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 ID fan sits
- Grate or fluidised bed
- Boiler
- Economiser
- Lime and carbon injection
- Bag filter
- ID fan
- Stack
The fan is installed after the bag filter. Ahead of it, dry sorbent injection reacts lime or sodium bicarbonate with HCl, producing calcium or sodium chloride salts, most of which the filter captures. On a fluidised bed unit the cyclone and denser furnace add resistance. Typical suction at the fan inlet is 300 to 450 mmWC on grate units and 400 to 550 mmWC on fluidised bed units. The fan runs at 140 to 170°C, which dry sorbent injection needs, so there is little margin before deposits become damp.
Gas at the ID fan
| Gas temperature, normal | 140 to 170°C after the bag filter |
| Maximum or upset | Up to 230°C with economiser fouling or a sorbent system trip |
| Dust at the fan | Below 10 to 20 mg/Nm³ of fine chloride salts, unreacted lime and fly ash |
| Moisture and dew point | 10 to 15% water vapour; HCl raises the acid dew point to about 110 to 130°C |
| Corrosive species | HCl, SO₂ and hygroscopic CaCl₂; occasional ammonium chloride with SNCR |
| Density at the design temperature | About 0.83 kg/m³ at 155°C, 69% 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 RDF fired power plants
| Blade form | Backward curved with smooth blade surfaces that shed fine salt deposits |
| Material of construction | Mild steel with corrosion allowance on dry duty; SS 316L for the inlet box and impeller where the gas runs close to dew point |
| Drive and arrangement | Direct coupled, arrangement 8, with a gas-tight shaft seal |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace pressure |
| Also specified | Insulated casing, cleaning doors, casing drain and a warm air purge for shutdowns |
What this ID fan has to cope with
Salt build-up on blades
Fine chlorides that pass the bag filter stick to the blades as soon as the surface is damp, and the deposit grows unevenly. Vibration then rises over weeks. Keeping the casing warm and inspection at each outage, with water washing followed by thorough drying, controls it.
Chloride pitting
Chlorides concentrate under deposits and cause pitting in mild steel and even in 304 stainless. Where deposits cannot be avoided, SS 316L or duplex grades and smooth welds without crevices reduce attack. Inspection at each outage shows early pitting before it threatens the impeller.
Fuel quality swings
RDF quality varies with the source waste, and a batch rich in plastics raises gas volume and temperature. The fan is sized for the highest calorific value RDF the plant is designed to fire, with the maximum excess air. Fuel specification limits are agreed at the design stage.
Energy and control
RDF plants often run at constant output, but fuel quality changes the gas volume hour to hour. Variable speed control on furnace pressure follows these changes efficiently. On fluidised bed RDF units the ID and primary air fans together form a large share of auxiliary power, so matching the ID fan closely to the measured duty after commissioning usually pays back.
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 Forced Draft (FD) Fan for RDF fired power plants.
Questions buyers ask
How is an ID fan for an RDF boiler different from one for a coal boiler?
Dust is lower, because the fan follows a bag filter, but the gas contains HCl and chloride salts. Corrosion and sticky deposits near the dew point, rather than erosion, set the material choice and the need for insulation and purging. Erosion is rarely the limiting factor.
What causes vibration in the ID fan of an RDF fired power plant?
Usually uneven build-up of hygroscopic chloride salts on the blades. The fine dust that passes the filter becomes sticky when damp. Keeping the gas and casing warm, and cleaning at each outage, restores balance. Vibration trend alarms show build-up early, before bearings suffer. In-situ balancing then restores smooth running.
Can an RDF ID fan be made of mild steel?
Yes for many dry-duty plants running well above the acid dew point with good insulation. Where the gas temperature is close to dew point or deposits are frequent, stainless steel for the inlet box and impeller is preferred. Insulation and a warm air purge are needed either way.
Information needed to quote an ID fan for RDF fired power plants
- 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.
- RDF analysis. Chlorine, sulphur and moisture content of the design and worst-case RDF.
- Sorbent system. Sorbent used, injection point and the gas temperature at the bag filter.
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.