How groundnut shell AFBC boilers work
In an atmospheric fluidised bed combustion boiler, crushed groundnut shell is fed into a bed of sand held in suspension by air blown up through a distributor plate. The bed runs at about 800 to 850°C, and the shell burns within it and in the freeboard above. Groundnut shell ash is rich in potassium, which can react with silica sand and form sticky agglomerates if the bed runs hot, so operators keep bed temperature down and drain and replenish bed material. The flue gas carries fine bed sand from attrition as well as shell ash through the boiler bank, economiser, air heater, multiclone and ESP or bag filter to the ID fan. Shell-fired AFBC units are typically 8 to 30 TPH, and many were converted from coal.
Where the ID fan sits
- Fluidised bed and freeboard
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan is placed after the ESP or bag filter. Fine sand from bed attrition does not settle in the boiler passes, and the multiclone catches only the coarser part of it. Inlet suction is typically 200 to 300 mmWC: the freeboard is held at a slight negative pressure above a bed pressurised by the fluidising air fan, so every pressure drop downstream of the furnace falls on the ID fan.
Gas at the ID fan
| Gas temperature, normal | 150 to 175°C after the air heater |
| Maximum or upset | About 220°C during bed temperature excursions or air heater bypass |
| Dust before the collectors | Typically 10 to 30 g/Nm³ of ash and bed fines, higher than on grate firing |
| Dust at the fan | Low behind a working ESP or bag filter; highly abrasive if a field trips or bags fail |
| Moisture and dew point | Water dew point about 45 to 50°C |
| Density at the design temperature | About 0.81 kg/m³ at 165°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 groundnut shell AFBC boilers
| Blade form | Radial tipped, or backward curved with wear liners, chosen on how reliable the final collector is |
| Material of construction | Mild steel casing; impeller in wear-resistant plate with replaceable liners |
| Drive and arrangement | Direct coupled, arrangement 8, impeller speed kept moderate |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive holding freeboard draft |
| Also specified | Liners in the inlet box and scroll, an inspection door and continuous vibration monitoring |
What this ID fan has to cope with
Bed sand erosion
Bed sand is ground down by constant motion and leaves the furnace as fine, sharp particles. They pass the multiclone and wear the ID fan quickly whenever an ESP field trips or bags fail. Wear-resistant plate and liners make the fan tolerant of these collector upsets, which occur more often than design figures suggest.
Draft swings from agglomeration
If the bed starts to agglomerate because of potassium in the shell ash, fluidisation becomes uneven and freeboard pressure fluctuates. The ID fan must respond quickly so that the freeboard stays under suction and hot gas and ash do not escape at the feed points. A fast draft loop on a variable speed drive is the usual answer.
Coal design, shell fuel
Many shell-fired AFBC units were designed for coal. Shell produces more flue gas per unit of heat because of its lower calorific value and higher excess air, so the original ID fan falls short at full steam. Recalculating the duty from the shell analysis and site measurements shows whether a new impeller is enough or the fan must be replaced.
Energy and control
AFBC boilers run more steadily than grate units but rarely at full load all season. A variable speed drive trims ID fan power as load falls, and because the fluidising air fan sets the bed air rather than the ID fan, the two drives need coordinated control. Keeping air heater and collector leakage low pays off, since every cubic metre of leakage air is gas the ID fan must handle at high suction.
ID fan and FD fan in groundnut shell AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Feeding the air box under the bed |
| What it handles | Flue gas with shell ash and bed sand | Ambient fluidising air |
| Temperature at the fan | 150 to 175°C | Ambient, 25 to 45°C |
| Pressure | Suction of 200 to 300 mmWC | Discharge of 800 to 1,200 mmWC |
| Main design concern | Sand erosion | Stable pressure and fluidisation |
| Control | Freeboard draft | Steam demand and compartment slumping |
See also the Forced Draft (FD) Fan for groundnut shell AFBC boilers.
Questions buyers ask
Why does the ID fan on an AFBC boiler wear faster than on a grate boiler?
An AFBC bed throws fine sand into the flue gas through constant attrition, on top of the fuel ash. The dust load before the collectors is several times higher than on a grate. If the final collector bypasses or fails, the fan takes the full sand load and wears quickly, so wear protection is part of the design from the start.
Can a coal AFBC boiler be converted to groundnut shell?
Many have been, but the bed, feed system and fans all need checking. Groundnut shell ash is rich in potassium and lowers the temperature at which the bed agglomerates, and shell produces more gas per unit of heat than coal. The ID fan often needs a larger impeller or a new fan to reach full steam on shell.
What suction does the ID fan develop on a groundnut shell AFBC boiler?
Typically 200 to 300 mmWC at the fan inlet, covering the boiler passes, economiser, air heater, multiclone and ESP or bag filter. The figure depends on collector type and air heater size, so a site measurement of suction at full load is the best basis for sizing a replacement fan.
Information needed to quote an ID fan for groundnut shell AFBC 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.
- Bed material. Type and size of bed sand and the rate of bed make-up.
- Original fuel. Whether the boiler was designed for coal, and the original ID fan duty.
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.