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 ID fan sits
- Step or reciprocating grate
- Boiler bank
- Economiser
- Air heater
- Multiclone
- Bag filter or ESP
- ID fan
- Chimney
The fan is placed after the bag filter or ESP. Pith carries sand and short fibre that a multiclone only partly removes, so the residual dust depends on the final collector. Because the gas is wet, the air heater and collector are run above the dew point and the fan sits on the warm side of that line. Suction at the inlet is typically 180 to 260 mmWC, built up by the deeper fuel bed, the air heater and two stages of dust collection.
Gas at the ID fan
| Gas temperature, normal | 160 to 180°C after the air heater |
| Maximum or upset | About 230°C when wet fuel forces harder firing |
| Dust at the fan | Fine ash with sand and fibre; low after a bag filter, abrasive if the fan follows a multiclone only |
| Moisture and dew point | Water vapour of 18 to 25% by volume; water dew point about 60 to 65°C |
| Corrosive species | Chlorides and alkali salts, aggressive wherever condensate forms |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 coconut husk and coir pith fired boilers
| Blade form | Backward curved behind a bag filter or ESP, radial tipped where sand reaches the fan; both sized on the wet gas volume |
| Material of construction | Mild steel with a heat-resistant protective coating; SS 316 at the inlet box and drain areas where condensate collects |
| Drive and arrangement | Direct coupled, arrangement 8, with the motor checked for cold, dense gas at start-up |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft, without inlet vanes that catch fibre |
| Also specified | Casing drain, external insulation to keep the casing above dew point, and an access door |
What this ID fan has to cope with
Gas volume underestimated
Design figures often assume dry biomass. Coir pith at 50% moisture produces far more water vapour per tonne of steam, and the fan has to move all of it. A fan sized on dry-fuel combustion runs out of capacity and the boiler cannot reach full load. The duty point is built from the as-fired moisture, the real excess air and the measured air ingress.
Chloride attack at start-up and shutdown
Each time the boiler cools, chloride-laden condensate forms on cold steel in the fan and duct. It pits mild steel quickly, and the damage concentrates at the casing bottom and the inlet box. Insulation, a drain, warming the fan with low gas flow before full load, and corrosion-resistant material at the low points slow the attack.
Sand erosion
Pith lifted from open yards brings sand. The sand passes the grate and leaves with the fly ash, and where the bag filter is bypassed or a multiclone is the only collector, it cuts the blade tips and scroll. Wear liners at the leading edge and a lower tip speed are the remedy where sand loading is high.
Fibre catching at the inlet
Short coir fibres can lift from the grate and travel with the gas. They mat on inlet vanes, guard screens and damper blades, choke the gas path and unbalance the wheel. Leaving out inlet vane assemblies and screens at the fan inlet, and controlling by speed instead, removes the places where fibre catches.
Energy and control
Coir pith boilers swing with fuel moisture as much as with steam demand. In the monsoon the pith is wetter, the gas volume rises and the ID fan runs harder for the same steam. A variable speed drive covers this range without the losses of a throttled damper. Drying pith in the sun or with waste heat before firing is the most effective energy measure of all, because it cuts fuel use and ID fan load together.
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 Forced Draft (FD) Fan for coconut husk and coir pith fired boilers.
Questions buyers ask
Why does my coir pith boiler not reach full load?
The most common cause is that the ID fan cannot move the wet gas. Pith moisture adds a large volume of water vapour, and a fan sized for dry biomass runs out of draft before the boiler reaches rated steam. Measuring gas volume and suction at the fan inlet at full firing shows whether the fan or the boiler is the limit.
What material should the ID fan be for a coconut husk fired boiler?
Mild steel is normal for the impeller and casing while the gas stays well above dew point. Chlorides from the husk make any condensate aggressive, so the low points where water collects are protected with coating or SS 316, and the casing is insulated. Where the plant stops and starts often, a fully stainless wheel can be justified.
Can coir pith be burnt in a boiler designed for wood?
It can, with limits. Pith is wetter and finer than wood, so the grate, air distribution and ID fan all have to cope with more gas and more fines. Many wood-fired boilers lose a fifth or more of their rating on pith. Checking the ID fan capacity at pith moisture is the first step before changing fuel.
Information needed to quote an ID fan for coconut husk and coir pith 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 moisture. Moisture of the husk and pith as fired, in the dry season and in the monsoon.
- Chloride exposure. Whether the fuel is retted in brackish water, and any corrosion seen in the existing duct or fan.
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.