How palm fibre and shell fired boilers work
Palm oil mills burn their own mesocarp fibre and kernel shell to raise steam for the sterilisers, digesters and back-pressure turbine. Fibre leaves the press at 35 to 45% moisture, with 5 to 8% ash and a calorific value of about 2,600 to 3,000 kcal/kg as fired; shell is drier and denser at about 4,000 kcal/kg with 2 to 4% ash. They are fired together, typically about 70 parts fibre to 30 parts shell, on fixed or reciprocating grates in water tube boilers of 10 to 45 TPH. Fibre burns largely in suspension, shell on the grate. Gas passes the boiler bank, economiser or air heater where fitted, a multiclone and a wet scrubber, ESP or bag filter, then the ID fan. Steam demand swings with batch sterilisation cycles.
Where the ID fan sits
- Grate furnace
- Boiler bank
- Economiser or air heater
- Multiclone
- Scrubber, ESP or bag filter
- ID fan
- Chimney
The fan is after the multiclone and final collector. Many older mills used a multiclone only, and the fan then sees fibre ash and fine shell grit at several grams per cubic metre; newer mills add a wet scrubber, ESP or bag filter to meet emission limits. Suction at the fan inlet is typically 150 to 250 mmWC. The fan usually stands outdoors near the chimney in a hot, humid climate.
Gas at the ID fan
| Gas temperature, normal | 180 to 230°C at the fan |
| Maximum or upset | Around 280°C at peak firing with fouled surfaces |
| Dust at the fan | Fibre ash with hard shell grit; several g/Nm³ after a multiclone only |
| Moisture and dew point | Wet fibre gives a water dew point of about 55 to 60°C |
| Corrosive species | Potassium and chlorides in the ash; low sulphur |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 palm fibre and shell fired boilers
| Blade form | Radial tipped where the fan follows a multiclone; backward curved with liners after a filter or ESP |
| Material of construction | Mild steel casing; wear-resistant plate on the blades and at the scroll cut-off |
| Drive and arrangement | Direct coupled, arrangement 8, at moderate tip speed |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on a fast furnace draft loop |
| Also specified | Bolted liners, an inspection door, weatherproof motor and terminal box |
What this ID fan has to cope with
Shell grit erosion
Kernel shell is hard, and fragments that escape the grate are as abrasive as sand. Where the fan follows only a multiclone, the leading edges and the outer part of the blades wear quickly. Wear-resistant plate, replaceable liners and a lower tip speed extend impeller life, and an efficient final collector removes most of the problem.
Load swings from sterilisers
When a batch steriliser opens, steam demand jumps, the firing rate rises and the gas volume follows within a minute or two. A fan on a slow damper lets the furnace go positive and blow ash out of the doors. A variable speed drive on a fast draft loop keeps the furnace under suction through each cycle.
Fan sized before mill expansion
Mills raise their throughput over the years, and boilers are pushed beyond their original rating with more fibre. The ID fan becomes the bottleneck. Measured gas volume and suction at today's peak, rather than the original nameplate, form the basis for an uprated impeller or a new fan.
Energy and control
A palm oil mill runs in step with fruit arrival, with long days in peak crop and short weeks in the lean months. Steam demand cycles with the sterilisers within each hour. A variable speed drive handles both the hourly swings and the seasonal drop, and saves the power a throttled damper would waste. Reducing air ingress at the furnace doors and ash hoppers also lowers the gas the fan must move.
ID fan and FD fan in palm fibre and shell fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multiclone and final collector | Beside the furnace, under the grate |
| What it handles | Flue gas with fibre ash and shell grit | Ambient humid air |
| Temperature at the fan | 180 to 230°C | Ambient, 25 to 38°C |
| Main design concern | Shell grit erosion and fast draft response | Following steriliser load swings |
| Control | Fast furnace draft loop | Steam pressure with secondary air |
See also the Forced Draft (FD) Fan for palm fibre and shell fired boilers.
Questions buyers ask
Why does the ID fan wear fast on a palm oil mill boiler?
Kernel shell fragments and fibre ash are abrasive, and many mill boilers have only a multiclone ahead of the fan. The grit erodes the blade leading edges and scroll. Wear plate, liners, lower tip speed and a better final collector all extend impeller life between repairs.
How does the ID fan cope with steriliser steam swings?
Each steriliser cycle changes steam demand quickly, and the furnace firing follows. A variable speed drive with a fast furnace draft loop changes fan speed within seconds and keeps the furnace under suction. A damper on a slow actuator often cannot keep up, and the furnace goes positive.
Can the ID fan be uprated for a higher mill throughput?
Often, yes. A new impeller of larger diameter or different blade form on the existing casing, sometimes with a larger motor, can give more volume and suction. The duty should be based on measured gas flow and suction at the present peak, and the shaft, bearings and foundation checked for the new load.
Information needed to quote an ID fan for palm fibre and shell 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 ratio. Proportion of fibre and shell fired, and any empty fruit bunch added.
- Load cycle. Number of sterilisers and the steam swing during a cycle.
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.