How rice husk fired travelling grate boilers work
Rice husk is fed pneumatically or by spreader onto a slowly moving chain or travelling grate, where it dries, devolatilises and burns as a thin bed. Husk is light, with a bulk density near 110 kg/m³, a calorific value of about 3,000 to 3,400 kcal/kg and 18 to 22% ash, almost all of it silica. Most of the husk burns in suspension above the grate, so a large share of the ash and a fraction of unburnt char leave with the flue gas. The gas passes the boiler bank, economiser and air heater, then a multiclone and an ESP or bag filter before the ID fan and chimney. Undergrate air comes from the FD fan through zoned compartments, with overfire air from a separate secondary air fan.
Where the ID fan sits
- Furnace and grate
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan sits after the final collector so that it handles the residual dust and not the full ash load. On older plants with only a multiclone, or where the bag filter is bypassed during start-up, the fan sees several grams per cubic metre of silica and wears in months. The suction at the fan inlet is the sum of the furnace draft, the boiler and economiser losses, the air heater and both collectors, typically 150 to 250 mmWC on a 10 to 20 TPH unit.
Gas at the ID fan
| Gas temperature, normal | 150 to 175°C after the air heater |
| Maximum or upset | Up to 220°C with fouled heat surfaces or air heater bypass |
| Dust at the fan | Fine, angular silica with some glowing char; 50 to 150 mg/Nm³ after a bag filter or ESP, several g/Nm³ if the fan follows a multiclone only |
| Moisture and dew point | Husk moisture of 8 to 12% gives a water dew point well below the gas temperature; low sulphur, so acid dew point is not usually limiting |
| Corrosive species | Minor; chlorides from some husk varieties can attack the cold end of the air heater, not normally the fan |
| 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 rice husk fired travelling grate boilers
| Blade form | Backward curved with wear liners behind a bag filter or ESP; radial tipped with replaceable liners where the fan follows a multiclone |
| Material of construction | Mild steel casing; impeller in wear-resistant plate (such as Hardox or SAILHARD class) or mild steel with hard-faced leading edges |
| Drive and arrangement | Direct coupled, arrangement 8, impeller at or below 1,000 rpm to limit tip speed and erosion |
| Bearings and cooling | Pedestal bearings outside the gas stream with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft, with an inlet damper for isolation |
| Also specified | Bolted wear liners at the scroll cut-off, an inspection door, and a casing drain |
What this ID fan has to cope with
Silica erosion of the blades
Husk ash is harder than mild steel. Erosion concentrates on the blade leading edge and the outer third of the blade where velocity is highest. Lower tip speed, liners on the pressure face and a wheel designed for liner replacement at site keep the fan running between annual shutdowns.
Char carry-over and fire risk
Unburnt husk char can still be glowing when it reaches the collector. A bag filter needs spark arrestors upstream, and the fan casing needs a drain and access door so smouldering deposits can be cleared before restart.
Unbalance from uneven wear
Erosion removes metal unevenly, and vibration rises gradually over months. Vibration monitoring on the bearings and in-situ balancing at each shutdown avoids bearing failures that would otherwise end the run.
Air ingress into a low-density gas
Leaks at the air heater and collector hoppers add cold air that the fan must also handle. The fan is sized on measured gas volume at the fan inlet, not on theoretical combustion air, so the margin is real.
Energy and control
Husk boilers often run at part load through the milling season, and an ID fan sized for the peak with a throttled damper wastes power for most of the year. A variable speed drive controlled from furnace draft typically cuts ID fan energy by a third or more at 60 to 80% load. Where an old radial fan has been worn and repatched, a new backward curved impeller in wear-resistant plate on the existing casing can recover efficiency without changing the foundation.
ID fan and FD fan in rice husk fired travelling grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the dust collector, before the chimney | At the air heater inlet, ahead of the grate |
| What it handles | Flue gas with silica ash and char | Clean ambient air |
| Temperature at the fan | 150 to 175°C, up to 220°C upset | Ambient, 25 to 45°C |
| Main design concern | Erosion and unbalance | Stable pressure across the grate zones |
| Usual blade form | Backward curved with liners, or radial tipped | Backward curved |
| Control | Furnace draft | Steam pressure and excess oxygen |
See also the Forced Draft (FD) Fan for rice husk fired travelling grate boilers.
Questions buyers ask
How often does an ID fan impeller wear out on a rice husk boiler?
It depends on the dust reaching the fan. Behind a well-maintained bag filter or ESP, a lined impeller can run several years. Behind a multiclone alone, unlined mild steel blades can lose thickness in a few months. Wear plate, lower tip speed and replaceable liners extend this considerably.
Can a backward curved fan be used on a rice husk boiler?
Yes, when the fan sits after a bag filter or ESP. Backward curved blades are more efficient but more sensitive to erosion, so they are supplied with wear liners. Where the fan follows only a multiclone, a radial tipped wheel is the safer choice.
Why does my ID fan current rise when the air heater is choked?
A fouled air heater adds resistance and raises the flue gas temperature. The damper or drive opens further to hold furnace draft, the fan moves more gas at a higher point on its curve, and power rises. Cleaning the heat surfaces restores the design point.
Information needed to quote an ID fan for rice husk fired 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.
- Collector. Whether the fan follows a multiclone, ESP or bag filter, and the measured dust at the fan inlet if available.
- Wear history. How long the present impeller lasts between repairs and photographs of the worn blades.
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.