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 FD fan sits
- Inlet filter
- FD (combustion air) fan
- Air preheater (if fitted)
- Torgas burner
- Combustor
The fan sits beside the combustor and supplies ambient air to the torgas burner and the support fuel burner. Some plants preheat the air in a small exchanger in the flue to help the wet, lean torgas burn steadily. Pressure is moderate, typically 150 to 350 mmWC, covering the preheater, dampers and burner. Because the combustor is held under suction by the ID fan, the FD fan only needs to deliver air to the burner, not push gas through the plant.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 150 to 300°C where fitted, downstream of the fan |
| Pressure at the fan outlet | Typically 150 to 350 mmWC |
| Dust | Clean air; biomass dust in the area |
| Turndown | Wide; support fuel flow and torgas flow vary |
| 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 biomass torrefaction plants
| Blade form | Backward curved, direct driven |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted or direct coupled |
| Bearings and cooling | Standard bearings; no cooling |
| Flow control | Variable speed drive with oxygen trim |
| Also specified | Inlet filter, non-return damper and burner management interlocks |
What this FD fan has to cope with
Lean, variable torgas
Torgas is wet and lean, and its heat content changes with feed moisture and reactor temperature. Air supply that does not follow these changes causes either CO and acid slip or excess air that cools the combustor. Oxygen trim on the fan speed keeps the combustor hot and the acids burnt, which also protects the ID fan downstream.
Start-up on support fuel
At start-up the combustor runs on oil, gas or biomass until the reactor is hot enough to release torgas, and the air needs of the support fuel differ widely from those of torgas. The fan must cover both duties, and the switch-over between them, which favours a variable speed drive over a fixed damper setting.
Back-flow at trip
On a fan stop, hot gas from the combustor can flow back into the air duct and reach the fan, especially while the reactor is still releasing torgas. A non-return damper at the fan outlet, an interlock that closes the torgas valve and diverts gas to a flare, and a heat-tolerant duct section protect the fan and the plant.
Energy and control
The combustion air fan is small, but excess air in the torgas combustor directly increases the gas volume the ID fan must move and the heat lost to the stack. Oxygen-trimmed speed control reduces both. Where a preheater is fitted, it improves torgas burnout and reduces support fuel, and its extra pressure drop costs far less than the fuel it saves.
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 Induced Draft (ID) Fan for biomass torrefaction plants.
Questions buyers ask
What air is needed to burn torgas?
Torgas is lean and wet, and needs relatively little air per cubic metre compared with a fuel gas, but its composition varies with feed moisture and reactor temperature. The air is therefore set by measured oxygen in the combustor flue gas rather than by a fixed ratio, typically with a few percent of oxygen left to ensure the acids burn out.
Does the FD fan handle torgas?
No. The FD fan on a torrefaction plant handles only ambient air and pushes it to the combustor. The torgas is drawn from the reactor by its own path, under the suction created by the ID fan or by the recirculation loop, so the combustion air fan stays clean, cool and simple in mild steel construction.
Is a separate fan needed for the support burner?
Sometimes. Many plants use one combustion air fan with separate dampers for the support burner and the torgas burner, because the two rarely run at full rate together. Where the support burner is a packaged unit with its own blower, the FD fan is sized for torgas alone plus the cooling air the support burner needs while idle.
Information needed to quote an FD fan for biomass torrefaction plants
- 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.
- Burner arrangement. Torgas burner, support burner and their air needs.
- Support fuel. Type and maximum firing rate of the support fuel.
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.