How coconut husk and coir pith fired boilers work
Coconut husk and coir pith come from coir fibre extraction. Husk is fibrous and bulky; pith is the fine, spongy residue left once the fibre is removed, and holds 50 to 70% moisture when fresh. Both carry chloride and sodium from retting in brackish water, and pith from open yards often contains sand. Dried or blended, the fuel has a calorific value of about 2,500 to 3,500 kcal/kg as fired and 5 to 12% ash. It is usually burnt on a reciprocating or inclined step grate with a drying zone, in boilers of 2 to 15 TPH. Gas passes the boiler, economiser and air heater, a multiclone and a bag filter or ESP, then the ID fan. The FD fan pushes air through the air heater so that hot air reaches the wet fuel on the grate.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Drying zone
- Burning zones
The fan draws ambient air and pushes it through the tubular air heater, where it is warmed to about 180 to 220°C before reaching the grate. The first grate zone is a drying zone, where hot air lifts moisture from the pith before ignition. The FD fan has to overcome the air heater, the zone dampers and a compact bed of wet pith that packs more tightly than wood chips, so its pressure requirement is higher than on a dry-fuel grate of the same size.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 40°C, humid in coastal areas |
| Air after the air heater | 180 to 220°C, downstream of the fan |
| Pressure at the fan outlet | Typically 120 to 200 mmWC |
| Dust | Clean air; pith fines from the yard can be drawn in |
| Atmosphere | Salt and humidity on coastal plants |
| 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 husk and coir pith fired boilers
| Blade form | Backward curved, with pressure reserve for a wet, packed bed |
| Material of construction | Mild steel, coated for humid coastal air |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings; no cooling needed |
| Flow control | Variable speed drive on steam pressure, with separate dampers for the drying and burning zones |
| Also specified | Inlet screen against pith fines and an inlet silencer where needed |
What this FD fan has to cope with
Wet fuel not igniting
If hot air cannot pass through the drying zone, pith reaches the burning zone still damp and the fire dies back towards the discharge end. The cause is often an FD fan without the pressure to push through a packed bed, or air leaking past the air heater. A fan selected for the bed resistance of the wettest season keeps ignition steady.
Packing and channelling
Fine pith packs tightly, and air finds a few easy paths through the bed instead of spreading evenly. Those patches burn fiercely and blow fines into the gas, while the rest smoulders. Higher plenum pressure from the FD fan, combined with grate movement and zone dampers, breaks the pattern.
Pith fines on the air side
Pith yards are dusty, and the fine particles are light enough to be carried into the FD fan suction. They collect on the impeller and on the air side of the air heater tubes, cutting heat transfer just when the wet fuel needs it most. A screened intake taken from outside the fuel yard is the simple fix.
Energy and control
The FD fan load on a coir pith boiler follows fuel moisture as much as steam demand, and both change through the year. Speed control on steam pressure, with oxygen trim, lets the fan supply what the bed needs without throttling losses. Keeping the air heater clean matters as much: a fouled air heater lowers the hot air temperature, the wet fuel dries slowly, and operators compensate with excess air that loads both fans.
ID fan and FD fan in coconut husk and coir pith fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Ahead of the air heater |
| What it handles | Wet, chloride-bearing flue gas with sand | Ambient air, preheated downstream |
| Temperature at the fan | 160 to 180°C | Ambient, 25 to 40°C |
| Main design concern | Gas volume and cold-end corrosion | Pressure through a wet, packed bed |
| Control | Furnace draft | Steam pressure and bed condition |
See also the Induced Draft (ID) Fan for coconut husk and coir pith fired boilers.
Questions buyers ask
Why does a coir pith boiler need hot combustion air?
Pith arrives with 50% or more moisture. Before it can burn, that water has to be driven off, and hot undergrate air does this on the drying zone of the grate. Without an air heater the fuel ignites late, unburnt pith falls into the ash pit and steam output drops, however much air the FD fan supplies.
What pressure should the FD fan develop for a coconut husk boiler?
Typically 120 to 200 mmWC at the fan outlet, covering the air heater, ducts, zone dampers and the fuel bed. Bed resistance varies with moisture and fines, so the fan is selected with reserve for the wettest fuel expected. The grate maker's plenum pressure is the best starting point for the duty.
Does the forced draft fan need special materials on coastal coir plants?
Not special alloys, but good corrosion protection. Salt-laden humid air corrodes mild steel casings, cones and fasteners. A zinc-rich primer with an epoxy finish and stainless fasteners are usually enough, with an annual check of the impeller hub and inlet cone, where corrosion tends to start.
Information needed to quote an FD fan for coconut husk and coir pith 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.
- Air heater. Hot air temperature and the air side pressure drop of the air heater at full load.
- Grate zones. Number of grate zones and whether the drying zone has its own damper.
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.