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 ID fan sits
- Step grate furnace
- Boiler tubes
- Economiser
- Air heater
- Multiclone or bag filter
- ID fan
- Chimney
The fan sits after the dust collector, with a suction of typically 120 to 200 mmWC at its inlet. The husk hopper at the top of the step grate is open to the furnace through the column of husk in the chute. If draft is too strong, the fan pulls air down through that column, cooling the bed and raising gas volume. If draft is too weak, flame and smoke creep back up the chute. The fan and its control set this balance.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 230°C with fouled heat surfaces |
| Dust at the fan | Lower share of fly ash than spreader firing; fine silica and light char, 50 to 150 mg/Nm³ after a bag filter |
| Air ingress | Through the husk hopper and chute, varying with hopper level |
| Moisture and dew point | Dry gas on normal husk; low sulphur |
| 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 inclined step grate boilers
| Blade form | Backward curved with liners, for efficiency and a stable curve at the moderate dust level |
| Material of construction | Mild steel casing; impeller in wear-resistant plate or with hard-faced leading edges |
| Drive and arrangement | Direct coupled, arrangement 8 on larger units, belt driven on small ones |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft measured at the grate top |
| Also specified | Access door and drain; draft tapping located near the hopper chute |
What this ID fan has to cope with
Air drawn through the husk hopper
When the hopper level drops, the husk column in the chute becomes a leak path. The ID fan pulls cold air down through it, the bed top cools and the boiler loses output while the fan moves extra gas. Draft control from a tapping near the top of the grate, rather than the furnace roof, keeps the hopper seal stable.
Backfire up the chute
If the ID fan is throttled or a damper sticks, smoke and flame can travel up the husk chute and ignite the hopper. A fan with adequate margin, a reliable drive, and an alarm on low furnace draft guard against this, along with a rotary valve or flap in the chute where the layout allows.
Fine light char at the fan
Although less ash leaves with the gas than on a spreader-fired grate, step grates producing carbon-rich ash also release fine, light char particles. These can smoulder in hoppers and the fan casing. A casing drain, access door and good collector housekeeping limit fire risk at restart.
Energy and control
Step grate husk boilers usually run steadily through the milling and parboiling season with gradual load changes, so a variable speed drive on draft saves a steady share of fan power. The bigger lever is often air ingress: a well-sealed hopper and chute reduce gas volume and fan power together, and raise boiler efficiency. Checking oxygen at the boiler outlet against the ID fan speed is a simple routine test.
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 Forced Draft (FD) Fan for rice husk inclined step grate boilers.
Questions buyers ask
Why does smoke come back up the husk hopper on our step grate boiler?
The furnace is losing negative pressure at the top of the grate, so gas finds its way up the husk chute. The ID fan may be short of capacity, its damper may be sticking, or the draft is being measured in the wrong place. Measuring draft at the grate top usually identifies the cause.
What ID fan is used on a rice husk step grate boiler?
Usually a backward curved ID fan with wear liners, after a bag filter or multiclone, designed for about 150 to 180°C. Because a step grate releases less fly ash than spreader firing, a lined backward curved wheel is often practical even behind a good multiclone, with radial tipped blades kept for very dusty duties.
Does a step grate boiler need a strong ID fan?
It needs a steady one more than a strong one. The suction is moderate, typically 120 to 200 mmWC at the fan, but the draft at the grate top must be held within a narrow band so that air does not leak through the hopper and flame does not creep up the chute.
Information needed to quote an ID fan for rice husk inclined step 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.
- Hopper and chute. Type of husk feed from hopper to grate and any seal or rotary valve fitted.
- Ash product. Whether carbon-rich black ash is produced for sale, which affects excess air and char carry-over.
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.