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 ID fan sits
- Grate furnace
- Boiler bank
- Economiser
- Air heater
- Multiclone
- Bag filter
- ID fan
- Chimney
The fan is after the bag filter. Cob ash is low, so dust at the fan is light, and even a fan following only a multiclone sees a modest load. Suction at the inlet is typically 150 to 220 mmWC on a 5 to 10 TPH unit. Because erosion is not severe, the fan can be selected for efficiency rather than ruggedness, with a backward curved or aerofoil wheel.
Gas at the ID fan
| Gas temperature, normal | 150 to 170°C after the air heater |
| Maximum or upset | About 210°C with fouled surfaces |
| Dust at the fan | Low; light ash with some silica and char |
| Moisture and dew point | Water dew point about 45 to 55°C depending on cob moisture |
| Corrosive species | Minor; some potassium and chloride in the ash |
| 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 corn cob fired boilers
| Blade form | Backward curved or aerofoil, for efficiency on a low-dust duty |
| 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 for the alternative fuels used off-season |
What this ID fan has to cope with
Oversized fans from cautious selection
Because biomass is often assumed to be dusty, many cob-fired boilers carry radial fans with large margins. On a low-ash fuel they run throttled at poor efficiency for years. A correctly sized backward curved wheel, or a new impeller in the existing casing, can cut fan power noticeably without any loss of draft.
Draft swings from a lumpy bed
Crushed cob pieces vary in size, and the bed collapses unevenly as they burn. Furnace pressure swings with each collapse, and the ID fan must respond to keep the furnace under suction. A variable speed drive on a well-tuned draft loop keeps these swings small.
Off-season fuel changes
When cob stocks run out, plants switch to wood, husk or briquettes. A fan selected tightly for cob may be short of capacity on a wetter or higher-ash fuel. The duty is checked at the worst alternative fuel, and wear protection added if a high-ash fuel will be used for long periods.
Energy and control
Seed plants and starch units run hard after harvest, then at lower load or on other fuels. Because the dust duty is light, an efficient backward curved fan on a variable speed drive is the right choice, and the savings over an oversized throttled radial fan can be substantial. Regular cleaning of the air heater and multiclone keeps resistance, and so fan power, down.
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 Forced Draft (FD) Fan for corn cob fired boilers.
Questions buyers ask
What type of ID fan is suitable for a corn cob fired boiler?
Cob ash is low, so a backward curved or aerofoil fan in mild steel, direct coupled with a variable speed drive, is usually the best choice. It gives good efficiency without much erosion risk. If the boiler also fires higher-ash fuels, wear protection can be added to the blades.
Can I replace a radial ID fan with a more efficient one on a corn cob boiler?
Often yes. Many cob-fired boilers have radial fans chosen for dusty biomass. On low-ash cobs a backward curved impeller is more efficient. In many cases a new impeller can be fitted in the existing casing after checking the speed, shaft and motor. The saving usually justifies the change within a few seasons.
Why does furnace draft fluctuate on a corn cob boiler?
Crushed cobs form an uneven bed that collapses as pieces burn through, causing short changes in gas volume and pressure. A variable speed drive with a fast draft controller handles this. More uniform cob sizing and steady feeding also reduce the swings. Overfire air also helps.
Information needed to quote an ID fan for corn cob 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 sizing. Cob size as fired, whole or crushed, and its moisture.
- Alternative fuels. Other fuels fired when cobs are not available.
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.