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 ID fan sits
- Pellet burner or grate
- Boiler passes
- Economiser
- Multicyclone or bag filter
- ID fan
- Chimney
The fan sits after the multicyclone or bag filter, near the chimney base, and on packaged boilers is often mounted on the boiler skid or a small frame beside it. Suction at the inlet covers the furnace draft, the boiler passes, the economiser and the collector, typically 80 to 180 mmWC on a 2 to 6 TPH unit. Where an oil or gas boiler is converted to pellets, the original design relied on the burner fan alone, so an ID fan and collector are added and the boiler becomes balanced draft.
Gas at the ID fan
| Gas temperature, normal | 150 to 190°C after the economiser |
| Maximum or upset | Up to 250°C with fouled tubes, or at full load where no economiser is fitted |
| Dust at the fan | Light, fine ash; usually below 100 mg/Nm³ after a bag filter, higher with agro pellets behind a cyclone |
| Moisture and dew point | Dry fuel, so water vapour is moderate and the water dew point is about 45 to 55°C |
| Corrosive species | Low with wood pellets; agro pellets bring potassium chloride, which deposits mainly on the economiser |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 biomass pellet fired boilers
| Blade form | Backward curved, for efficiency on lightly loaded gas |
| Material of construction | Mild steel casing and impeller; abrasion-resistant liners only where agro pellets are fired behind a cyclone |
| Drive and arrangement | Direct coupled arrangement 8, or overhung on the motor shaft for small packaged units |
| Bearings and cooling | Pedestal bearings with a cooling disc and a heat shield at the casing |
| Flow control | Variable speed drive on furnace draft, interlocked with burner modulation |
| Also specified | Acoustic lagging or a silencer where the boiler house adjoins process blocks, and an inspection door |
What this ID fan has to cope with
Agro pellets bought in place of wood pellets
Pellet quality varies between suppliers and seasons. An agro pellet with 10% potassium-rich ash produces far more fly ash, slag in the burner pot and deposits on the economiser, which raise draft loss and gas temperature at the fan. The fan is selected with pressure margin for the dirtier pellet the plant may end up buying, not only the test fuel.
Converted oil boilers short of draft
A boiler built for oil ran with a pressurised furnace and low flue losses. After conversion, the collector and any new economiser add resistance that the chimney cannot overcome, and the burner puffs back at high fire. The ID fan is sized on the measured or calculated resistance of the converted system, with the higher excess air pellets need.
Draft swings at burner steps
Pellet burners step or modulate between low and high fire. If the fan follows slowly, the furnace goes briefly positive and smoke escapes at sight glasses and doors. A drive with a fast draft loop, and a minimum speed that holds suction at low fire, keep the furnace steady through each change.
Noise near clean production areas
Pellet boilers often stand beside pharma or food blocks with tight noise limits. A backward curved wheel at moderate tip speed, casing lagging or an enclosure, and a silencer between fan and chimney reduce noise without adding much resistance or power. Noise is best dealt with at selection, because treating an existing fan afterwards costs more.
Energy and control
Pellet boilers in pharma and food plants usually follow production shifts, with long periods at 30 to 70% load and short peaks. Pellets are dear per unit of heat, so excess air costs more than fan kilowatts, but both respond to the same control. A variable speed ID fan holding furnace draft, paired with oxygen trim on the combustion air fan, cuts fan power at low fire and stops the fan pulling extra cold air through the furnace.
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 Forced Draft (FD) Fan for biomass pellet fired boilers.
Questions buyers ask
Does a biomass pellet fired boiler need an ID fan?
Most do once a dust collector and economiser are fitted. Very small pellet burners on smoke tube boilers sometimes run on burner fan pressure and chimney draft alone, but the resistance of a cyclone or bag filter needs an ID fan to keep the furnace at suction across the burner's range.
What changes when an oil fired boiler is converted to pellets?
Oil boilers run pressurised with no collector. Pellets need more excess air, so the flue gas volume rises, and the new cyclone or bag filter adds resistance. An induced draft fan is added to give balanced draft, sized on the converted system's actual gas volume and pressure drop, not on the original oil duty.
Can the same ID fan handle wood pellets and agro pellets?
Yes, if it is selected for the agro pellet case. Agro pellets give more ash and foul the heat surfaces faster, so the fan needs pressure and temperature margin and, where it follows only a cyclone, wear protection on the blade inlet. A wood pellet duty alone leaves no room for a fuel change.
Information needed to quote an ID fan for biomass pellet fired 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.
- Pellet type. Wood or agro pellet, its ash content and the supplier's fuel analysis, and any change of supplier expected.
- Conversion details. Whether the boiler was converted from oil or gas, and which collector and economiser were added.
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.