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 ID fan sits
- Spreader stoker and travelling grate
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan follows the ESP or bag filter, with a multiclone ahead of them that also feeds cinder reinjection. Because the air heater, two collectors and a spreader furnace all add resistance, suction at the fan inlet is typically 200 to 300 mmWC on 20 to 40 TPH units. The fan handles gas at 150 to 180°C after the air heater, much cooler than on small chain grate boilers.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 220°C with fouled air heater tubes |
| Dust at the fan | Fine, hard coal ash with some char; 50 to 150 mg/Nm³ after the ESP, much more during field trips |
| Moisture and dew point | Water dew point 40 to 50°C |
| Corrosive species | Low sulphur; cold-end air heater corrosion more likely than fan corrosion |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 Indian coal travelling grate boilers
| Blade form | Backward curved with wear liners behind an ESP or bag filter |
| Material of construction | Mild steel casing; impeller with wear-resistant liners on the blade leading edge and the backplate |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc and vibration monitoring |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Liners at the scroll cut-off, access door, and margin for reinjection air |
What this ID fan has to cope with
Spreader carry-over and ESP trips
Spreader firing throws a large share of high-ash Indian coal into suspension, so the dust load ahead of the collectors is heavy. When an ESP field trips or a bag fails, the fan suddenly sees grams per cubic metre of hard ash. Wear liners sized for these periods, and an interlock or alarm on collector failure, prevent rapid blade loss.
Reinjection adds air and dust
Cinder reinjection blows char from the boiler and multiclone hoppers back into the furnace with high pressure air. This adds air to the gas volume and, if the multiclone is worn, recirculates fine ash that raises the load on the ESP and the fan. The fan duty includes reinjection air.
Air heater leakage raises the gas volume
Tubular air heaters on coal travelling grate boilers erode and corrode at the cold end. Leaking tubes pass FD air directly to the gas side, so the ID fan moves more gas while the grate receives less. Fans that run at full speed on an otherwise healthy boiler often point to air heater leakage.
Energy and control
Travelling grate boilers in paper and process plants run around the clock, often at 60 to 90% load. A variable speed ID fan on furnace draft typically saves a third of the power used with damper control. Fixing air heater leakage and sealing ash and reinjection hoppers can reduce the gas volume by 10% or more, which shows up directly in fan power.
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 Forced Draft (FD) Fan for Indian coal travelling grate boilers.
Questions buyers ask
How is a travelling grate ID fan different from a chain grate ID fan on Indian coal?
Travelling grate boilers are usually larger and spreader fired, with air heaters and ESPs. The fan handles cooler gas, but the carry-over from spreader firing is heavier, so collector failures hit it harder. Chain grate boilers are smaller, often without air heaters, and their fans run hotter.
Why does my ID fan current rise when the ESP trips?
The fan sees much more dust, and the gas density and volume do not change much, so power rises a little. The bigger concern is wear: a few hours of high dust load can remove more blade metal than months of normal running. Restoring the ESP quickly protects the fan.
What ID fan suction does a 30 TPH coal travelling grate boiler need?
Typically 200 to 300 mmWC at the fan inlet on a 30 TPH unit, depending on the air heater, the multiclone and the ESP or bag filter, and on how much reinjection air enters the furnace. We calculate the duty from the boiler data and fuel analysis, and check it against measured suction and gas temperature on existing plants before selecting.
Information needed to quote an ID fan for Indian coal travelling grate 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.
- Firing system. Spreader type, reinjection system and reinjection air quantity.
- Collector performance. ESP or bag filter outlet dust, and frequency of field trips or bag failures.
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.