How imported coal chain grate boilers work
Small chain grate boilers firing imported coal, from about 2 to 15 TPH, are common in textile dyeing and process units. Sized Indonesian or South African coal flows from a hopper over a gate onto a chain of cast links, burning as a bed 80 to 120 mm deep as it moves to the ash discharge. Indonesian coal is high in moisture and volatiles and low in ash, so the ash layer protecting the links is thin and the coal ignites quickly at the front. Many of these units have no air heater, only an economiser. The gas passes a multiclone, then a bag filter or a wet scrubber, with the ID fan placed before the scrubber or after the bag filter.
Where the FD fan sits
- Inlet screen
- FD fan
- Undergrate plenum
- Zone dampers
- Chain grate links
- Coal bed
The fan delivers ambient air directly into the undergrate plenum on most small units, without an air heater. The plenum has two to four zones. Pressure needed is modest because the bed is thin, typically 50 to 100 mmWC. Some units have a small overfire air fan; many do not, and that is a common cause of soot with high volatile imported coal.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the fan | Ambient; no air heater on most small units |
| Pressure at the fan outlet | Typically 50 to 100 mmWC |
| Dust | Coal dust in the boiler house |
| Zones | Two to four 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 imported coal chain grate boilers
| Blade form | Backward curved, compact direct-mounted unit |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling |
| Flow control | Variable speed drive on steam pressure |
| Also specified | Inlet screen and zone dampers |
What this FD fan has to cope with
Link overheating with thin ash
Imported coal leaves little ash to shield the chain links, and the rear of the grate carries almost bare links. Throttling rear zones to reduce excess air overheats them. The fan and dampers must deliver cooling air to every zone, even where there is little coal.
Soot from volatiles without overfire air
Volatile imported coal ignites and releases gas quickly at the front of the grate. Without overfire air, much of that gas cracks into soot that coats the tubes and raises stack temperature. Some plants add a small overfire air fan, or a branch from the FD fan with nozzles above the grate, which reduces soot and saves fuel.
Clinker with low fusion ash
Low ash fusion temperatures in many imported coals cause clinker on hot spots in the bed, which blocks the air passages between the links and leaves patches of unburnt coal. The fan needs pressure margin to keep some air flowing through these patches until the clinker reaches the end of the grate and is discharged with the ash.
Energy and control
Small chain grate boilers often run with high excess air and coarse damper control. A variable speed FD fan with oxygen trim reduces excess air, saving fuel and reducing the load on the ID fan. Adding overfire air where it is missing often improves efficiency more than any fan change.
ID fan and FD fan in imported coal chain grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multiclone, before the scrubber or after the bag filter | Beside the boiler, feeding the undergrate plenum |
| What it handles | Hot, humid gas with light ash and soot | Ambient air |
| Temperature at the fan | 170 to 210°C | Ambient |
| Main design concern | Hot gas and scrubber position | Link cooling with a thin bed |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Induced Draft (ID) Fan for imported coal chain grate boilers.
Questions buyers ask
Why do the chain links burn on my imported coal boiler?
Imported coal has little ash, so the links at the rear of the grate are almost bare and exposed to furnace radiation. If the rear zones are throttled to reduce excess air, there is no air left to cool them and they overheat and distort. Keeping enough undergrate air through every zone, including the rear, protects the links.
Does a small imported coal chain grate boiler need overfire air?
It helps considerably. Imported coal is high in volatiles, which are released at the front of the grate and need air above the bed to burn cleanly. Without overfire air, soot and CO rise, tubes foul quickly and efficiency falls. A small fan or a branch from the FD fan with a row of nozzles is usually enough on these boilers.
What FD fan pressure does a small chain grate boiler need?
Typically 50 to 100 mmWC at the fan outlet with imported coal, because the coal bed is thin and the plenum and dampers are short. Units with a thicker bed or blended coal need the upper end of the range. We confirm the figure from the grate design and measured plenum pressure before selecting or replacing the fan.
Information needed to quote an FD fan for imported coal chain 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.
- Grate data. Grate size, number of zones and plenum pressure.
- Overfire air. Whether overfire air is fitted or planned.
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.