How producer gas fired boilers work
Producer gas is made in a gasifier from biomass, coal or charcoal by partial combustion with air, and fired in a boiler as a low calorific fuel gas. It typically contains 18 to 22% CO, 12 to 20% H₂, a few percent CH₄ and about half nitrogen, with a calorific value near 1,000 to 1,300 kcal/Nm³. Close-coupled systems fire the hot raw gas at 300 to 500°C straight into the boiler, carrying tar, char and dust; cold gas systems scrub and cool it first. The large nitrogen content means a high flue gas volume per unit of heat. Balanced draft is usual, with an ID fan after the boiler and an FD fan for combustion air.
Where the FD fan sits
- Air intake
- FD fan
- Air damper
- Producer gas burner
- Furnace
The FD fan stands near the burner and supplies air to the burner register or a windbox around the gas nozzles. It takes ambient air. Because the gas arrives in large volume at low pressure, the burner depends on air velocity for mixing and flame stability, so the register takes a meaningful share of the fan pressure, typically 80 to 150 mmWC in total on balanced draft units. The gasifier has its own air blower; the FD fan serves only combustion.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Typically 80 to 150 mmWC |
| Air requirement | About 1.0 to 1.3 m³ of air per m³ of producer gas, plus excess air |
| Dust | Biomass or coal dust near the gasifier feed |
| Turndown | Set by gasifier turndown, typically 2:1 to 3:1 |
| 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 producer gas fired boilers
| Blade form | Backward curved, stable at reduced flow |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or direct mounted |
| Bearings and cooling | Motor or pedestal bearings |
| Flow control | VFD in ratio with gas flow, with O₂ trim |
| Also specified | Inlet screen against fuel dust, air pressure switch, interlock with the gasifier |
What this FD fan has to cope with
Gas quality changes with feedstock
Changes in moisture or size of the biomass alter the CO and H₂ content of the gas within minutes. A fixed air setting then gives either CO in the stack or excess air that cools an already weak flame. O₂ trim, or CO measurement, acting on the FD fan control corrects this as it happens.
Fuel dust at the intake
Gasifier fuel handling generates dust close to the boiler, and it builds up on an unscreened impeller, causing unbalance and lost air flow. An inlet screen and an intake on the clean side of the building prevent this. Where the area is very dusty, a ducted intake from outside is worth the small extra pressure.
Flame stability at low gas flow
When the gasifier is turned down, gas velocity at the burner drops and the flame can lift or flash back, depending on burner design. The FD fan must reduce air in step and hold register velocity. A fan sized close to the actual duty, rather than with a large margin, gives better control at low flow.
Energy and control
The FD fan on a producer gas boiler is small, because air demand per unit of gas is low. Its power saving is modest, but its effect on fuel use is not: excess air directly cools a low calorific flame and reduces boiler output. VFD control with O₂ trim keeps excess air low and steady, and that matters more for steam output than the fan power itself.
ID fan and FD fan in producer gas fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the economiser and collector | At the gas burner |
| What it handles | High-volume flue gas, char on raw gas systems | Ambient air |
| Temperature at the fan | 170 to 220°C | Ambient, 25 to 45°C |
| Relative size | Large, due to fuel nitrogen | Small, low air per m³ of gas |
| Main design concern | Gas volume and tar deposits | Air ratio with changing gas quality |
| Control | Furnace draft, coordinated with gasifier | Gas flow and O₂ |
See also the Induced Draft (ID) Fan for producer gas fired boilers.
Questions buyers ask
Why does producer gas need so little combustion air?
Producer gas is mostly nitrogen with CO and H₂, so its calorific value is low and each cubic metre needs only about 1 to 1.3 m³ of air. Natural gas needs nearly 10. The FD fan on a producer gas fired boiler is therefore small, but it must mix air well with a large gas flow.
What is the difference between the gasifier blower and the boiler FD fan?
The gasifier blower supplies the limited air that partly burns the fuel inside the gasifier to make producer gas. The FD fan supplies combustion air at the boiler burner to burn that gas completely. They are controlled separately but must be coordinated through the plant control.
Can the FD fan be reused if the boiler is converted to direct biomass firing?
Sometimes, but direct firing needs far more air per unit of heat delivered to the furnace, at a different pressure set by the grate or bed. The fan must be checked against the new air flow and resistance, and in most cases a larger fan is needed.
Information needed to quote an FD fan for producer gas fired boilers
- 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.
- Gas flow and analysis. Producer gas flow at full load and its composition.
- Burner design. Burner type and the air pressure needed at the register.
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.