overfire air |
| 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 imported coal travelling grate boilers
| Blade form | Backward curved for efficiency |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8 or direct mounted |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on steam pressure with oxygen trim |
| Also specified | Inlet screen and zone damper actuators |
What this FD fan has to cope with
Grate cooling with low ash
Imported coal leaves a thin ash layer, so the grate bars are exposed to radiant heat at the rear zones. Undergrate air is the only cooling. If zones are throttled to cut excess air, grate bars overheat and warp. The fan must supply enough air at every zone to keep the bars cool, even where little coal remains.
Clinker from low fusion ash
Many Indonesian coals have ash fusion temperatures around 1,150 to 1,250°C, well below Indian coal. Where the bed burns hot, ash fuses into clinker that blocks the air passages in the grate bars and stops the bed burning out. The fan needs pressure in hand to keep air flowing through partly clinkered areas until they travel off the end of the grate.
Air split between undergrate and overfire
Shifting more air to overfire jets reduces soot and CO from the volatiles, but less undergrate air means less cooling for a grate that already has a thin ash cover. Fans and dampers must allow the two flows to be set independently, and combustion tuning must check both grate bar temperature and flue gas CO before settling on a split.
Energy and control
The FD fan on an imported coal travelling grate boiler should hold excess air low enough to save fuel without overheating the grate. A variable speed drive with oxygen trim does this better than dampers. Correct coal sizing, with limited fines, gives an even bed that needs less excess air.
ID fan and FD fan in imported coal travelling grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP, before the chimney | Before the air heater, feeding the grate zones |
| What it handles | Humid flue gas with light ash and soot | Clean ambient air |
| Temperature at the fan | 150 to 175°C | Ambient |
| Main design concern | Gas volume with coal moisture | Grate cooling with thin ash cover |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Induced Draft (ID) Fan for imported coal travelling grate boilers.
Questions buyers ask
Why do grate bars burn out on imported coal travelling grate boilers?
Imported coal has little ash, so the protective ash layer on the grate is thin, and the bars depend on undergrate air for cooling. If zones are throttled, especially at the rear, the bars overheat. Keeping enough FD air to every zone protects the grate.
How much overfire air does imported coal need?
Typically 20 to 30% of total air, more than with Indian coal, because high volatile imported coal releases much of its combustible gas above the grate. The exact share depends on the coal, the furnace height and the nozzle arrangement, and is set during combustion tuning by measuring CO and oxygen at the boiler outlet across the load range.
Does the FD fan on an imported coal travelling grate boiler handle hot air?
No. On an imported coal travelling grate boiler the FD fan pushes ambient air into the air heater, which heats it to 140 to 190°C downstream of the fan. The fan is therefore selected at ambient density, built in plain mild steel, and needs no high-temperature construction, shaft cooling or thermal expansion allowances.
Information needed to quote an FD fan for imported 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 and overfire air quantities and the overfire fan arrangement.
- Grate condition. Grate bar failures or overheating history, and zone damper arrangement.
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.