How bamboo chip fired boilers work
Bamboo chips, rejects and bamboo dust are burnt in pulp and paper mills that use bamboo as a raw material, and in bamboo processing units in the north-east, Karnataka and Andhra Pradesh. Fresh chips hold 30 to 45% moisture, while air-dried rejects and dust fall to 15 to 20%. Bamboo has 2 to 5% ash, unusually rich in silica for a wood-like fuel, and a calorific value of about 3,000 to 3,600 kcal/kg as fired. Chips are long and splintery and burn on travelling or reciprocating grates, or in AFBC boilers alongside coal. Gas passes the boiler, economiser, air heater, multiclone and an ESP or bag filter to the ID fan.
Where the ID fan sits
- Grate or fluidised bed furnace
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan is placed after the ESP or bag filter. Bamboo ash is light but its silica is hard, so whatever passes the collectors still erodes the blades over time. Suction at the fan inlet is typically 180 to 260 mmWC on a mill boiler. In paper mills the boiler may run alongside a recovery boiler and other steam sources, so it often carries swing load.
Gas at the ID fan
| Gas temperature, normal | 160 to 180°C after the air heater |
| Maximum or upset | About 230°C with fouled surfaces |
| Dust at the fan | Fine silica-rich ash; low after an ESP or bag filter |
| Moisture and dew point | Water dew point 50 to 62°C depending on chip moisture |
| Corrosive species | Some potassium and chloride; low sulphur unless coal is co-fired |
| 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 bamboo chip fired boilers
| Blade form | Backward curved with wear liners, sized for the wettest chips |
| Material of construction | Mild steel casing; wear-resistant plate at the blade leading edges |
| Drive and arrangement | Direct coupled, arrangement 8, moderate tip speed |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Inspection door, replaceable liners and vibration pick-ups |
What this ID fan has to cope with
Silica in the fly ash
Bamboo concentrates silica in its skin, and the fly ash it leaves is harder than most wood ash. A fan selected as for clean wood firing wears faster than expected, particularly if the ESP or bag filter underperforms. Wear liners at the leading edges and a moderate tip speed give the fan a reasonable life between repairs.
Gas volume swings with chip moisture
Fresh chips at 40% moisture and dry dust at 15% produce very different gas volumes per tonne of steam. The fan must deliver full draft on the wettest fuel without running inefficiently on dry fuel. A variable speed drive with a selection centred on the typical mix covers both.
Co-firing with coal
Many mill boilers fire bamboo with coal. Coal adds ash and sulphur, raising both erosion and the acid dew point. The fan duty and materials are checked for the worst co-firing ratio, and the gas temperature at the fan kept above the acid dew point when coal is used.
Energy and control
A mill boiler often swings load as recovery boiler output and paper machine demand change. A variable speed drive on the ID fan follows these swings and saves power at lower loads. Drying chips with waste heat before firing reduces gas volume and so ID fan power, and also improves combustion and boiler efficiency.
ID fan and FD fan in bamboo chip fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Ahead of the air heater |
| What it handles | Flue gas with silica-rich ash | Ambient air, preheated downstream |
| Temperature at the fan | 160 to 180°C | Ambient, 15 to 40°C |
| Main design concern | Silica erosion and moisture range | Pressure through a matted chip bed |
| Control | Furnace draft | Steam pressure |
See also the Forced Draft (FD) Fan for bamboo chip fired boilers.
Questions buyers ask
Why does the ID fan wear on a bamboo fired boiler?
Bamboo ash is rich in silica, which is harder than steel and erodes the fan blades even at low dust loads. Wear is worst if the ESP or bag filter underperforms. Wear liners, wear-resistant plate on the leading edges and a moderate tip speed extend the life of the impeller.
Can a wood chip boiler fire bamboo chips?
Usually yes, but bamboo ash is more abrasive and fresh bamboo can be wetter than seasoned wood. The ID fan should be checked for the wettest fuel and for wear protection. Long splintery chips may also need changes to the fuel feed and grate. Bamboo dust needs suitable feeding too.
What happens to the ID fan when coal is co-fired with bamboo?
Coal adds ash and sulphur. The dust at the fan becomes more abrasive and the acid dew point rises, so the fan needs wear protection and the gas must stay warmer. The fan duty also changes, since coal produces less gas per unit of heat than wet bamboo.
Information needed to quote an ID fan for bamboo chip 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 fresh chips, dried rejects, bamboo dust and any coal co-fired.
- Collector. Type and performance of the ESP or bag filter ahead of the 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.