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 ID fan sits
- Grate furnace
- Boiler and economiser
- Semi-dry scrubber
- Lime and carbon injection
- Bag filter
- ID fan
- Stack
The fan is the last item before the stack, after the bag filter, so the dust it sees is low. The suction at its inlet covers the furnace draft, the boiler, the scrubber and the bag filter, typically 350 to 550 mmWC. The scrubber cools the gas to 140 to 160°C to make its chemistry work, which leaves the gas only a margin above the acid dew point of HCl and SO₃. Some plants use a wet scrubber after the bag filter, and in that case the fan is placed either before it, on hot dry gas, or after it on saturated gas.
Gas at the ID fan
| Gas temperature, normal | 140 to 160°C after the scrubber and bag filter |
| Maximum or upset | Up to 220°C on scrubber water failure; 60 to 70°C saturated if after a wet scrubber |
| Dust at the fan | Low, typically below 10 mg/Nm³ of fine salts and lime after the bag filter |
| Moisture and dew point | Wet waste gives 15 to 25% water vapour; acid dew point often 120 to 140°C |
| Corrosive species | HCl, SO₂, residual chlorides and hygroscopic calcium chloride from the scrubber |
| 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 municipal solid waste to energy plants
| Blade form | Backward curved, for efficiency on clean, low-dust gas |
| Material of construction | Mild steel with corrosion allowance and an insulated casing on dry duty; SS 316L or duplex stainless after a wet scrubber |
| Drive and arrangement | Direct coupled, arrangement 8, with a gas-tight shaft seal |
| Bearings and cooling | Pedestal bearings outside the gas path with a cooling disc |
| Flow control | Variable speed drive on furnace pressure, with an inlet damper for isolation |
| Also specified | Full insulation and cladding, casing drain, and a warm air or recirculation connection to keep the fan dry during shutdown |
What this ID fan has to cope with
Acid condensation during shutdown
When the plant stops, the casing cools below the acid dew point and residual gas condenses as dilute hydrochloric and sulphuric acid. Insulation, purging the fan with warm dry air, and a drain at the casing low point stop corrosion pitting between runs.
Hygroscopic deposits
Calcium chloride from the scrubber absorbs moisture and forms a sticky, corrosive paste on blades and inside the casing if the gas temperature drops. Keeping the gas above its dew point and cleaning doors for inspection avoid both imbalance and corrosion under deposits.
Variable waste and furnace pressure
Waste calorific value changes hour by hour, and wet loads cause sudden steam and gas volume changes. The fan responds quickly through speed control to hold the furnace negative and prevent fugitive emissions at the feed chute. The fan is sized for the wettest and the richest waste in the design range.
Energy and control
MSW plants run continuously at a set steam output, but gas volume varies with waste moisture and excess air. An ID fan on variable speed control follows furnace pressure directly and avoids the losses of a throttled damper. Since auxiliary power is a large share of gross output on small MSW units, a fan selected close to the real duty point noticeably improves net export.
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 Forced Draft (FD) Fan for municipal solid waste to energy plants.
Questions buyers ask
What material is used for an ID fan in a waste to energy plant?
On dry duty after a bag filter at 140 to 160°C, mild steel with corrosion allowance and good insulation is common. Where the fan follows a wet scrubber and handles saturated gas, SS 316L or duplex stainless is used. The inlet box, which runs coldest, is often upgraded first.
Why does the ID fan corrode in an MSW incineration plant?
Mostly during shutdowns. The gas contains HCl and SO₂, and when the fan cools below the acid dew point the condensate is strongly acidic. Calcium chloride deposits also attract moisture. Keeping the fan warm and dry when stopped is the main protection. Water washing must be followed by thorough drying.
Where is the ID fan placed in an MSW plant?
Normally after the bag filter and before the stack, so it handles cleaned gas and keeps the whole system under suction. Where a wet scrubber is added, the fan may be before or after it, which changes the material choice. It is the last item in the gas cleaning train.
Information needed to quote an ID fan for municipal solid waste to energy 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.
- Gas cleaning train. Scrubber type, sorbents used, and whether the fan is before or after any wet stage.
- Acid gas loading. Raw gas HCl and SO₂ concentrations and the gas temperature leaving the scrubber.
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.