How municipal solid waste to energy plants work
A municipal solid waste to energy plant burns mixed household waste, usually on a reciprocating or moving grate, to raise steam for a turbine. Indian MSW is wet and low in heating value, typically 40 to 55% moisture and 1,200 to 2,000 kcal/kg as received, so it is aged in the bunker to drain leachate before firing. The furnace holds the gas above 850°C for at least two seconds. The gas then passes the boiler, often with SNCR for NOx, a semi-dry or dry scrubber using lime and activated carbon for acid gases and dioxins, and a bag filter before the ID fan and stack. Primary air is drawn from the waste bunker hall, which keeps the bunker under suction and controls odour.
Where the FD fan sits
- Bunker hall intake
- Screen
- FD fan
- Steam air preheater
- Zone dampers
- Grate zones
The fan takes its suction from high in the bunker hall, through a screen, and discharges through a steam coil air preheater that heats the air to 150 to 220°C for burning wet waste. From there the air divides into zoned plenums under the grate. A separate secondary air fan, often also drawing from the bunker or the boiler house, supplies air above the grate. The FD fan works against the preheater, ducts, dampers and a waste bed of varying depth, typically 250 to 400 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient to bunker hall temperature, 30 to 45°C |
| Air after the preheater | 150 to 220°C, downstream of the fan |
| Dust | Paper, plastic film and fibres from the bunker; light fibrous material collects on screens |
| Humidity and odour | High humidity and organic acids from decomposing waste |
| Pressure at the fan outlet | Typically 250 to 400 mmWC |
| 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 municipal solid waste to energy plants
| Blade form | Backward curved with open, non-clogging blades and no narrow passages |
| Material of construction | Mild steel with an epoxy or similar protective coating inside, SS for the inlet screen |
| Drive and arrangement | Direct coupled, arrangement 8, motor outside the bunker hall |
| Bearings and cooling | Pedestal bearings with sealed housings against dust and moisture |
| Flow control | Variable speed drive on total primary air, split by zone dampers |
| Also specified | Inlet screen with access for cleaning, a pressure drop gauge across the screen, and a drain at the casing low point |
What this FD fan has to cope with
Screen and impeller fouling
Light plastic and paper fragments from the bunker block the inlet screen and wrap on the impeller hub. A large-area screen with low face velocity, regular cleaning and a differential pressure alarm avoid loss of air. Mesh size is chosen so the screen traps film without high pressure loss.
Corrosion from bunker air
Bunker air is humid and carries organic acids and ammonia from decomposing waste. These corrode bare mild steel, worst in the casing bottom and inlet cone where condensate collects during cool nights and shutdowns. An internal protective coating, drains at the casing low points, and stainless steel for the screen and fasteners extend life considerably.
Air demand on wet waste
Wet waste needs preheated air and more of it under the drying zone. The fan has margin for the wettest waste in the monsoon, when the bunker air is also at its most humid. Air demand per tonne of waste is highest in the monsoon, and that sets the fan size.
Energy and control
MSW firing is controlled on steam output and furnace temperature, and the primary air demand varies with waste moisture. A variable speed FD fan allows the combustion control to adjust air smoothly. Recovering heat into the primary air with the steam preheater is fuel-neutral, but optimising excess air through better air control reduces flue gas volume and therefore ID fan power.
ID fan and FD fan in municipal solid waste to energy plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | Bunker hall intake, ahead of the air preheater |
| What it handles | Cleaned flue gas with HCl and SO₂ | Humid, odorous bunker air |
| Temperature at the fan | 140 to 160°C | 30 to 45°C |
| Main design concern | Acid corrosion below dew point | Fouling by light waste and moisture |
| Control | Furnace pressure | Primary air flow by grate zone |
See also the Induced Draft (ID) Fan for municipal solid waste to energy plants.
Questions buyers ask
Why is primary air taken from the waste bunker in an MSW plant?
Drawing combustion air from the bunker hall keeps the bunker under slight suction, so odours and dust do not escape. The odorous compounds are then burned in the furnace. It is the normal design in modern waste to energy plants. The FD fan duty includes this extraction role.
Does the FD fan in a waste to energy plant need special materials?
It handles air, but bunker air is humid, dusty and mildly acidic. Mild steel with internal coating, a stainless steel screen and good drainage is usually sufficient. Stainless steel throughout is rarely necessary. Drainage and coatings matter more than the base material, because the problems start where condensate sits.
How is combustion air controlled on an MSW grate?
The FD fan supplies total primary air, and zone dampers divide it between drying, burning and burnout zones under the grate. Secondary air above the grate is controlled separately to keep the furnace temperature above 850°C. The FD fan speed follows total primary air demand from the combustion control.
Information needed to quote an FD fan for municipal solid waste to energy 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.
- Air intake. Location of the intake in the bunker hall and any screens or filters provided.
- Grate zones. Number of undergrate zones and the design air split between them.
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.