How producer gas fired reheating furnaces work
Many re-rolling and forging units fire their reheating furnaces on producer gas made on site in a coal gasifier. Coal, usually low ash imported or washed coal, is gasified with air and steam blown through the bed by a gasifier air blower, giving a low calorific gas of about 1,100 to 1,500 kcal/Nm³ rich in CO, hydrogen and nitrogen. Hot raw gas, at 400 to 600°C with tar and dust, can be fired directly through a short insulated main, or the gas can be cooled, cleaned of tar and dust and fired cold. Because the fuel is so diluted with nitrogen, the flue gas volume per tonne of steel is higher than with oil or natural gas, and it carries SO₂ from the coal sulphur, plus soot when hot raw gas is used.
Where the ID fan sits
- Furnace zones
- Charging end flue
- Recuperator
- Flue gas duct
- ID fan
- Chimney
The fan sits after the recuperator. With hot raw gas firing, soot and condensed tar foul the recuperator tubes, so the suction rises steadily between cleanings, from perhaps 80 mmWC clean to considerably more fouled. The fan must have margin for the fouled condition. Gas reaches it at about 300 to 450°C; on furnaces without a recuperator it is diluted with air before the fan.
Gas at the ID fan
| Gas temperature, normal | 300 to 450°C after the recuperator |
| Maximum or upset | 550 to 650°C when the recuperator is bypassed for cleaning |
| Dust at the fan | Soot and fine ash, heavier with hot raw gas firing; can be sticky with tar |
| Corrosive species | SO₂ from coal sulphur, with an acid dew point often around 120 to 150°C |
| Flue gas volume | Higher per tonne of steel than oil or natural gas firing |
| Density at the design temperature | About 0.53 kg/m³ at 400°C, 44% 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 reheating furnaces
| Blade form | Backward inclined flat plate or radial tipped, shedding soot more easily than aerofoil blades |
| Material of construction | Boiler quality plate up to about 400°C; heat-resisting steel where the fan stays in service during recuperator bypass |
| Drive and arrangement | Direct coupled, with sliding supports and the motor clear of the hot casing |
| Bearings and cooling | Cooling disc, heat shield and water-cooled bearings on hot duties |
| Flow control | Variable speed drive on furnace pressure |
| Also specified | Inspection and cleaning doors, insulation to stay above acid dew point, and a casing drain |
What this ID fan has to cope with
Tar soot on the impeller
With hot raw gas firing, unburnt tar and soot condense on cooler surfaces, including the fan blades. The layer builds unevenly and the fan goes out of balance. Cleaning doors, a blade form that sheds deposits, keeping the gas hot enough at the fan, and better burner mixing to burn the tar all reduce the problem.
Acid corrosion at low load
Coal sulphur leaves the furnace as SO₂ and some SO₃. When the fan runs at low gas temperature during holds or start-up, acid condenses on the casing and duct, and corrosion starts at joints and stiffeners. Insulation and keeping the gas above the acid dew point protect the fan.
Fan sized for oil, not producer gas
Furnaces converted from oil to producer gas to cut fuel cost often keep the old ID fan. Producer gas brings much more nitrogen into the flue gas, so the fan runs out of capacity and the furnace pressurises. The duty must be recalculated from the producer gas composition and the actual fuel rate.
Energy and control
Producer gas furnaces in re-rolling mills run with frequent holds and gasifier load changes. A variable speed drive on the ID fan, holding furnace pressure, avoids wasting power against a closed damper. Because the flue gas volume is large, excess air control has a strong effect on fan power as well as on gasifier coal consumption.
ID fan and FD fan in producer gas fired reheating furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the recuperator, before the chimney | Before the recuperator |
| What it handles | Large volume flue gas with soot and SO₂ | Ambient air |
| Temperature at the fan | 300 to 450°C | Ambient |
| Main design concern | Tar soot fouling and acid dew point | Air ratio as gas quality changes |
| Control | Furnace pressure | Linked to producer gas flow |
See also the Forced Draft (FD) Fan for producer gas fired reheating furnaces.
Questions buyers ask
Why does a producer gas furnace need a bigger ID fan than an oil-fired one?
Producer gas contains about half nitrogen, which passes straight through combustion. For the same heat input the flue gas volume is much larger than with furnace oil, so the induced draft fan must handle more gas. A furnace converted from oil to producer gas often needs a larger fan.
How do we stop soot buildup on our coal gasifier furnace ID fan?
Improve burner mixing so tar burns completely, keep the flue gas hot enough that tar does not condense at the fan, use a blade form that sheds deposits, and fit cleaning doors. Switching from hot raw gas to cleaned cold gas removes most of the problem.
What is the acid dew point on a producer gas fired furnace?
It depends on coal sulphur and excess air, but is often around 120 to 150°C. The ID fan and ducts should run above this, with insulation, to avoid acid condensation and corrosion during low load operation and start-up. Furnaces firing hot raw gas from high sulphur coal sit at the upper end of the range, and need the most margin.
Information needed to quote an ID fan for producer gas fired reheating furnaces
- 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 and fuel. Coal type and sulphur content, hot raw or cleaned cold gas, and gas composition.
- Recuperator fouling. Suction at the fan inlet with a clean and with a fouled recuperator.
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.