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 ID fan sits
- Grate or AFBC furnace
- Superheater
- Economiser
- Air heater
- ESP or bag filter
- ID fan
- Chimney
The fan follows the ESP or bag filter. Dust reaching it depends on the fuel of the day: silica from husk, fine potassium salts from stalks and straw, and sand from field-collected residues. On grate units suction at the fan is typically 200 to 300 mmWC; on AFBC units with cyclones, 300 to 400 mmWC. Most plants of 10 MW and above run two fans in parallel, each with its own drive.
Gas at the ID fan
| Gas temperature, normal | 140 to 175°C after the air heater |
| Maximum or upset | Up to 220°C with dry, high-volatile fuels and fouled surfaces |
| Dust at the fan | Varies with fuel: silica, potassium salts and field sand; low after an ESP or bag filter |
| Moisture and dew point | Water vapour from about 10% with dry husk to 25% with wet residues; water dew point up to about 65°C |
| Corrosive species | Chlorides from stalks and straw raise the acid dew point and form deposits |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 mixed agro-Residue fired boilers
| Blade form | Backward curved with wear liners, selected to stay efficient across the fuel envelope |
| Material of construction | Mild steel casing, insulated; wear-resistant plate at the blade inlet |
| Drive and arrangement | Direct coupled, arrangement 8, usually two fans in parallel |
| Bearings and cooling | Pedestal bearings with a cooling disc and vibration monitoring |
| Flow control | Variable speed drives on furnace draft |
| Also specified | Wear liners at the cut-off, washing doors, casing drain and cladding |
What this ID fan has to cope with
Designing for the average fuel
A fan sized for the average blend runs short when the plant fires wet cane trash or stalks in the monsoon, and is oversized when it fires dry husk. Listing each fuel the plant expects to buy, with moisture, ash and chlorine, and checking the fan at the worst case of each, gives a fan that covers the real year.
Erosion and deposits together
Husk silica erodes the impeller, and chloride-rich straw and stalks deposit salts on it. A blade that is both worn and coated unbalances quickly. Wear-resistant blade inlets, smooth surfaces, washing doors and vibration trending let the plant manage both. Treating only one, for example fitting thick liners without washing doors, leaves the other to end the run.
Parallel fans at changing duty
As the fuel changes, the gas volume moves, and two parallel fans may be pushed toward the unstable part of their curves at low volume and high pressure. Stable curves, matched drives and a control strategy that switches to one fan when volume falls keep the pair running smoothly.
Energy and control
Mixed fuel power plants dispatch to the fuel they can buy. When fuel is short or wet the boiler runs below rating for weeks. Variable speed ID fans keep auxiliary power down through these periods, and every kilowatt saved is exported. A fan selected for an envelope with good efficiency at the common middle fuel, rather than only at the extreme, saves the most.
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 Forced Draft (FD) Fan for mixed agro-Residue fired boilers.
Questions buyers ask
How do you size an ID fan for a boiler firing many different agro residues?
We list the fuels the plant expects to fire with their moisture, ash and composition, and calculate gas volume, temperature and pressure for each. The induced draft fan is selected to cover the largest volume at the highest resistance, with good efficiency at the most common fuel.
Why does my mixed biomass boiler lose output in the monsoon?
Wet residues raise the flue gas volume per tonne of steam. If the ID fan was sized for dry husk or an average blend, it reaches its limit and the boiler derates. Checking the fan at the wettest fuel, and adding speed or a larger impeller, usually restores output.
Is wear or corrosion the bigger problem for agro-residue boiler ID fans?
It depends on the fuel mix. Husk-rich blends wear the impeller; straw and stalk-rich blends deposit chlorides that corrode during shutdowns. A mixed fuel plant usually sees both, so the fan needs wear protection at the blade inlet, washing doors and full insulation. Fuel records over a year show which risk dominates at a given plant.
Information needed to quote an ID fan for mixed agro-Residue 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.
- Fuel envelope. List of fuels fired through the year, with moisture, ash and chlorine of each.
- Operating pattern. Months at full load and at reduced load, and whether one fan runs alone.
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.