How paddy straw fired boilers work
Paddy straw is the stalk left after rice harvest. It is baled in the field and fired mainly in 10 to 30 MW biomass power plants and large industrial boilers in north India, where straw would otherwise be burnt in the open. Baled straw carries about 10 to 20% moisture, 12 to 18% ash and a calorific value of about 3,000 to 3,500 kcal/kg. The ash is mostly silica but rich in potassium, with chlorine often 0.3 to 0.8%, and it softens from about 800 to 900°C. Bales are broken and shredded, then fed onto a water-cooled vibrating grate or a travelling grate. The flue gas passes the superheaters, economiser and air heater, then an ESP or bag filter, before the ID fan.
Where the ID fan sits
- Vibrating grate furnace
- Superheaters
- Economiser
- Air heater
- ESP or bag filter
- ID fan
- Chimney
The fan is placed after the ESP or bag filter. Straw ash has two parts: coarse silica-rich particles that the collector removes well, and a submicron fume of potassium chloride and sulphate formed in the furnace, part of which slips through a weak ESP. On a 10 to 15 MW unit the suction at the fan is typically 250 to 350 mmWC, because the furnace, the widely pitched convective banks, the air heater and the collector all add resistance. Most plants run two fans in parallel, each sized for 50 to 60% of full load.
Gas at the ID fan
| Gas temperature, normal | 140 to 160°C after the air heater |
| Maximum or upset | Up to 200°C as superheater fouling builds between sootblowing |
| Dust at the fan | Fine silica and submicron potassium salts; typically 30 to 100 mg/Nm³ after a bag filter or well-sized ESP |
| Moisture and dew point | Water dew point about 50 to 60°C; HCl and SO₃ raise the acid dew point, so cold spots on the fan can corrode |
| Corrosive species | Chlorides and some SO₂; casing corrosion where insulation is missing or the fan stands idle |
| 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 paddy straw fired boilers
| Blade form | Backward curved, with smooth surfaces that shed salt deposits |
| Material of construction | Mild steel or weathering steel casing, fully insulated; wear plate at the blade inlet against silica |
| Drive and arrangement | Direct coupled, arrangement 8, usually two fans in parallel |
| Bearings and cooling | Pedestal bearings with cooling disc and temperature monitoring |
| Flow control | Variable speed drives on furnace draft, with tight-shutoff isolating dampers so one fan can run alone |
| Also specified | Washing doors and a casing drain, vibration transmitters, and cladding over the insulation |
What this ID fan has to cope with
Salt deposits on the blades
Potassium chloride fume that slips through the collector condenses on the impeller and builds a hard, hygroscopic layer, thickest on the suction side of the blades. When patches break away the rotor goes out of balance. Smooth backward curved blades, access doors for water washing at outages and vibration trending let the plant clean before the fan trips.
Corrosion while standing
When the boiler stops, chloride deposits on the casing and impeller absorb moisture from the air and turn acidic. A fan idle through a straw shortage can corrode faster than one in service. Full casing insulation, a drain, and either washing the fan or keeping it warm and dry during outages limit the damage.
Running one fan at low load
Straw stocks run down late in the year and bales arrive wetter, so plants often run below rating. Two parallel fans with variable speed let the plant run one fan efficiently, provided the idle fan's dampers shut tight and the fan curves are stable enough to run in parallel without hunting.
Air heater leakage
Corrosion at the cold end of the air heater opens leaks that pass air into the gas side. The ID fan then handles more gas at a lower temperature, closer to the acid dew point. Margin in the fan selection covers some leakage, but the measured volume at the fan inlet, not the theoretical figure, must set the duty.
Energy and control
Straw power plants run at base load while fuel is plentiful and below rating when stocks are thin or the straw is wet. The ID fans are among the largest auxiliary loads, and every kilowatt they draw is deducted from exported power. Variable speed drives on each fan, controlled from furnace draft, recover a large share of the auxiliary power at part load, and switching to one fan at low load saves more where the system curve allows it.
ID fan and FD fan in paddy straw fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | Before the air heaters, feeding the grate |
| What it handles | Flue gas with silica and potassium chloride fume | Ambient air carrying straw fibre |
| Temperature at the fan | 140 to 160°C, up to 200°C | Ambient, 5 to 45°C |
| Main design concern | Salt deposits and corrosion | Clean intake and grate cooling air |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Forced Draft (FD) Fan for paddy straw fired boilers.
Questions buyers ask
Why does the ID fan vibrate on a paddy straw fired boiler?
Paddy straw ash contains potassium chloride that passes the collector as a fine fume and deposits on the impeller. The layer builds and breaks away unevenly, which unbalances the rotor. Water washing at outages, smooth blade surfaces and online vibration monitoring keep the fan running between planned stops.
Should a straw boiler ID fan be made of stainless steel?
Not usually. In normal running the gas is above its acid dew point, so insulated mild steel works. Corrosion happens mostly during shutdowns or where insulation is missing. Stainless steel is considered for parts that stay cold, such as drains and inlet boxes, rather than for the whole fan.
What ID fan pressure does a straw fired power plant need?
Typically 250 to 350 mmWC at the fan on a 10 to 15 MW boiler, depending on the convective pass arrangement, air heater and ESP or bag filter. We size the induced draft fan from the boiler maker's pressure drops or site measurements, with margin for fouling between cleanings.
Information needed to quote an ID fan for paddy straw 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.
- Straw analysis. Moisture, ash, chlorine and potassium content of the straw as fired.
- Fan arrangement. Single fan or two in parallel, and whether the plant runs on one fan at low load.
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.