How RDF fired power plants work
Refuse derived fuel is the combustible fraction of municipal and industrial waste, shredded, dried and separated from inerts, metals and wet organics. It is lighter and drier than raw MSW, typically 15 to 25% moisture and 2,500 to 4,000 kcal/kg, but rich in plastics, so its chlorine content is often 0.5 to 1.5%. RDF is fired on a travelling or reciprocating grate, or in a bubbling or circulating fluidised bed, sometimes co-fired with coal or biomass. The flue gas passes the boiler with conservative superheater temperatures to limit chloride corrosion, then dry or semi-dry lime injection, activated carbon and a bag filter, before the ID fan and stack.
Where the FD fan sits
- Inlet filter
- FD fan
- Air preheater
- Air plenum
- Grate or bed
- Furnace
The fan draws air through a filter or screen, often from the fuel storage hall for odour and dust control, and pushes it through a steam or flue gas air preheater to the undergrate plenum or the fluidised bed air box. On grate units the pressure needed is typically 200 to 350 mmWC. On bubbling or circulating fluidised beds the primary air must fluidise the bed and the fan works at 800 to 1,400 mmWC, which needs a high-pressure design.
Air at the FD fan
| Air temperature at the fan | Ambient or fuel hall air, 25 to 45°C |
| Air after the preheater | 150 to 250°C, downstream of the fan |
| Dust | Light RDF fluff, film and fibres from the storage hall |
| Bed condition | Variable resistance with RDF bed depth on a grate, or bed density on a fluidised bed |
| Pressure at the fan outlet | 200 to 350 mmWC on grates; 800 to 1,400 mmWC on fluidised beds |
| 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 RDF fired power plants
| Blade form | Backward curved for grate duty; high-pressure backward curved or radial for fluidising duty |
| Material of construction | Mild steel casing and impeller; stainless steel inlet screen |
| Drive and arrangement | Direct coupled, arrangement 8; high-pressure fans with a stiff shaft and heavy pedestal |
| Bearings and cooling | Pedestal bearings with labyrinth seals against dust |
| Flow control | Variable speed drive on air flow, with zone dampers |
| Also specified | Inlet filter with differential pressure alarm, inlet silencer on high-pressure fans |
What this FD fan has to cope with
Fluff at the intake
Light RDF fragments and plastic film from the storage hall are drawn towards the intake, block the screen and wrap round the impeller hub. A large screen at low face velocity, a differential pressure alarm and access for cleaning while the fan runs keep air flow steady and stop the combustion control chasing a falling air supply.
Defluidisation on fluidised beds
If the primary air fan cannot hold pressure when bed density rises from inerts in the RDF, the bed slumps and agglomerates. The fan is selected with margin over the maximum bed pressure drop and a steep characteristic. Bed pressure is logged to show when margin is running out.
Air distribution on grates
Light RDF burns fast and unevenly on a grate, with blow holes where the bed is thin and dense patches where it is thick, so air bypasses the burning fuel. Zoned undergrate air with independent dampers, fed from a fan that holds steady plenum pressure as dampers move, lets the operator correct distribution along the grate.
Energy and control
The FD fan follows steam load, but on fluidised bed units it also has a minimum flow set by fluidisation, so turndown is limited. A variable speed drive still saves power at partial loads above that minimum. Controlling excess air closely also lowers flue gas volume, which reduces ID fan load and sorbent consumption.
ID fan and FD fan in RDF fired power plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | Ahead of the air preheater |
| What it handles | Chloride-rich cleaned flue gas | Ambient or fuel hall air |
| Temperature at the fan | 140 to 170°C | 25 to 45°C |
| Main design concern | Salt deposits and chloride corrosion | Fluff at the intake, bed pressure |
| Control | Furnace pressure | Air flow and bed condition |
See also the Induced Draft (ID) Fan for RDF fired power plants.
Questions buyers ask
What pressure does an FD fan need for an RDF fluidised bed boiler?
Typically 800 to 1,400 mmWC, because the air must pass the distributor and support the bed. Grate fired RDF boilers need much less, usually 200 to 350 mmWC. The fluidising fan is a high-pressure design with a stiff shaft, selected for the maximum bed pressure drop rather than the average.
Why take combustion air from the RDF storage hall?
It keeps the storage hall under suction, controls dust and odour, and burns the odorous compounds. The air carries light fluff, so the fan needs a well-designed screen. Keeping the storage hall under suction also reduces fire risk from dust build-up in the building. The intake screen needs regular cleaning.
What fans does an RDF power plant need?
Typically a primary air or FD fan, a secondary air fan, an ID fan after the bag filter, and on fluidised beds sometimes a separate fluidising or loop seal blower. Each has a different pressure and duty. On grate fired units the FD fan carries undergrate air only.
Information needed to quote an FD fan for RDF fired power plants
- 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.
- Firing system. Grate, bubbling or circulating fluidised bed, and the air split between primary and secondary air.
- Air intake. Whether air is drawn from the RDF storage hall and what screening is provided.
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.