How biomass gasifier close-Coupled boilers work
In a close-coupled gasifier boiler, biomass such as wood chips, briquettes or coconut shell is partly oxidised in an updraft or downdraft gasifier to make producer gas, which flows straight into a combustion chamber attached to the boiler and burns there with secondary air. Because the gas is burnt hot, the tar it carries is burnt too, and the ash stays mostly in the gasifier. The flue gas leaves the combustion chamber at 1,000 to 1,200°C, passes the boiler tubes and an economiser, and reaches a small cyclone or bag filter at 160 to 220°C. Gasification air is supplied by a small blower or drawn in by suction, combustion air by an FD fan, and an ID fan maintains suction through the whole train.
Where the FD fan sits
- Inlet screen
- FD (combustion air) fan
- Air preheater (if fitted)
- Air control damper
- Gas burner or nozzles
- Combustion chamber
The fan sits near the combustion chamber and delivers ambient air to the burner or air nozzles around the producer gas inlet, sometimes through a small air preheater in the flue. Its pressure is modest, typically 100 to 250 mmWC, covering the ducts, control damper and burner. On pressurised gasifiers, a separate gasification air blower feeds the gasifier grate at a controlled rate.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | Up to 150 to 250°C where a preheater is fitted |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Dust | Clean air; screened against biomass dust |
| Turndown | Gas flow follows steam demand; low flows must stay stable |
| 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 gasifier close-Coupled boilers
| Blade form | Backward curved, small direct-driven unit |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft, arrangement 4 |
| Bearings and cooling | Motor bearings only; no cooling |
| Flow control | Variable speed drive ratio-linked to gas flow or oxygen |
| Also specified | Inlet screen and a non-return flap to stop hot gas back-flow |
What this FD fan has to cope with
Air-gas ratio drift
Producer gas from wood is lean, typically around 1,000 to 1,300 kcal/Nm³, and its quality moves with fuel moisture and bed condition. Too little air leaves tar and CO in the flue gas, and too much cools the flame below the point where tar burns. Speed control linked to flue gas oxygen holds the ratio as gas quality changes.
Back-flow of hot gas
When the combustion air fan stops while the combustion chamber is hot, producer gas and hot flue gas can flow back into the air duct and reach the fan. This can scorch the duct lining and create a flammable pocket. A non-return damper at the fan outlet and an interlock that closes the gas line valve on fan failure protect the fan and the duct.
Oversized fans at low load
Many gasifier boilers are supplied with combustion air fans selected with generous margins, then throttled hard by a damper for most of their life. At the low flows these plants often run at, a heavily throttled fan gives poor resolution and unstable air. A correctly sized fan or a speed drive gives finer control and better burnout.
Energy and control
The combustion air fan is small, but its control decides fuel efficiency. A variable speed drive with oxygen trim reduces excess air, which cuts both the stack loss and the ID fan power. On plants with a combustion air preheater, the added pressure drop is small and the fuel saving outweighs the extra fan power several times over.
ID fan and FD fan in biomass gasifier close-Coupled boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the economiser and cyclone, before the chimney | At the combustion chamber, feeding the gas burner |
| What it handles | Low-dust flue gas with some soot | Clean ambient air |
| Temperature at the fan | 170 to 220°C | Ambient |
| Main design concern | Steady suction through the gasifier and boiler | Accurate air-gas ratio |
| Control | Combustion chamber suction | Gas flow and oxygen |
See also the Induced Draft (ID) Fan for biomass gasifier close-Coupled boilers.
Questions buyers ask
What is the FD fan on a gasifier boiler for?
On a gasifier fired boiler the FD fan supplies the combustion air that burns the producer gas in the combustion chamber. It does not handle the gas itself. In a pressurised gasifier, a separate blower supplies the air used for gasification, while in a suction gasifier that air is drawn in by the ID fan through the fuel bed.
How much air does producer gas need to burn?
Roughly 1 to 1.3 Nm³ of air per Nm³ of wood producer gas, plus excess air of 10 to 30% to complete combustion of tar and soot. The exact figure depends on the gas composition, which varies with fuel moisture, so it is best set by oxygen measurement in the flue rather than a fixed ratio.
Can the combustion air fan and gasifier blower be combined?
In some small designs one fan feeds both the gasifier and the burner through separate dampers. Separate fans give independent control of gasification rate and combustion air, which improves gas quality, tar burnout and turndown. On anything above a small packaged unit, two fans are usually the better arrangement.
Information needed to quote an FD fan for biomass gasifier close-Coupled 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.
- Producer gas data. Gas flow, typical composition or calorific value, and burner type.
- Gasification air. Whether gasification air comes from this fan, a separate blower, or ID fan suction.
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.