How Juliflora wood fired boilers work
Prosopis juliflora is a thorny invasive shrub harvested across the dry districts of Tamil Nadu, Gujarat and Rajasthan, and is the main fuel for many biomass power plants of about 7.5 to 20 MW in Tamil Nadu. The wood is chipped to 25 to 50 mm, with 20 to 35% moisture as fired, 2 to 4% ash plus soil and sand from uprooted stumps, and a calorific value of about 3,000 to 3,500 kcal/kg. Power plant boilers of 30 to 90 TPH run at high pressure and temperature with superheaters, firing on travelling grates or in AFBC and CFBC furnaces. Gas passes the superheater, boiler bank, economiser, air heater and ESP to the ID fans, usually two in parallel, then the chimney.
Where the ID fan sits
- Furnace
- Superheater
- Boiler bank
- Economiser
- Air heater
- ESP
- ID fans
- Chimney
The fans are placed after the ESP, typically as two fans of about 60% capacity each, sharing a common duct to the chimney. Fine ash and sand that pass the ESP reach the fans. Suction at the inlet is typically 250 to 350 mmWC at full load. The fans run continuously for months, so efficiency and reliability matter as much as the duty point, and motors are usually 200 kW or more, often at medium voltage.
Gas at the ID fan
| Gas temperature, normal | 140 to 160°C after the air heater |
| Maximum or upset | About 200°C with fouled surfaces or air heater bypass |
| Dust at the fan | Fine ash with sand; low after a working ESP, abrasive if a field trips |
| Moisture and dew point | Water dew point about 55 to 62°C depending on wood moisture |
| Operating pattern | Continuous base load, often 7,500 hours a year or more |
| Density at the design temperature | About 0.84 kg/m³ at 150°C, 70% 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 Juliflora wood fired boilers
| Blade form | Backward curved, aerofoil where dust is low, selected for stable parallel operation |
| Material of construction | Mild steel casing; impeller with wear protection at the leading edges |
| Drive and arrangement | Direct coupled, arrangement 8, with medium voltage motor on larger units |
| Bearings and cooling | Sleeve or anti-friction bearings with oil lubrication and temperature monitoring |
| Flow control | Variable speed drive or hydraulic coupling, with inlet dampers for isolation |
| Also specified | Outlet isolating dampers, vibration monitoring and an online condition monitoring interface |
What this ID fan has to cope with
Unstable parallel operation
Two fans in parallel can hunt, with one taking more load while the other moves towards stall. Each fan must have a stable pressure curve across the operating range, and the control must keep their loads balanced. Selection checks the stall margin with one fan running alone at full load as well as both together.
Sand from uprooted stumps
Juliflora is often harvested by pulling the whole plant, roots and all. Soil and sand come with the wood and leave as fly ash. If an ESP field trips, the fans see a sudden rise in abrasive dust. Wear protection on the blades and quick ESP repair limit the damage.
Loss of one fan
If one ID fan trips, the remaining fan must carry as much load as it can without overloading its motor. The boiler then runs down to a reduced load. The selection shows what load can be held on one fan and the protection settings for that case.
Energy and control
A biomass power plant sells its output, and the auxiliary power used by the fans comes straight off that. A few percent of fan efficiency, over thousands of hours a year, is worth a significant sum. Variable speed drives, efficient aerofoil or backward curved wheels, and good sealing of the air heater and ESP to cut air ingress are the main measures.
ID fan and FD fan in Juliflora wood fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, usually two in parallel | Ahead of the air heater, often two in parallel |
| What it handles | Flue gas with fine ash and sand | Ambient air |
| Temperature at the fan | 140 to 160°C | Ambient, 25 to 42°C |
| Main design concern | Parallel stability and erosion life | Efficiency at base load |
| Control | Furnace draft | Air flow or steam demand |
See also the Forced Draft (FD) Fan for Juliflora wood fired boilers.
Questions buyers ask
Why do biomass power plants use two ID fans?
Two fans of about 60% capacity each allow the plant to keep running at reduced load if one fan trips or needs maintenance. In a power plant, losing all generation for a fan fault is costly. Each fan is selected to run stably both alone and in parallel.
How does sand in juliflora wood affect the ID fan?
Uprooted stumps carry soil and sand into the fuel, which leaves as abrasive fly ash. Behind a working ESP the dust at the fan is low, but if a field trips the fans see much more sand. Wear protection on the blades limits the damage.
Should an ID fan on a juliflora power plant use a VFD or hydraulic coupling?
Both are used. A variable speed drive usually gives better efficiency across the load range and finer control. A hydraulic coupling is simpler and suits plants where the fan runs mostly at a fixed high load. The choice depends on load profile and the plant's electrical system.
Information needed to quote an ID fan for Juliflora wood 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.
- Fan configuration. Number of ID fans and the share of full load each must carry.
- Motor supply. Supply voltage and whether the fan will run on a VFD or hydraulic coupling.
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.