How wood fired crematoria work
A wood fired crematorium replaces the open pyre with an enclosed furnace. Logs, typically 150 to 300 kg per cremation against several hundred kilograms on an open pyre, are stacked on a cast iron or refractory grate under or around the body. Wood at 15 to 30% moisture burns with underfire air through the grate and overfire air above it, and the primary chamber reaches about 700 to 900°C. The gas passes a secondary chamber where unburnt volatiles and tar are completed, then a settling chamber or cyclone to drop ash and sparks, and usually a wet scrubber. An ID fan draws the gas to a chimney, often 30 m tall. Wood ash, 1 to 3% of the wood, collects under the grate with the bone remains.
Where the FD fan sits
- Inlet
- FD fan
- Air header
- Underfire air below grate
- Overfire air ports
- Secondary chamber air
The fan sits beside the furnace and feeds a header with three branches: underfire air to a plenum below the grate, overfire air through ports in the side walls above the wood, and secondary air into the afterburner. Underfire air must pass the grate and the ash layer, so the pressure needed is moderate, about 100 to 250 mmWC. Dampers on each branch set the split.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clean air; some ash can fall back into the underfire plenum if the fan stops |
| Air split | Underfire high at ignition; overfire and secondary high during the main burn |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Operating pattern | Cyclic, with a long burnout at low air |
| 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 wood fired crematoria
| Blade form | Backward curved, quiet at moderate pressure |
| Material of construction | Mild steel, painted |
| Drive and arrangement | Direct drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive with branch dampers for underfire, overfire and secondary air |
| Also specified | Non-return damper in the underfire line to stop hot gas and ash backing into the fan |
What this FD fan has to cope with
Smoky burn from wrong air split
Too much underfire air drives the wood to burn hard and release volatiles faster than the overfire air can finish them. Too little and the wood smoulders. Separate control of underfire and overfire air, set through the cycle, gives a clear chimney and an even burn.
Hot gas back into the fan
When the fan stops at the end of a cremation, hot gas and ash can flow back through the underfire plenum into the air duct and the fan casing, scorching paint and filling the scroll with ash. A non-return damper in the underfire line and a short positive purge at shutdown keep the fan clean and stop heat damage at the outlet.
Grate blockage
Ash, charcoal and unburnt wood build up on the grate as the burn progresses, so the underfire resistance rises towards the end of the cycle. A fan selected with pressure margin keeps air flowing through the bed until burnout is complete, and a grate design that allows raking and ash removal stops the air path closing altogether.
Energy and control
Air demand on a wood fired crematorium changes through the cycle and the fan runs at full flow only for part of the burn. A variable speed drive on chamber temperature or oxygen lets it slow down during burnout. Getting the air right also reduces the wood per cremation, which is the main running cost and an environmental concern in its own right.
ID fan and FD fan in wood fired crematoria
| ID fan | FD fan | |
|---|---|---|
| Position | After the scrubber, at the chimney | Beside the furnace, feeding grate and ports |
| What it handles | Saturated smoke with ash and tar | Ambient air |
| Temperature at the fan | 55 to 80°C | Ambient |
| Main design concern | Tar build-up and wet corrosion | Underfire and overfire air split |
| Control | Chamber draft | Chamber temperature or oxygen |
See also the Induced Draft (ID) Fan for wood fired crematoria.
Questions buyers ask
What is underfire and overfire air in a wood crematorium?
Underfire air is blown up through the grate into the wood to sustain burning, and it controls how fast the wood burns. Overfire air is blown above the wood to burn the gases and volatiles it releases. In a wood crematorium both come from the combustion air fan, with dampers that set the split through the cremation cycle.
Does a wood fired crematorium need a forced draft fan?
Most enclosed wood fired crematoria use one, since the grate and the ash bed resist natural air flow and the operator needs control over the burn rate. The forced draft fan works together with the ID fan, which holds the furnace under suction, so that air enters only where it is wanted and smoke cannot escape.
How much wood does an enclosed wood crematorium use?
Typically 150 to 300 kg per cremation, much less than an open pyre, because the refractory chamber retains heat between cremations and the air supply is controlled. Wood moisture, the size of the logs, the number of cremations run back to back and the operator's handling of the air dampers all affect the consumption in practice.
Information needed to quote an FD fan for wood fired crematoria
- 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.
- Grate and air branches. Grate area, underfire plenum arrangement and number of overfire and secondary air ports.
- Control. Manual dampers or automatic control from temperature or oxygen.
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.