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 ID fan sits
- Gasifier
- Producer gas line
- Combustion chamber
- Boiler tubes
- Economiser
- Cyclone or bag filter
- ID fan
- Chimney
The fan sits at the end of the train after a small cyclone or bag filter. In suction-type systems it is the only fan pulling gas through the fuel bed of the gasifier, so its suction must cover the gasifier bed, the gas line, the burner, the boiler and the collector, often 150 to 250 mmWC. In pressurised designs, a gasification blower pushes air into the gasifier and the ID fan only handles the boiler side, at lower suction.
Gas at the ID fan
| Gas temperature, normal | 170 to 220°C after the economiser |
| Maximum or upset | Up to 280°C if the economiser is bypassed or fouled with soot |
| Dust at the fan | Low; fine char and soot, typically below 200 mg/Nm³ after a cyclone |
| Moisture and dew point | Water dew point 50 to 60°C with chipped wood |
| Corrosive species | Tar and soot if combustion is poor; low sulphur and chlorine with wood |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 gasifier close-Coupled boilers
| Blade form | Backward curved, for efficiency on lightly loaded gas |
| Material of construction | Mild steel casing and impeller; stainless shaft sleeve where tar is a concern |
| Drive and arrangement | Direct coupled or belt driven on small units, arrangement 8 or 9 |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on combustion chamber suction |
| Also specified | Access door for soot cleaning, casing drain, and a gas-tight shaft seal |
What this ID fan has to cope with
Suction sets the gas rate
Where the ID fan pulls air through the gasifier, any change in fan speed changes the gasification rate and the producer gas quality as well as the furnace draft. A fan controlled only by a manually set damper makes the flame hunt as the fuel bed settles. A speed-controlled fan with a fast pressure loop on the combustion chamber gives steady gas production.
Tar and soot after poor combustion
If secondary air is short or the combustion chamber runs cold at low load, unburnt tar from the producer gas condenses as soot and sticky deposits in the economiser and on the fan blades. Smooth backward curved blades, an access door for cleaning, and correct combustion air control at low load keep deposits manageable and avoid unbalance.
Back-flow and gas safety
Producer gas is toxic and flammable. Loss of the ID fan while the gasifier is hot can push gas back through the feed and air inlets into the boiler house. Interlocks that stop the fuel feed and open a flare or vent on fan failure are part of the system design, and the fan motor is often given a dependable supply for this reason.
Energy and control
Gasifier boilers are usually small, 1 to 10 TPH, but the ID fan runs continuously, often against a damper set during commissioning. A variable speed drive on combustion chamber suction gives finer control of the gasification rate and saves power during low steam demand. Keeping the economiser clean holds the gas temperature, and therefore the gas volume at the fan, close to design.
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 Forced Draft (FD) Fan for biomass gasifier close-Coupled boilers.
Questions buyers ask
Does a biomass gasifier boiler need an ID fan?
Most do. In suction gasifiers the ID fan draws air through the fuel bed and moves the gas to the burner. In pressurised designs a blower feeds the gasifier and the ID fan handles the boiler side. Natural draft alone is rarely enough for the combined resistance of the gasifier bed, boiler, economiser and collector.
Why is there soot in my gasifier boiler ID fan?
Soot comes from producer gas tar that did not burn fully, usually because secondary air was short, the gas was too wet, or the combustion chamber ran cold at low load. Correcting air control, keeping the chamber refractory hot, and cleaning the economiser and fan at regular intervals prevent the build-up that leads to unbalance.
What happens if the ID fan trips on a gasifier boiler?
The gasifier keeps producing gas for some time after the fan stops, because the fuel bed is still hot. Without suction, producer gas can leak into the boiler house through feed doors and air inlets. Fuel feed must stop at once, and the gas should be vented or flared through a dedicated safety path controlled by interlocks.
Information needed to quote an ID fan for biomass gasifier close-Coupled boilers
- 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.
- Gasifier type. Updraft or downdraft, suction or pressurised, and the gasifier bed pressure drop.
- Safety interlocks. How the plant handles a fan trip, and whether a flare or vent is fitted.
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.