How coal pulsating grate boilers work
A pulsating grate is a stepped, inclined grate, usually water cooled, that is given a short pulse at timed intervals by a pneumatic or hydraulic actuator. Each pulse shakes the coal bed down the slope, so the fuel moves forward in steps rather than continuously. Coal sized about 6 to 25 mm is fed from a hopper at the top; ash falls off the lower end into a pit. Units are mostly small and medium, about 2 to 25 TPH, and often designed to take biomass as well. Undergrate air comes from the FD fan, overfire air from a secondary fan. The gas passes the furnace, boiler bank or smoke tubes, economiser and sometimes an air heater, then a multiclone and bag filter or ESP before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater (if fitted)
- Undergrate plenum
- Pulsating grate
- Fuel bed
The fan sits beside the boiler at floor level and delivers air either directly to the grate plenum or first through a tubular air heater. The plenum is sealed against the moving grate with flexible seals, and these leak more as they wear. The fan outlet pressure is typically 60 to 120 mmWC, set mainly by the bed depth and any air heater. Overfire air comes from a separate, higher pressure fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 100 to 160°C, where an air heater is fitted |
| Dust | Clean air; coal fines in the boiler house |
| Pressure at the fan outlet | Typically 60 to 120 mmWC |
| Grate seal leakage | A few percent of flow, rising as seals wear |
| 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 coal pulsating grate boilers
| Blade form | Backward curved, with a steadily rising pressure curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft for small units; direct coupled above about 30 kW |
| Bearings and cooling | Motor bearings or standard pedestal bearings |
| Flow control | Variable speed drive on steam pressure, trimmed by flue gas oxygen |
| Also specified | Inlet screen and silencer; isolating damper |
What this FD fan has to cope with
Changing bed resistance after each pulse
Just before a pulse the bed is deepest and densest; just after, it is loose. A fan running on a flat part of its curve sees large flow changes as resistance changes. Selecting the duty on the steep, stable part of the curve keeps air flow nearly constant through the cycle.
Grate seal leakage
Worn side seals let air bypass the bed. The fan delivers its rated flow but the fuel receives less, and clinkers form. Measuring plenum pressure and flue gas oxygen together shows when the seals need attention rather than a larger fan. Replacing seals restores the air to the bed at no extra fan power.
Overheated grate bars at low air flow
Even on water cooled grates, the air passing through the bars helps keep them cool. Turning the FD fan too far down at low load can overheat the bars. A minimum speed or minimum air setting protects the grate. The grate supplier usually specifies this minimum, and the drive is set so it cannot go below it.
Energy and control
These boilers often run in a daily cycle, near full load on day shifts and well below at night. A variable speed FD fan tracking steam pressure uses much less power at low load than one held at full speed with a throttled damper. Setting the excess oxygen target correctly, usually about 6 to 8% on coal for this firing system, avoids heating air that the coal does not need.
ID fan and FD fan in coal pulsating grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Below the grate, ahead of the plenum |
| What it handles | Flue gas with fine coal ash | Clean ambient air |
| Temperature at the fan | 160 to 200°C, up to 250°C upset | Ambient, 25 to 45°C |
| Main design concern | Fast draft response to grate pulses | Stable air through a changing bed |
| Control | Furnace draft | Steam pressure and excess oxygen |
See also the Induced Draft (ID) Fan for coal pulsating grate boilers.
Questions buyers ask
What pressure does an FD fan need on a pulsating grate boiler?
Typically 60 to 120 mmWC at the fan outlet, depending on bed depth and whether an air heater is fitted. The exact figure comes from the grate supplier's plenum pressure plus duct and air heater losses, and it should include margin for the deepest bed before a pulse.
Why does my pulsating grate clinker even with the forced draft fan at full speed?
Often the air is not reaching the bed where it is needed. Worn grate seals, a blocked plenum or uneven coal size let air bypass the heavy areas. Check plenum pressure, grate seal condition and coal sizing before increasing fan capacity. A larger fan only pushes more air through the paths that already have too much.
Is a combustion air fan the same as an FD fan?
On a pulsating grate boiler, yes. The FD fan supplies the undergrate combustion air. A separate secondary or overfire air fan supplies air above the grate, and the two are sized together from the boiler's air split. On some smaller units a single fan supplies both, with a branch duct and damper for the overfire air.
Information needed to quote an FD fan for coal pulsating 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.
- Plenum pressure. Design undergrate plenum pressure at full load and with the deepest bed.
- Air heater. Whether an air heater is fitted and its air-side pressure drop.
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.