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 ID fan sits
- Pulsating grate furnace
- Boiler bank
- Economiser
- Multiclone
- Bag filter or ESP
- ID fan
- Chimney
The fan sits after the bag filter or ESP at ground level near the chimney. Upstream are the furnace, the convection surfaces, the economiser and two stages of dust collection, which together give an inlet suction of about 120 to 200 mmWC on a typical unit. With a bag filter, the filter resistance rises between cleaning cycles, adding a slow swing on top of the pulse disturbance. Keeping the fan after the collector limits it to fine residual ash.
Gas at the ID fan
| Gas temperature, normal | 160 to 200°C after the economiser |
| Maximum or upset | Up to 250°C with fouled tubes or when the economiser is bypassed |
| Dust at the fan | Fine coal ash, usually 30 to 100 mg/Nm³ after a bag filter; brief peaks just after each grate pulse |
| Moisture and dew point | Water dew point about 45 to 55°C on coal; rises when wet biomass is mixed in |
| Pressure transients | Short furnace pressure swings of a few mmWC at each pulse |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 coal pulsating grate boilers
| Blade form | Backward curved, with liners at the leading edges for fine coal ash |
| Material of construction | Mild steel casing and impeller; wear-resistant plate on the blade tips where the collector is a multiclone only |
| Drive and arrangement | Direct coupled, arrangement 8; belt driven on the smallest units |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive with a fast furnace draft loop; inlet damper for start-up |
| Also specified | Inspection door, drain, and a flexible connection on the inlet duct |
What this ID fan has to cope with
Furnace puffing at each grate pulse
When the grate pulses, a burst of fresh coal and disturbed ash meets the air and burns fast, and furnace pressure jumps for a few seconds. If the draft loop is slow, flame and ash puff from the peep holes. A variable speed fan with headroom above the normal operating speed and a tuned draft loop absorbs these transients.
Bag filter resistance building between pulses
Pulse jet bag filters on these boilers see their pressure drop climb as the cake builds. A fan sized only for clean bags runs out of suction near the end of the cleaning cycle. The duty includes the bag filter at its maximum allowable differential pressure.
Low gas temperature at part load
Small pulsating grate boilers often idle at low load overnight. Gas temperature at the fan can fall towards the acid dew point, and moist ash cakes on the blades. Keeping the economiser outlet above about 140°C, or bypassing it at low load, protects both the bag filter and the fan.
Energy and control
Pulsating grate boilers usually serve a single process plant with a daily load cycle, running near full load on shifts and at low fire at night or weekends. A damper-controlled ID fan wastes most of its power at those low loads. A variable speed drive on furnace draft saves energy and also handles the pulse transients better than a damper actuator, so it improves both running cost and furnace stability.
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 Forced Draft (FD) Fan for coal pulsating grate boilers.
Questions buyers ask
Why does the furnace go positive when the pulsating grate moves?
Each pulse drops fresh coal into the hot zone and breaks up the ash layer, so combustion rises suddenly and more gas is produced for a few seconds. If the ID fan cannot respond quickly, furnace pressure goes positive. A variable speed drive with a fast draft loop is the usual cure.
What ID fan is used on a coal pulsating grate boiler?
Usually a backward curved centrifugal ID fan behind a bag filter or ESP, with a variable speed drive. The fan is sized on the measured gas volume, including air ingress, and on the bag filter pressure drop at the end of its cleaning cycle rather than with clean bags.
Can the induced draft fan handle biomass if we change fuel?
Often yes, but biomass with high moisture produces more flue gas per tonne of steam and lower gas density. The fan may need more speed or a larger impeller. Send us the fuel analysis and steam output for the new fuel and we will check the fan against it.
Information needed to quote an ID fan for coal pulsating 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.
- Grate pulse. The pulse interval and the observed furnace pressure swing at each pulse.
- Collector. Type of final collector and its maximum allowable 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.