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 ID fan sits
- Grate and primary chamber
- Secondary chamber
- Spark arrestor or cyclone
- Wet scrubber
- ID fan
- Chimney
The fan sits after the wet scrubber, at the chimney base. A cyclone or settling chamber ahead of the scrubber removes the coarse ash and burning embers, so the fan sees saturated gas with fine ash and soot carried through as droplets. Suction must cover the grate bed, both chambers, the cyclone and the scrubber, typically 150 to 300 mmWC. The fan also has to pull enough draft through the wood stack at ignition, when the fire is small and smoky.
Gas at the ID fan
| Gas temperature, normal | 55 to 80°C after the wet scrubber |
| Maximum or upset | 300 to 450°C if scrubber water fails during a hot burn |
| Dust at the fan | Fine wood ash and soot in scrubber carry-over; higher if the cyclone is bypassed |
| Moisture and dew point | Saturated; wood moisture and combustion water add 15 to 25% before the scrubber |
| Tar and condensables | Tar and pyroligneous acids from wet or poorly burnt wood |
| Density at the design temperature | About 1.02 kg/m³ at 75°C, 85% 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 wood fired crematoria
| Blade form | Radial-tipped or backward inclined, open shape that sheds wet ash and tar |
| Material of construction | SS 316 impeller and casing, or FRP casing with stainless impeller |
| Drive and arrangement | Belt drive, arrangement 9, motor outside the gas stream |
| Bearings and cooling | Standard bearings with shaft seal against wet gas |
| Flow control | Variable speed drive on primary chamber draft |
| Also specified | Wash nozzles, casing drain, large inspection door and a high-temperature trip |
What this ID fan has to cope with
Tar and soot build-up on the blades
Wet logs and a smoky start release tar and acetic acid that pass the secondary chamber when it is still cool. They stick to the impeller with fine ash and unbalance it within weeks. An open radial or backward inclined blade, wash nozzles in the casing and preheating the secondary chamber before ignition reduce the build-up.
Sparks and embers reaching the scrubber
Wood burning on a grate throws burning embers into the gas stream. Without a spark arrestor or cyclone they damage scrubber packing and FRP linings and can reach the fan. A drop-out chamber with a baffle and a cyclone ahead of the scrubber keep embers out of the wet section.
Weak draft at ignition
When the wood is first lit, the fire is small and smoky, and the gas is cool, so the chimney gives little natural draft. A fan sized only for the main burn condition at high temperature can leave the hall smoky at start. The fan curve is checked at the cold, low-flow start condition.
Wet corrosion in the fan
Scrubber exit gas is saturated and carries acetic and other organic acids from wood smoke, together with fine ash. Mild steel casings corrode at the bottom of the scroll and along the seams, and thinning blades lose balance. Stainless or FRP construction, a drain at the lowest point, and a mist eliminator upstream prevent rapid wastage in this duty.
Energy and control
A wood fired crematorium runs a strongly varying cycle: a cool, smoky ignition, an intense main burn, and a long burnout. A variable speed drive holding chamber draft lets the fan add suction at ignition and during charging, then ease back as the fire settles. Controlled draft also cuts the wood needed per cremation, since excess air drawn through the grate cools the chamber.
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 Forced Draft (FD) Fan for wood fired crematoria.
Questions buyers ask
Why does a wood fired crematorium need an ID fan?
An enclosed wood fired crematorium has a grate, a secondary chamber, a cyclone and usually a wet scrubber. Their combined resistance is more than a chimney can overcome, especially at ignition when the gas is cool. The ID fan provides steady draft so the wood burns cleanly and smoke does not escape into the hall.
How do you stop tar build-up on a crematorium ID fan?
Burn dry, seasoned wood where possible, preheat the secondary chamber before lighting, and keep the scrubber water flowing. On the fan itself, an open blade shape that sheds deposits, wash nozzles in the casing and a large inspection door let the operator clean the impeller on a routine before the tar and ash cause vibration.
What is the difference between wood and gas fired crematoria for the ID fan?
Wood firing produces fly ash, sparks and tar, so the wood fired crematorium needs a cyclone or spark arrestor and a fan with an open, self-cleaning blade. Gas firing produces very little dust, so a backward curved fan is used and the main concern is volume swing.
Information needed to quote an ID fan for wood fired crematoria
- 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.
- Wood consumption. Wood per cremation, its typical moisture and the number of cremations per day.
- Gas cleaning. Cyclone or spark arrestor, scrubber type and the expected gas temperature at the fan.
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.