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 ID fan sits
- Gasifier
- Gas burner and furnace
- Boiler passes
- Economiser
- Cyclone or bag filter
- ID fan
- Stack
The ID fan sits after the economiser and, on raw gas systems, after a cyclone or bag filter that catches char and ash carried over from the gasifier. Suction covers the furnace, boiler passes, economiser and collector, typically 100 to 200 mmWC. In close-coupled plants the furnace draft also acts back through the burner on the gasifier, so the draft control must be coordinated with the gasifier air supply. Placing the fan after the collector means it handles only fine residual dust.
Gas at the ID fan
| Gas temperature, normal | 170 to 220°C after the economiser |
| Maximum or upset | Up to 280°C on gasifier upsets or with tar-fouled surfaces |
| Dust at the fan | Fine char and ash after the collector on raw gas systems; negligible on cleaned gas |
| Moisture and dew point | Moderate to high water vapour from fuel moisture and hydrogen combustion; low sulphur on biomass |
| Flue gas volume | Higher per unit of heat than conventional fuels because of the nitrogen in the fuel gas |
| 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 producer gas fired boilers
| Blade form | Backward curved, with wear liners on raw gas systems |
| Material of construction | Mild steel; wear plate on blade leading edges where char reaches the fan |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | VFD on furnace draft, coordinated with gasifier control |
| Also specified | Inspection door, casing drain, access for tar and soot cleaning |
What this ID fan has to cope with
Undersized fans from the wrong flue gas basis
Burning low calorific gas produces roughly 30 to 50% more flue gas per unit of heat than oil or natural gas, because of the nitrogen carried in the fuel. An ID fan sized by rule of thumb from conventional boilers cannot hold draft at full load. Sizing from the actual gas analysis and air ratio avoids this.
Tar deposits on the impeller
If raw gas cools before it burns, or combustion is incomplete, tar condenses in the boiler and the remaining aerosol reaches the fan. The sticky film collects char and unbalances the impeller. Good burnout, insulated hot gas lines, and an access door for cleaning the wheel in place limit the problem.
Draft coupled to the gasifier
In close-coupled systems the furnace draft acts back through the burner into the gasifier. An ID fan that swings draft upsets gasifier temperature and gas quality, which in turn upsets combustion. Slow, coordinated control between the gasifier air blower and the ID fan VFD keeps both stable.
Energy and control
Producer gas boilers often follow the gasifier's steady output, but start-up, feedstock changes and grate or bed cleaning bring long periods at reduced gas flow. A VFD on the ID fan saves energy through these periods and gives the slow, precise draft control the gasifier needs. Because flue gas volume is high, the ID fan is a larger share of auxiliary power than on a conventional boiler, so its efficiency deserves attention.
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 Forced Draft (FD) Fan for producer gas fired boilers.
Questions buyers ask
Why is the ID fan larger on a producer gas boiler than on an oil fired boiler?
Producer gas is about half nitrogen, which passes through the flame and leaves as flue gas without adding heat. The result is a much higher flue gas volume for the same steam output, so the ID fan must move more gas at the same draft than on an oil or natural gas boiler of the same rating.
Can tar from a gasifier damage the boiler ID fan?
Yes. Tar that is not burned condenses on cooler surfaces and on the impeller, collects char and dust, and causes unbalance and vibration. Complete combustion, keeping raw gas hot until it reaches the burner, and a fan with access for cleaning are the main defences.
How is ID fan draft controlled on a producer gas fired boiler?
Furnace draft is held by the ID fan VFD, but in close-coupled systems the draft also affects gasifier pressure. The loop is tuned slowly and coordinated with the gasifier air supply so that a change in draft does not upset gas quality and then combustion.
Information needed to quote an ID fan for producer gas fired 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. Raw hot gas or cleaned cold gas, and the feedstock.
- Gas analysis. CO, H₂, CH₄ and N₂ content, and tar and dust loading if measured.
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.