How biomass torrefaction plants work
A torrefaction plant heats dried biomass to 250 to 300°C without oxygen, in a rotary drum, screw reactor or moving bed, for 15 to 45 minutes. The biomass loses water and part of its volatiles as torgas, a mixture of water vapour, CO, CO₂, acetic acid, methanol and light tars, and leaves as a brittle, darker solid with higher calorific value. The torgas is drawn off and burnt in a combustor, often with support fuel, and the hot flue gas heats the reactor directly through a recirculating gas loop or indirectly through a jacket or thermic fluid heater. A predryer usually takes heat from the same flue gas. An ID fan exhausts the flue gas through a cyclone or bag filter to the stack, and an FD fan supplies combustion air to the torgas combustor.
Where the ID fan sits
- Torgas combustor
- Reactor heating loop
- Predryer
- Cyclone or bag filter
- ID fan
- Stack
The fan sits at the cold end, after the predryer and dust collector. By then the gas has given up most of its heat and is humid from the moisture driven off in the predryer. The fan holds suction on the combustor and on the reactor heating circuit, so that torgas and flue gas cannot leak into the plant. A separate recirculation fan, handling hotter gas at 300 to 400°C, moves gas around the direct heating loop; it is a process fan rather than the ID fan.
Gas at the ID fan
| Gas temperature, normal | 100 to 150°C after the predryer |
| Maximum or upset | Up to 250°C if the predryer is bypassed |
| Dust at the fan | Fine biomass dust and char; below 100 mg/Nm³ after a bag filter |
| Moisture and dew point | Very humid; water dew point 60 to 75°C from predryer moisture |
| Corrosive species | Acetic and formic acid from unburnt torgas, condensing at cold spots |
| Density at the design temperature | About 0.88 kg/m³ at 130°C, 73% 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 biomass torrefaction plants
| Blade form | Backward curved, smooth profile to shed dust |
| Material of construction | SS 304 or SS 316 wetted parts where acid condensate is expected; otherwise mild steel with insulation |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings outside the gas path with a shaft cooling disc |
| Flow control | Variable speed drive on combustor suction |
| Also specified | Gas-tight shaft seal, casing drain, insulated casing, and combustible dust precautions where required |
What this ID fan has to cope with
Acid condensate from torgas
Torgas contains acetic acid, formic acid and other organics. If the combustor runs cold at start-up or low load, these pass unburnt and condense with water vapour in the cool fan casing, corroding mild steel within months. Stainless wetted parts, casing insulation, a drain at the lowest point and a warm-up on hot gas before full flow address this.
Tar and dust deposits
Fine char from the reactor and predryer, combined with traces of tar, forms sticky deposits that build up on the blades and inside the scroll. These grow unevenly and cause unbalance. Smooth stainless blades with no pockets, a large access door, and a regular wash during planned stops keep vibration down between overhauls.
Gas tightness
Torgas and its flue gas can contain CO. The fan is under suction on the inlet side and slightly positive on the outlet side, so a leaking shaft seal or casing joint can either draw air into the system or push gas into the plant. Gasketed joints, a proper shaft seal and CO monitoring around the fan keep the area safe.
Energy and control
Torrefaction plants are energy balanced around the torgas: the more completely it is burnt and its heat recovered, the less support fuel is needed. The ID fan on a variable speed drive holds combustor suction steadily, which helps stable combustion. Reducing air leaks into the reactor loop and predryer lowers the flue gas volume, saving both fan power and heat.
ID fan and FD fan in biomass torrefaction plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the predryer and bag filter, before the stack | At the torgas combustor |
| What it handles | Humid flue gas with fine dust and possible acids | Clean ambient air |
| Temperature at the fan | 100 to 150°C | Ambient |
| Main design concern | Acid condensate and gas tightness | Air control for lean, variable torgas |
| Control | Combustor suction | Flue gas oxygen |
See also the Forced Draft (FD) Fan for biomass torrefaction plants.
Questions buyers ask
What fans are used in a biomass torrefaction plant?
Usually a combustion air fan for the torgas combustor, a recirculation fan that moves hot gas through the reactor in direct heating designs, a predryer fan or exhaust, and an ID fan that exhausts the flue gas to the stack after heat recovery. Some plants combine the predryer exhaust and the ID fan into one machine.
Why does a torrefaction plant ID fan need stainless steel?
Unburnt torgas carries acetic acid and other organic acids that condense with water at the low gas temperatures after the predryer. Stainless steel resists this condensate where mild steel would corrode quickly. If the combustor reliably burns all the torgas at every load, an insulated mild steel fan can be acceptable.
Is the recirculation fan the same as the ID fan?
No. The recirculation fan moves hot, low-oxygen gas around the reactor loop at 300 to 400°C and needs heat-resisting construction and a tight seal. The ID fan on a torrefaction plant exhausts only the surplus flue gas after the predryer and dust collector, at lower temperature but with higher moisture.
Information needed to quote an ID fan for biomass torrefaction 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.
- Heating method. Direct gas recirculation or indirect heating, and where the ID fan sits relative to the predryer.
- Torgas composition. Expected acid and tar content, and the combustor outlet temperature.
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.