How waste incinerators work
Waste incinerators vary with the waste. Municipal solid waste burns on moving grates; hazardous solids, sludges and drums go to rotary kilns; biomedical and small institutional waste to controlled-air two-chamber units; sewage and ETP sludge to fluidised beds; and liquid or solvent wastes to liquid injection chambers. Almost all have a secondary combustion zone held at about 850 to 1,100°C with around two seconds residence time, followed by a waste heat boiler or a quench, then gas cleaning: dry or semi-dry lime and activated carbon injection with a bag filter, a wet scrubber, or both. The ID fan sits at the end of this train, and FD fans supply primary and secondary combustion air.
Where the FD fan sits
- Air intake
- FD fan
- Air preheater
- Primary air ducts
- Secondary air nozzles
- Combustion chambers
Primary air fans sit near the furnace and deliver air under the grate, into the kiln hood or into the primary chamber, through an air preheater on larger plants. A separate secondary air fan supplies nozzles in the afterburning zone at higher pressure for mixing. Fluidised bed units use a high-pressure fluidising air blower instead. Required pressure ranges from about 100 to 250 mmWC for grates and two-chamber units, to 1,500 mmWC or more for fluidised beds.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | Up to 150 to 250°C on wet-waste grate plants, downstream of the fan |
| Dust | Bunker dust, odour and moisture where air is drawn from the waste bunker |
| Pressure at the fan outlet | 100 to 250 mmWC for grates; 300 to 600 mmWC for secondary air; 1,500 mmWC or more for fluid beds |
| Load profile | Follows waste feed and calorific value, which can change quickly |
| 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 waste incinerators
| Blade form | Backward curved for primary air; high-pressure radial or backward curved for secondary and fluidising air |
| Material of construction | Mild steel; coated or stainless steel where bunker air is humid and acidic |
| Drive and arrangement | Direct coupled or direct mounted |
| Bearings and cooling | Standard pedestal or motor bearings |
| Flow control | Variable speed drive on the combustion control loop with oxygen and temperature trim |
| Also specified | Inlet screen, airflow measurement and silencer |
What this FD fan has to cope with
Changing calorific value
Waste calorific value can change from one charge to the next. If combustion air cannot follow, CO spikes and secondary chamber temperature falls below the regulatory minimum. Airflow measurement and a fast drive tied to oxygen and temperature control keep combustion within limits.
Corrosion from bunker air
Air drawn from the waste bunker is humid and contains organic acids and odour compounds. Coated or stainless steel construction and an inlet screen protect the fan where bunker extraction is used.
Secondary air mixing
Poor mixing in the afterburning zone leaves pockets of unburnt gas. The secondary air fan must deliver enough pressure for high nozzle velocity across the full turndown, which often means a separate high-pressure fan rather than a branch from the primary air fan.
Energy and control
Combustion air demand follows the waste feed and its calorific value. Variable speed drives on primary and secondary air fans, with oxygen trim, reduce fan power and also reduce flue gas volume, which in turn lowers ID fan power and reagent use in the gas cleaning train. On fluidised bed units the fluidising blower is the largest air-side load, and its efficiency at the normal operating point deserves attention.
ID fan and FD fan in waste incinerators
| ID fan | FD fan | |
|---|---|---|
| Position | End of the gas cleaning train, before the stack | Ahead of the combustion chambers |
| What it handles | Cleaned flue gas, dry or saturated | Ambient or bunker air |
| Temperature at the fan | 140 to 200°C dry, 60 to 80°C wet | Ambient, 25 to 45°C |
| Main design concern | Corrosion and high system resistance | Following changing waste calorific value |
| Control | Furnace pressure | Oxygen and chamber temperature |
See also the Induced Draft (ID) Fan for waste incinerators.
Questions buyers ask
What is the difference between primary and secondary air on an incinerator?
Primary air is supplied to the burning waste, under the grate, into the kiln or into the primary chamber, often sub-stoichiometric. Secondary air is injected at high velocity into the afterburning zone to complete combustion of gases and meet CO and residence time requirements.
Does the FD fan on a waste incinerator handle hot gas?
No. It handles ambient air, or air drawn from the waste bunker on larger grate plants. Air preheating, where used for wet waste, happens downstream of the fan, so the fan is sized at ambient density and built in mild steel or coated steel.
Why do fluidised bed incinerators need a high-pressure FD fan?
The air must pass through the distributor and support the weight of the sand bed. This typically needs 1,500 mmWC or more at the fan, far more than a grate or two-chamber incinerator.
Information needed to quote an FD fan for waste incinerators
- 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 split. Primary, secondary and any fluidising air flows and pressures.
- Air source. Whether air is drawn from the waste bunker or from outside.
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.