How biomass pellet fired boilers work
Biomass pellets are pressed from sawdust, agro residues or a blend into 6 to 10 mm cylinders with about 8 to 10% moisture and a calorific value of 3,800 to 4,300 kcal/kg. Wood pellets carry under 1 to 2% ash; agro pellets made from husk, straw or shells carry 5 to 15%, with more potassium and silica. Because the fuel is dry and uniform, pellets suit automatic burners on smoke tube boilers and fixed or reciprocating grates on small water tube units, typically 1 to 10 TPH, and they often replace furnace oil or gas in pharma and food plants. The flue gas passes the boiler passes, an economiser and a multicyclone or bag filter before the ID fan. An FD fan supplies primary and secondary air to the burner or grate.
Where the FD fan sits
- Inlet screen
- FD fan
- Air duct and damper
- Primary and secondary air ports
- Pellet burner or grate
On a packaged pellet boiler the FD fan is mounted on the burner or beside the furnace front and supplies air straight to the burner pot or grate plenum. Larger grate units have a separate fan feeding an undergrate plenum, with secondary air nozzles above the bed. Few pellet boilers have an air heater, so the fan works against the burner or grate, ducts and dampers, typically 60 to 150 mmWC, and the air enters the furnace close to ambient temperature.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the fan | Ambient; an air heater is uncommon on small pellet boilers |
| Dust | Clean air, though pellet fines from the day silo and feeder can be drawn in |
| Pressure at the fan outlet | Typically 60 to 150 mmWC |
| Turndown | Burners modulate down to about 30% of full fire |
| 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 biomass pellet fired boilers
| Blade form | Backward curved, for a stable curve across the turndown |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Variable speed drive with oxygen trim, and a damper setting the primary and secondary split |
| Also specified | Inlet screen, inlet silencer and a flexible connection to the burner |
What this FD fan has to cope with
Excess air drift
Pellets burn cleanly within a narrow air band. Too little air leaves smoke and unburnt carbon in the ash; too much carries heat to the chimney and adds to the ID fan load. A drive linked to an oxygen analyser at the boiler outlet holds the band as pellet batches and burner output change through the day.
Clinker from agro pellets
Agro pellet ash softens at a lower temperature than wood ash. If primary air falls too far at low fire, the bed overheats and fuses into clinker in the burner pot or on the grate. The fan must hold enough primary air at minimum load to keep the bed cool, which sets its minimum flow as firmly as its maximum.
Pellet dust at the fan inlet
Fines from pneumatic silo filling and screw feeders settle around the boiler front. A fan inlet in this area draws wood dust that coats the screen and impeller and reduces air delivery. Ducting the inlet from a cleaner point, and cleaning the screen on a fixed routine, keeps the airflow and balance steady.
Energy and control
A pellet boiler that replaced oil often keeps the old habits: fixed fan speed and a damper. Variable speed on the combustion air fan with oxygen trim gives a measurable fuel saving, because pellets cost more per unit of heat than most loose biomass, and fan power falls steeply at the low fire common on night shifts. The FD and ID drives should be tuned together so the furnace stays at slight suction through each load step.
ID fan and FD fan in biomass pellet fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the collector, before the chimney | At the burner or grate plenum |
| What it handles | Flue gas with light pellet ash | Clean ambient air |
| Temperature at the fan | 150 to 190°C, up to 250°C | Ambient, 25 to 45°C |
| Main design concern | Draft stability, noise and efficiency | Excess air control across turndown |
| Control | Furnace draft | Burner output and oxygen trim |
See also the Induced Draft (ID) Fan for biomass pellet fired boilers.
Questions buyers ask
Is the FD fan on a pellet boiler the same as the burner fan?
On packaged pellet boilers, yes. The combustion air fan built into or beside the burner is the forced draft fan. On larger grate fired pellet boilers it is a separate fan with ducting to the undergrate plenum and to the secondary air nozzles above the bed.
Why does my biomass pellet fired boiler form clinker at low load?
Usually because primary air falls too far at low fire and the bed overheats, especially with agro pellets whose ash softens near 900 to 1,000°C. Holding a minimum primary air flow with the FD fan, and checking the ash content of the pellets being delivered, usually solves it.
Is oxygen trim worth fitting on a pellet boiler FD fan?
On boilers above about 2 TPH or with long running hours, usually yes. Oxygen trim adjusts the FD fan speed to hold excess air near its target, which keeps combustion clean as pellet batches change and reduces both fuel use and ID fan power. On smaller boilers a well-set fixed air ratio is often enough.
Information needed to quote an FD fan for biomass pellet fired 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.
- Burner or grate. Type of pellet burner or grate and the primary and secondary air split it needs.
- Control. Present burner modulation steps and whether an oxygen analyser is fitted.
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.