How rice husk inclined step grate boilers work
In an inclined step grate boiler, rice husk slides by gravity from a hopper at the top down a series of fixed or slightly moving steps. It dries on the upper steps, burns on the middle steps and leaves as ash at the bottom into an ash pit. The bed is thicker and burns more on the grate than with a spreader, so less husk burns in suspension and a smaller share of the ash leaves with the flue gas. Many step grate units produce the black, carbon-rich husk ash sold to steel plants and cement grinding units. Boilers range from about 2 to 15 TPH, often with a refractory-lined furnace, economiser, air heater and multiclone or bag filter ahead of the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate duct
- Zone dampers
- Step grate
The fan sits at the side of the boiler, pushing ambient air through the air heater into a duct that feeds compartments under the upper, middle and lower steps. Each compartment has a damper. Air to the upper steps dries the incoming husk, the middle steps take most of the combustion air, and the lower steps burn out the char. Some designs take a share of air over the bed through secondary nozzles from the same fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | About 100 to 180 mmWC, higher than a thin-bed grate |
| Air after the air heater | 120 to 180°C, downstream of the fan |
| Bed resistance | Varies with husk depth and moisture along the steps |
| Dust | Clean air; husk dust near the hopper |
| 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 rice husk inclined step grate boilers
| Blade form | Backward curved, selected for a rising curve to cope with bed resistance changes |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or motor-mounted |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on steam pressure, zone dampers for air split |
| Also specified | Inlet screen, pressure tappings in each zone compartment |
What this FD fan has to cope with
Air short-circuiting through thin patches
Where the husk bed thins on one step, air rushes through that patch and the rest of the bed starves. A fan that delivers enough pressure to work through a full bed, with zone dampers set by measured compartment pressure, gives even burning along the grate rather than holes and clinker.
Wet husk raising resistance
Husk stored in the open during the monsoon arrives wetter and packs more densely, raising bed resistance. A fan selected only on dry-season figures runs out of pressure and the boiler loses output. We include a pressure margin for wet husk on the drying steps.
Too much air for black ash
Plants selling carbon-rich ash deliberately limit burnout air on the lower steps. If the FD fan and its dampers cannot be turned down cleanly, carbon in the ash falls and its value drops. A variable speed drive with a good damper on the lowest zone gives that control.
Energy and control
The FD fan on a step grate husk boiler is a moderate load that runs continuously. A variable speed drive tied to steam pressure, combined with properly set zone dampers, cuts both fan power and excess air. Lower excess air also reduces the gas volume through the ID fan, so the saving appears on both fans. Recording zone pressures once a shift helps operators keep the air split right.
ID fan and FD fan in rice husk inclined step grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the dust collector | Before air heater and zone compartments |
| What it handles | Flue gas with fine silica and char | Ambient air |
| Temperature at the fan | 150 to 180°C | Ambient, 25 to 45°C |
| Main design concern | Steady draft at the hopper seal | Air split through a thick bed |
| Control | Draft at the grate top | Steam pressure and zone dampers |
See also the Induced Draft (ID) Fan for rice husk inclined step grate boilers.
Questions buyers ask
How is air distributed on an inclined step grate?
The FD fan feeds compartments under the upper, middle and lower steps through separate dampers. The upper zone dries the husk, the middle zone burns it and the lower zone burns out the char. The split is set by damper position and checked by compartment pressure and the colour of the ash leaving the grate.
What FD fan pressure does a rice husk step grate boiler need?
Typically about 100 to 180 mmWC at the fan outlet. A step grate carries a thicker husk bed than a travelling or spreader grate, so the bed resistance is higher, and wet husk raises it further. The air heater and ducts add to that figure.
Does the FD fan affect black ash quality?
Yes. The air supplied to the lower steps decides how much carbon burns out before the ash leaves the grate. Plants that sell carbon-rich husk ash control this zone closely, and a fan with a variable speed drive and good zone dampers makes it easier to hold a consistent product.
Information needed to quote an FD fan for rice husk inclined step 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.
- Zone arrangement. Number of air compartments under the steps and any overbed air from the same fan.
- Husk condition. Range of husk moisture through the year, especially monsoon husk.
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.