How eucalyptus bark fired boilers work
Eucalyptus bark is stripped from pulpwood logs in the drum debarkers of paper mills and fired, often with wood chips, sawdust and other biomass, in the mill's power boiler. It is wet, at 45 to 55% moisture, with a calorific value of about 1,800 to 2,400 kcal/kg as fired. Clean bark has 3 to 6% ash, rich in calcium, but logs dragged and stacked on bare ground bring sand and soil that can push total ash to 10% or more. The long, stringy bark is shredded before firing on a travelling or reciprocating grate, or in a bubbling fluidised bed. The flue gas passes the superheater, economiser and air heater, then an ESP, before the ID fan and chimney.
Where the ID fan sits
- Grate or fluidised bed
- Superheater
- Economiser
- Air heater
- ESP
- ID fan
- Chimney
The fan follows the ESP. Sand and grit from the woodyard are coarser than bark ash and the ESP catches most of them, but where a field is out of service, or an older mill has only a multicyclone, the fan sees abrasive grit. Suction at the fan is typically 200 to 300 mmWC on a 20 to 50 TPH boiler. Paper mills usually install two fans or a standby, because the boiler supplies process steam continuously and a fan trip stops production.
Gas at the ID fan
| Gas temperature, normal | 160 to 190°C after the air heater |
| Maximum or upset | Up to 230°C with fouled surfaces |
| Dust at the fan | Fine calcium-rich ash with sand; abrasive when the ESP underperforms |
| Moisture and dew point | 20 to 25% water vapour by volume; water dew point about 60 to 65°C |
| Corrosive species | Low sulphur and chlorine from bark alone; higher when mill sludge or plastic rejects are co-fired |
| Density at the design temperature | About 0.79 kg/m³ at 175°C, 66% 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 eucalyptus bark fired boilers
| Blade form | Backward curved with wear liners, sized on wet gas volume |
| Material of construction | Mild steel casing; impeller in wear-resistant plate or with bolted blade liners |
| Drive and arrangement | Direct coupled, arrangement 8, often two fans or a standby |
| Bearings and cooling | Pedestal bearings with a cooling disc and vibration monitoring |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Liners at the scroll cut-off, casing insulation, drain and access doors |
What this ID fan has to cope with
Sand erosion
Bark from logs dragged across the yard carries sand far harder than wood ash. Grit that passes the ESP erodes the blade inlet and the casing at the cut-off. Wear-resistant blades, cut-off liners, a moderate tip speed and woodyard practice that keeps logs off bare earth together set the impeller life, and the first three are in the fan maker's hands.
Wet gas volume in the monsoon
At 50% moisture, water from the bark forms a large share of the flue gas. A fan sized on drier chips runs out of capacity when bark arrives soaked in the monsoon or its share in the fuel rises. Selecting the fan on the wettest expected blend, with a drive to trim it back in dry months, avoids the derate.
Co-firing sludge and rejects
Many mills add primary sludge or plastic rejects to the bark. Sludge brings moisture and fine ash; rejects bring chlorine, which raises the acid dew point and attacks cold metal. The fan selection, casing insulation and drain material are checked against the full fuel mix, not bark alone.
Energy and control
Paper mill boilers run continuously, and the ID fan is the largest boiler auxiliary. Variable speed control saves power at part load and as the fuel moisture moves through the year. Pressing or drying bark with flue gas cuts the gas volume and fan power together. Replacing a worn radial impeller with a lined backward curved wheel in the same casing can also cut power noticeably.
ID fan and FD fan in eucalyptus bark fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP | Before the air heater |
| What it handles | Wet flue gas with ash and sand | Ambient air |
| Temperature at the fan | 160 to 190°C, up to 230°C | Ambient, 25 to 45°C |
| Main design concern | Sand erosion and wet gas volume | Pressure through air heater and wet bed |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Forced Draft (FD) Fan for eucalyptus bark fired boilers.
Questions buyers ask
Why is ID fan wear high on a bark fired boiler?
Bark from logs handled on the ground carries sand and grit. Sand is much harder than bark ash and erodes the impeller where gas velocity is highest, at the blade inlet and tip. A healthy ESP, wear-resistant blades and cleaner log handling in the woodyard all reduce the wear.
How does wet bark affect the boiler ID fan?
Water in the bark leaves as vapour and raises the flue gas volume per tonne of steam. The induced draft fan must be sized on the wet gas volume at the wettest expected fuel, otherwise the boiler derates in the monsoon when bark comes in soaked from open storage.
Does a eucalyptus bark fired boiler need a standby ID fan?
In a paper mill, often yes, or two fans in parallel. The boiler supplies steam to the paper machines and a fan trip stops production. With two fans the plant can run at reduced load while one is repaired, which usually justifies the cost. Where space is short, a spare balanced rotor kept at site is the minimum.
Information needed to quote an ID fan for eucalyptus bark 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 mix. Share of bark, chips, sludge and rejects, with moisture and ash of each.
- Redundancy. Whether one fan, two in parallel or a standby is required.
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.