How coconut shell fired boilers work
Coconut shell from desiccated coconut units, copra driers and shell charcoal yards is a dense, hard fuel. Crushed to 10 to 30 mm, it carries 8 to 15% moisture, a calorific value of about 4,200 to 4,600 kcal/kg and well under 2% ash. It burns hot on fixed, travelling or reciprocating grates, and most units in the coastal belt are smoke tube or packaged water tube boilers of 1 to 10 TPH, many without an economiser. Flue gas leaves the boiler at 200°C or more on the simpler units, passes a cyclone or multiclone and sometimes a bag filter, then the ID fan and chimney. The FD fan supplies undergrate air, which also keeps the grate bars cool under the hot shell bed.
Where the FD fan sits
- Inlet screen
- FD fan
- Air duct and damper
- Undergrate plenum
- Grate and shell bed
The FD fan sits beside the furnace and blows ambient air into the plenum under the grate. Most coconut shell boilers have no air heater, so the air enters the bed cold. The fan works against the duct, the grate bars and a bed of dense shell pieces, a resistance that rises as the bed thickens or clinker forms. Some furnaces take part of the FD air as overfire air through side nozzles to burn the volatiles cleanly.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 40°C, humid in coastal areas |
| Air heater | Usually none; air enters the grate at ambient |
| Pressure at the fan outlet | Typically 60 to 120 mmWC, rising with bed depth and clinker |
| Dust | Clean air; shell grit from crushing can be drawn in |
| Atmosphere | Salt-laden air on plants near the sea |
| 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 coconut shell fired boilers
| Blade form | Backward curved, with pressure margin for a thick or clinkered bed |
| Material of construction | Mild steel with a marine-grade paint system on coastal plants |
| Drive and arrangement | Impeller on the motor shaft, or direct coupled |
| Bearings and cooling | Standard pedestal or motor bearings |
| Flow control | Variable speed drive or inlet damper on steam pressure, with manual overfire air dampers |
| Also specified | Inlet screen, stainless fasteners, and a branch to overfire nozzles where the furnace has them |
What this FD fan has to cope with
Grate bar burning
A dense shell bed burns hot, and a grate that gets too little air at low load loses its cooling. Bars warp, gaps open and fuel falls through into the ash pit. An FD fan that holds a minimum undergrate flow, rather than a damper closed almost shut, protects the grate as well as the combustion.
Pressure lost to clinker
Potassium-rich shell ash fuses into clinker that closes the air spaces in the grate. If the fan has no pressure reserve, air flow falls away and the bed burns unevenly, with hot spots that make more clinker. A fan curve that rises steeply towards shut-off keeps air flowing while the operator breaks the clinker.
Corrosion in coastal air
Many coconut-based plants are within a few kilometres of the sea. Salt-laden humid air corrodes mild steel casings, inlet cones and motor fittings, and rust scale can unbalance a small impeller. A zinc-rich primer with an epoxy finish, stainless fasteners and a yearly check of the hub and inlet cone are cheap protection.
Energy and control
Shell boilers in coconut processing often run at half to two-thirds load for long stretches. With an inlet damper the FD fan wastes much of its power at these loads. A variable speed drive, combined with an oxygen check at the boiler outlet, lowers fan power and also trims excess air, since operators tend to run shell boilers with far more air than the fuel needs to avoid smoke.
ID fan and FD fan in coconut shell fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone or bag filter | Under the grate, beside the furnace |
| What it handles | Hot flue gas with light alkali ash and soot | Ambient air, often humid and salty |
| Temperature at the fan | 200 to 250°C without an economiser | Ambient, 25 to 40°C |
| Main design concern | Gas temperature and bearing heat | Air through a dense, hot bed |
| Control | Furnace draft | Steam pressure |
See also the Induced Draft (ID) Fan for coconut shell fired boilers.
Questions buyers ask
How much FD air does a coconut shell fired boiler need?
Coconut shell needs roughly 5.5 to 6 kg of air per kg of fuel for complete combustion, plus excess air of typically 40 to 60% on a grate. The FD fan duty then depends on the share taken as overfire air and on the grate resistance. Send the boiler output and fuel consumption and we will work out the fan size.
Why do the grate bars burn on my coconut shell boiler?
Shell burns at a high bed temperature, and the grate relies on undergrate air for cooling. At low load, or when clinker blocks part of the grate, air flow drops and bars overheat. Keeping a minimum FD air flow, cleaning clinker regularly and giving the fan enough pressure reserve prevent repeated grate damage.
Is a VFD worth fitting to a small forced draft fan?
On boilers of 2 TPH and above that run more than one shift, usually yes. The FD fan follows steam demand, and speed control saves energy compared with throttling at part load. On very small fans the saving is modest, but a drive still makes air control smoother and helps operators bring excess air down.
Information needed to quote an FD fan for coconut shell 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.
- Grate type. Fixed, travelling or reciprocating grate, and whether the furnace has overfire air nozzles.
- Site environment. Distance from the sea and whether the fan is indoors or outdoors.
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.