How Indian coal chain grate boilers work
Chain grate boilers firing Indian coal are common from about 2 to 20 TPH in textile processing, dyeing and food plants. Sized coal, typically 6 to 25 mm, flows from a hopper over a guillotine gate onto an endless chain of cast links, which carries it slowly through the furnace as a bed 100 to 150 mm deep. Indian coal is high in ash, often 30 to 45%, with a calorific value of about 3,500 to 4,500 kcal/kg, moderate volatiles and a high ash fusion temperature. The thick ash layer protects the links, and most ash falls off the end into the ash pit, but fines and light ash are carried into the gas. Flue gas passes the boiler, often an economiser, then a multiclone and bag filter or wet scrubber, the ID fan and chimney.
Where the FD fan sits
- Inlet screen
- FD fan
- Undergrate plenum
- Zone dampers
- Chain grate links
- Coal and ash bed
The FD fan is usually mounted beside the boiler and delivers ambient air directly into the undergrate plenum, often without an air heater on smaller units. The plenum is divided into two to four zones with dampers. The fan must overcome the duct, dampers, the chain links and the coal and ash bed, typically 60 to 120 mmWC. Coal fines that sift through the links collect in the plenum hoppers.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the fan | Ambient on most small units; 120 to 160°C where an air heater is fitted |
| Pressure at the fan outlet | Typically 60 to 120 mmWC |
| Dust | Coal dust in the boiler house; screened inlet recommended |
| Zones | Two to four undergrate zones with manual or automatic dampers |
| 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 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, arrangement 4 |
| Bearings and cooling | Motor bearings only; no cooling |
| Flow control | Variable speed drive on steam pressure with oxygen trim |
| Also specified | Inlet screen and a plenum with sound sealing at the grate sides |
What this FD fan has to cope with
Thick ash bed and uneven burning
High-ash Indian coal builds a thick layer of ash over the grate as it burns out. Air takes the path of least resistance through thin or burnt-out areas, often at the tail end, while the active burning zone runs short. Zone dampers and enough fan pressure let the operator force air where the coal is actually burning.
Siftings and plenum leakage
Fine coal falls through the chain links into the plenum, and gaps at the grate sides let air bypass the bed. Both waste air and fuel, and a fan that seems undersized is often just leaking. Good side seals, regular siftings removal and sizing on measured, not nominal, plenum pressure help.
Clinker from coal blending
When units blend Indian coal with lower-fusion coal or biomass, the bed can clinker and block air passages. The FD fan then needs more pressure to keep air flowing, and a fan with margin on a stable curve allows the operator to work through the clinker without starving the fire.
Energy and control
Small chain grate boilers often run high excess air because damper control is coarse. A variable speed FD fan with oxygen trim reduces excess air and the gas the ID fan must move, so the saving appears on both fans and in fuel. Coal sizing matters too: too many fines block air flow and raise the pressure the fan needs.
ID fan and FD fan in Indian coal chain grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multiclone or bag filter, before the chimney | Beside the boiler, feeding the undergrate plenum |
| What it handles | Flue gas with hard high-ash coal fly ash | Ambient air |
| Temperature at the fan | 180 to 230°C | Ambient |
| Main design concern | Erosion behind a multiclone and hotter gas | Even air through a thick ash bed |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Induced Draft (ID) Fan for Indian coal chain grate boilers.
Questions buyers ask
How much air pressure does a chain grate boiler FD fan need?
Usually 60 to 120 mmWC for Indian coal on chain grates, covering the ducts, zone dampers, grate links and the coal and ash bed. Thicker beds and coal with many fines need more. We set the figure from the grate design or measured plenum pressure.
Why does my chain grate boiler need zone dampers on the FD air?
Coal burns at different rates along the grate: it ignites near the front, burns hardest in the middle and is mostly ash at the rear. Zone dampers let the operator send more air where it is needed and less where only ash remains, which saves fuel and reduces unburnt carbon.
Can the FD fan on a small coal boiler be removed and run on natural draft?
Some very small, hand-fired units run on natural draft and ID fan suction alone. A mechanical chain grate firing high-ash Indian coal needs forced undergrate air to push air through the thick ash layer and to control combustion along the grate. Removing the FD fan usually cuts capacity, raises unburnt carbon in the ash and makes the boiler harder to control.
Information needed to quote an FD fan for Indian 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 width and length, number of zones and design plenum pressure.
- Coal. Coal source, size range and ash content.
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.