How biomass briquette fired boilers work
Biomass briquettes are made by compressing sawdust, groundnut shell, husk, bagasse or mixed agro-waste into cylinders of 60 to 90 mm diameter, without a binder. They are dense, with 8 to 12% moisture and a calorific value of about 3,800 to 4,300 kcal/kg, but their ash ranges from 3 to 15% depending on what went into them. Briquettes are widely used to replace coal, furnace oil and diesel in dyeing, textile, chemical and pharmaceutical plants, often in boilers converted from other fuels. They burn on fixed, travelling or reciprocating grates, swelling and breaking into fines as they heat. Gas passes the boiler, economiser, multiclone and bag filter to the ID fan.
Where the ID fan sits
- Grate furnace
- Boiler
- Economiser
- Multiclone
- Bag filter
- ID fan
- Chimney
The fan is after the bag filter. Briquette ash varies by raw material and supplier, from low-ash sawdust briquettes to high-ash mixed agro-waste ones, so the dust at the fan depends on the collector and the fuel bought that month. Suction at the inlet is typically 150 to 250 mmWC. Where an oil-fired boiler has been converted with an external grate furnace, the ID fan is new and must cover the furnace, boiler and full collector train.
Gas at the ID fan
| Gas temperature, normal | 160 to 190°C at the fan |
| Maximum or upset | About 240°C with sooted tubes |
| Dust at the fan | Varies with raw material; low after a bag filter |
| Moisture and dew point | Water dew point about 45 to 50°C |
| Fuel variability | Ash from 3 to 15% between suppliers |
| Density at the design temperature | About 0.79 kg/m³ at 175°C, 66% 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 briquette fired boilers
| Blade form | Backward curved, with wear liners where high-ash briquettes are used |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Inspection door, and a duty checked against the old fuel's fan rating |
What this ID fan has to cope with
Conversion from coal or oil
A coal-fired boiler's ID fan is sized for less gas per unit of heat than briquettes produce, and an oil-fired boiler's for far less. After conversion, the fan runs out of draft before full steam. The new duty is calculated from the briquette analysis, excess air and the added collector resistance, and the fan rerated or replaced.
Ash varies by supplier
Briquettes made from sawdust may have 3% ash, while those from mixed agro-waste or sandy groundnut shell can exceed 10%. Dust at the fan and the rate of wear change with each supplier. Wear liners and an efficient final collector let the fan cope with the worst briquettes bought.
Bag filter added at conversion
Plants switching from oil often need a bag filter for the first time. It adds 100 to 150 mmWC of resistance that the ID fan must overcome, and the filter's own temperature limits apply. The fan duty includes the filter at its fouled pressure drop.
Energy and control
Dyeing, chemical and pharmaceutical plants often run steady steam loads with periodic peaks. A variable speed drive on the ID fan saves power at the lower loads. Choosing briquettes with consistent low ash and moisture also helps, as it lowers gas volume, wear and collector resistance, reducing fan power over time.
ID fan and FD fan in biomass briquette fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Beside the furnace, under the grate |
| What it handles | Flue gas with variable ash | Ambient air |
| Temperature at the fan | 160 to 190°C | Ambient, 25 to 45°C |
| Main design concern | Gas volume after fuel conversion | Pressure through a compacting bed |
| Control | Furnace draft | Steam pressure |
See also the Forced Draft (FD) Fan for biomass briquette fired boilers.
Questions buyers ask
Will my existing ID fan work after converting to briquettes?
Often not at full load. Briquettes produce more flue gas per unit of heat than coal and much more than oil. If a bag filter is added, the resistance also rises. A calculation of the new duty, compared with the fan curve, shows whether a new impeller, higher speed or a new fan is needed.
How does briquette quality affect the ID fan?
Briquettes from different raw materials have different ash and moisture. High-ash briquettes put more dust through the collector and wear the fan faster; wetter ones raise gas volume. Specifying briquette quality, or designing the fan for the worst expected, avoids surprises. Ask suppliers for ash figures.
What size ID fan is needed for a biomass briquette fired boiler?
It depends on steam output, briquette analysis, excess air and the resistance of the boiler, economiser and collectors. Share the boiler data, fuel analysis and any existing fan nameplate, and we will calculate the duty and compare it with the present fan. Site measurements improve accuracy.
Information needed to quote an ID fan for biomass briquette 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.
- Previous fuel. Fuel the boiler was designed for and the original ID fan duty.
- Briquette supply. Raw material of the briquettes and their typical ash and moisture.
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.