How mixed agro-Residue fired boilers work
Mixed agro-residue boilers burn whatever local residue is available and affordable through the year: rice husk, cotton and mustard stalk, groundnut and soya husk, cane trash, maize cobs, wood chips and others. Most are 10 to 30 MW biomass power plants and large process boilers on travelling, vibrating or reciprocating grates, or on AFBC units. Across these fuels moisture ranges from about 10 to over 40%, ash from 2 to 20%, and the calorific value from about 2,500 to 4,000 kcal/kg, with chlorine and potassium varying widely. The boiler, collector and fans are designed for an envelope of fuels rather than one. The flue gas passes the superheater, economiser and air heater, then an ESP or bag filter, before the ID fan.
Where the FD fan sits
- Inlet filter or screen
- FD fan
- Air heater
- Undergrate plenum or air distributor
- Grate or AFBC bed
The FD fan sits near the air heater and pushes ambient air through it into the grate plenum or the bed distributor. On grate units it works against 150 to 250 mmWC; on AFBC units, 800 to 1,100 mmWC, with the overfire or secondary air from a separate fan. The air heater outlet temperature is set for the wettest fuel, and the FD fan pressure is set for the densest bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 15 to 45°C |
| Air after the air heater | 150 to 220°C, downstream of the fan |
| Dust | Husk, straw and stalk dust from the fuel yard |
| Pressure at the fan outlet | 150 to 250 mmWC for a grate; 800 to 1,100 mmWC for an AFBC |
| Flow range | Wide, as the air split changes from fuel to fuel |
| 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 mixed agro-Residue fired boilers
| Blade form | Backward curved, with a stable curve over a wide flow range |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air flow with oxygen trim |
| Also specified | Inlet filter or screen with pressure drop alarm, inlet silencer for AFBC duty |
What this FD fan has to cope with
Air split changing with fuel
Dense, wet stalks need high primary air and pressure; dry husk lifts off the grate in the same air. Operators who leave the FD fan at one setting get clinker with one fuel and carry-over with the next. A drive on measured air flow, with the split adjusted at each fuel change, keeps combustion in the right place.
Clinker from potassium-rich fuels
Straw and stalks have low ash melting points. Short primary air on a hot bed fuses the ash. The fan must hold minimum primary air for grate cooling, or fluidising air on an AFBC, at every load and for every fuel, which sets its minimum flow.
Yard dust at the inlet
A mixed fuel yard throws husk, straw and stalk dust into the air. A filter or screen with generous area, an intake away from fuel handling, and a pressure drop alarm protect the impeller and the air heater from fouling. Cleaning the screen at every fuel change is a simple habit that keeps the airflow steady.
Energy and control
Mixed fuel plants run at changing loads, so the FD fan rarely sits at its design point. A variable speed drive with oxygen trim saves power and keeps excess air near design as fuel changes, which reduces the gas volume through the boiler and the ID fan power. Tuning the air split for each main fuel is often worth more than the fan efficiency itself.
ID fan and FD fan in mixed agro-Residue fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Before the air heater, feeding the grate or bed |
| What it handles | Flue gas whose dust and moisture change with fuel | Ambient air with yard dust |
| Temperature at the fan | 140 to 175°C, up to 220°C | Ambient, 15 to 45°C |
| Main design concern | Fuel envelope: volume, wear and deposits | Air split and stable pressure across fuels |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Induced Draft (ID) Fan for mixed agro-Residue fired boilers.
Questions buyers ask
How is the FD fan set for different agro residues?
Denser, wetter fuels need more primary air and pressure through the bed, while light, dry fuels need less undergrate air and more overfire air. The forced draft fan is controlled on measured air flow, and the split is adjusted for each main fuel the plant fires.
What FD fan pressure does a mixed biomass boiler need?
Typically 150 to 250 mmWC on a grate fired boiler including the air heater, and 800 to 1,100 mmWC on an AFBC. The pressure is set by the densest, wettest fuel the plant expects to fire, with margin for air heater fouling and the rise in bed resistance as ash and sand accumulate between bed drains.
Why does clinker form only with some fuels in my agro-residue boiler?
Potassium-rich fuels such as straw and stalks have low ash melting points and form clinker if the bed gets too hot. Holding enough primary air from the FD fan for those fuels, and blending them with husk or wood, keeps the bed below its clinker temperature.
Information needed to quote an FD fan for mixed agro-Residue 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.
- Fuel range. The densest and wettest fuels expected, and their share of the year.
- Firing system. Grate or AFBC, and the overfire or secondary air arrangement.
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.