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 FD fan sits
- Inlet screen
- FD fan
- Air heater, where fitted
- Undergrate plenum
- Grate
The fan sits beside the furnace and supplies undergrate air, through an air heater where fitted. Briquettes form a dense bed that becomes more compact as they break into fines. A share of the air may go above the bed as overfire air. On oil boilers converted with an external grate furnace, the FD fan is a new unit sized for the grate rather than for the old burner.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 120 to 170°C where fitted, downstream of the fan |
| Pressure at the fan outlet | Typically 90 to 170 mmWC |
| Bed condition | Dense; compacts as briquettes break up |
| Dust | Clean air; briquette fines near the fuel store |
| 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 briquette fired boilers
| Blade form | Backward curved, with pressure reserve for a compacted bed |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled or impeller on the motor shaft |
| Bearings and cooling | Standard bearings; no cooling |
| Flow control | Variable speed drive on steam pressure |
| Also specified | Inlet screen and a branch for overfire air where the furnace has nozzles |
What this FD fan has to cope with
Bed compaction as briquettes break up
Briquettes swell and crumble as they heat, and the fines pack into the gaps. Bed resistance rises along the grate and air flow falls where it is needed for burnout. An FD fan with a rising curve and pressure reserve keeps air flowing through the compacted bed.
Oil burner fan kept after conversion
When an oil boiler is converted to briquettes with an external furnace, the old burner fan is sometimes reused. It was designed for a burner register, not for a grate, and usually lacks the flow and the right pressure range. A new FD fan sized for the grate avoids poor combustion.
Briquette size changes
Suppliers make briquettes of different diameters. Smaller ones form a denser bed needing more pressure; larger ones a more open bed with more excess air. Speed control lets the fan adjust as the supply changes. The motor is sized for the densest bed, and overfire air dampers let the operator trim the split for each size bought.
Energy and control
Briquette-fired boilers in process plants often run steady loads with daily peaks. A variable speed drive on the FD fan, tied to steam pressure with oxygen trim, saves power and keeps excess air under control. Good grate and air distribution reduce unburnt carbon in the ash, which lowers fuel use and so the air the FD fan must supply.
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 Induced Draft (ID) Fan for biomass briquette fired boilers.
Questions buyers ask
Can the burner fan of an oil-fired boiler be used after converting to briquettes?
Rarely without changes. An oil burner fan is designed for a burner register and a much smaller air volume per unit of heat. A grate furnace needs undergrate air at a different pressure and more total air. A new FD fan, or a rerated one, is usually needed.
Why does my briquette fired boiler leave unburnt fuel in the ash?
Briquettes break into fines that compact the bed and block air in the burnout zone. Adjusting the undergrate air split, giving the FD fan enough pressure and controlling grate speed usually improve burnout. Briquette quality and size also play a part. Overfire air also helps.
What FD fan is needed for a 5 TPH briquette fired boiler?
The duty depends on steam output, briquette analysis, excess air, grate type and any air heater. Send the boiler data and the present fan details if any, and we will calculate the air flow and pressure and recommend a suitable fan. Site checks refine it.
Information needed to quote an FD fan for biomass briquette 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.
- Conversion. Whether the boiler was converted from oil or coal and what furnace was added.
- Briquette size. Diameter and length of the briquettes and their raw material.
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.