How corn cob fired boilers work
Corn cobs are left after maize is shelled, at seed processing plants that dry cobs before shelling and at grain, feed and starch units. Cobs have 10 to 20% moisture, 1.5 to 3% ash and a calorific value of about 3,800 to 4,100 kcal/kg. Whole cobs are bulky but tough, so most plants crush them to pieces of 20 to 50 mm. The ash is low but contains potassium and some silica. Boilers of 2 to 15 TPH burn cobs on fixed, travelling or reciprocating grates, and seed plants often use cob-fired hot air generators for drying. Gas passes the boiler, economiser, air heater, multiclone and bag filter to the ID fan. Supply peaks after the kharif and rabi harvests, and plants switch to other fuels as stocks run out.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Grate
The fan sits near the air heater and pushes ambient air through it into the plenum under the grate. Crushed cobs form an open, lumpy bed with gaps through which air passes easily, so the plenum pressure is modest. Some furnaces add overfire air to burn volatiles above the bed. The fan works against the air heater, dampers and a low bed resistance.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the air heater | 130 to 170°C, downstream of the fan |
| Pressure at the fan outlet | Typically 70 to 140 mmWC |
| Bed resistance | Low; open lumpy bed |
| Dust | Clean air; cob dust from crushing |
| 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 corn cob fired boilers
| Blade form | Backward curved, for efficiency on clean air |
| 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, with zone dampers |
| Also specified | Inlet screen, and an oxygen tapping at the boiler outlet for trim |
What this FD fan has to cope with
Air bypassing through gaps
An open cob bed lets air pass through gaps instead of over the burning surfaces, so the oxygen measured in the flue gas rises while some cobs still burn slowly. Zone dampers with enough pressure drop, and grate movement that fills the gaps, keep air where it is needed.
Partly burnt cobs in the ash
Large cob pieces can reach the end of the grate with an unburnt core. Too little air in the burnout zone, or a grate that moves too fast, is usually the cause. Adjusting the FD air split towards the burnout zone and controlling cob size improve burnout.
High excess air
Because the bed offers little resistance, a fixed-speed FD fan easily delivers far more air than the cobs need. Excess air of 80 to 100% is common, which lowers efficiency and loads the ID fan. Speed control with oxygen trim brings it down to a sensible range.
Energy and control
Corn cob boilers often run at full load for a few months after harvest and at reduced load or on other fuels for the rest of the year. A variable speed drive on the FD fan, with oxygen trim, saves power across this pattern and stops the excess air creeping up that an open cob bed encourages. Lower excess air also lowers ID fan power.
ID fan and FD fan in corn cob fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Ahead of the air heater |
| What it handles | Flue gas with light, low ash | Ambient air |
| Temperature at the fan | 150 to 170°C | Ambient, 20 to 45°C |
| Main design concern | Efficiency and draft stability | Air distribution and excess air |
| Control | Furnace draft | Steam pressure with oxygen trim |
See also the Induced Draft (ID) Fan for corn cob fired boilers.
Questions buyers ask
How much undergrate air pressure does a corn cob grate need?
Usually less than for denser fuels. A crushed cob bed is open, and typical FD fan outlet pressure is 70 to 140 mmWC including the air heater. The grate design and cob size set the exact figure; finer cobs give a denser bed and need more pressure.
Why is excess air so high on my corn cob boiler?
An open cob bed offers little resistance, so a fixed-speed FD fan delivers more air than needed. Reducing fan speed with a variable speed drive and trimming on flue gas oxygen typically brings excess air down without causing smoke. A portable analyser check once a season is enough to show the gain.
Can the same FD fan serve corn cobs and other fuels?
Usually, with speed control. Denser fuels such as briquettes or wetter wood need more undergrate pressure, so the fan should have pressure reserve. Check the alternative fuels against the fan curve before switching. Wet wood also needs more total air volume per tonne of steam, so the flow margin matters as much as pressure. The fan curve and motor rating together show whether it is enough.
Information needed to quote an FD fan for corn cob 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 sizing. Cob size as fired and whether cobs are crushed.
- Air system. Whether overfire air is used and how it is supplied.
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.