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 FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate air ducts
- Zone dampers
- Grate compartments
The FD fan sits at ground level near the air heater and pushes ambient air through the air heater, where it picks up heat from the flue gas, and then into the undergrate air duct. Each grate compartment has its own damper so that the operator can match air to the fuel bed along the grate. Because husk burns largely in suspension, much of the air is supplied above the grate by the secondary air fan, and the FD fan carries the remainder against the resistance of the air heater, ducts, dampers and the thin husk bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 120 to 180°C, downstream of the fan |
| Dust | Clean air; husk dust from the boiler house can be drawn in without an inlet screen |
| Humidity | Monsoon humidity has negligible effect on the duty |
| Pressure at the fan outlet | Typically 100 to 200 mmWC including air heater and grate resistance |
| 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 fired travelling grate boilers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled or direct mounted on the motor shaft |
| Bearings and cooling | Standard pedestal or motor bearings; no cooling needed |
| Flow control | Variable speed drive, or inlet vane control on older plants, linked to steam pressure |
| Also specified | Inlet screen against husk and debris; inlet silencer where the fan is close to work areas |
What this FD fan has to cope with
Uneven air across the grate
If the FD fan pressure falls at part load, the zones farthest from the duct starve of air and the husk burns unevenly. A fan with a stable, rising pressure curve and a drive that holds plenum pressure keeps the air distribution right.
Husk dust drawn into the fan
Boiler houses that handle husk are dusty, and husk fibre can build up on an unscreened impeller and cause unbalance. An inlet screen and a duct inlet taken from a clean area solve this.
Air heater leakage
A leaking tubular air heater passes FD air directly into the flue gas, so the FD fan works harder and the ID fan has more gas to move. The FD selection includes the measured leakage, not only the theoretical air.
Energy and control
The FD fan on a husk boiler follows load closely, and throttling it with an outlet damper wastes power at the partial loads typical of rice mills and parboiling plants. A variable speed drive tied to the combustion control loop saves energy and keeps the air-fuel ratio stable. Checking excess oxygen at the economiser outlet is the quickest way to see whether the FD fan is supplying more air than the husk needs.
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 Induced Draft (ID) Fan for rice husk fired travelling grate boilers.
Questions buyers ask
What is the difference between the FD fan and secondary air fan on a husk boiler?
The FD fan supplies primary air through the grate to the burning bed. The secondary air fan blows air at higher pressure through nozzles above the grate to burn the volatiles and suspended husk. Husk needs a large share of secondary air because so much of it burns in suspension.
Does the FD fan handle hot air?
Usually no. The FD fan pushes ambient air into the air heater and the air is heated downstream. Its selection is therefore at ambient density, which makes it smaller and simpler than the ID fan.
What size FD fan does a 10 TPH rice husk boiler need?
It depends on the excess air, the share of secondary air, the air heater resistance and the grate design. Send us the boiler data or the existing fan nameplate and we will calculate the duty and confirm it against site measurements where possible.
Information needed to quote an FD fan for rice husk fired travelling 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.
- Air split. Share of total air supplied as undergrate air and as secondary or overfire air.
- Grate zones. Number of grate compartments and the design plenum pressure.
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.