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 FD fan sits
- Inlet filter or screen
- FD fan
- Steam coil air heater
- Flue gas air heater
- Grate air plenum
- Vibrating grate
The FD fan sits at ground level near the air heaters. Many straw boilers use a steam coil air heater ahead of the flue gas air heater, so that the gas-side tube metal stays above the acid dew point. The air goes to compartments under the water-cooled vibrating grate, while a separate secondary air fan supplies the overfire nozzles. The FD fan works against both air heaters, the ducts, the dampers and the grate and bed, typically 150 to 250 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 5 to 45°C over the year in north India |
| Air after the air heaters | 150 to 200°C, downstream of the fan |
| Dust | Straw fines and fibre in the boiler house air |
| Pressure at the fan outlet | Typically 150 to 250 mmWC |
| Ambient range | Winter air is much denser than summer air, so mass flow at a fixed speed varies through the year |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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. |
FD Fan selection for paddy straw fired boilers
| Blade form | Backward curved, for efficiency on filtered air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 8 or 4 |
| Bearings and cooling | Standard pedestal bearings; no cooling needed |
| Flow control | Variable speed drive on measured air flow, which corrects for ambient density |
| Also specified | Low-velocity inlet filter or fine screen with a differential pressure gauge, and an inlet silencer |
What this FD fan has to cope with
Straw fibre at the inlet
Bale handling throws light fibre into the air across the plant. Unfiltered, it mats on the inlet screen and impeller and on the steam coil fins, and the grate runs short of air. A generously sized filter or screen, an intake taken at height or away from the fuel shed, and a differential pressure alarm keep the airflow steady.
Clinker on the grate
Straw ash fuses at temperatures a poorly cooled bed reaches easily. Primary air cools the grate bars and limits bed temperature. The fan must deliver the air the grate maker specifies at every load, so the lowest useful flow is set by the minimum primary air for grate cooling, not by the lowest steam demand.
Seasonal change in air density
North Indian sites see air from near freezing in winter to above 40°C in summer. A fan sized for winter mass flow runs short in May; one sized for summer has surplus in January. Selecting at hot-weather density and controlling on measured mass flow covers the whole year.
Energy and control
Straw boilers run most of the year at steady output, so FD fan efficiency matters. A variable speed drive on measured air flow, with oxygen trim, removes damper losses and holds excess air near design as straw moisture changes between dry and damp bales. Lower excess air also cuts the gas volume the ID fans must handle, and that saving is usually larger than the FD fan saving itself.
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 Induced Draft (ID) Fan for paddy straw fired boilers.
Questions buyers ask
Does the FD fan on a straw fired boiler need an air filter?
In most straw plants, yes. Straw fines and fibre are everywhere around the fuel yard and boiler house. A low-velocity filter or fine screen at the forced draft fan inlet, cleaned regularly, stops fibre matting on the impeller and on the steam coil air heater fins.
How is air divided between the FD fan and the secondary air fan on a paddy straw boiler?
The FD fan delivers primary air up through the grate to the burning straw and keeps the grate cool. The secondary air fan blows air through nozzles in the furnace walls to burn the volatile gases released above the bed. Straw is highly volatile, so secondary air is a large share of the total.
Why do straw boilers use a steam coil air heater?
It warms the combustion air before the flue gas air heater, so the gas-side tube metal stays above the acid dew point and does not corrode. The FD fan pushes air through both heaters, so its pressure includes the steam coil resistance as well as the tubular air heater.
Information needed to quote an FD fan for paddy straw fired boilers
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Air heaters. Type of air heaters, steam coil and flue gas, and the air-side pressure drop of each.
- Site ambient. Minimum and maximum ambient temperature and the altitude of the plant.
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.