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 FD fan sits
- Inlet filter
- Combustion air (FD) fan
- Recuperator
- Hot air main
- Gas and air burners
The fan delivers ambient air to the recuperator and on to burners that mix producer gas and air at low pressure, typically 200 to 500 mmWC at the fan. The separate gasifier air blower supplies air and steam under the coal bed and sets the gas output; it is often a medium pressure blower and is matched to the gasifier, not to the furnace burners.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 250 to 400°C, downstream of the fan |
| Dust | Coal yard dust nearby; inlet filter recommended |
| Pressure at the fan outlet | Typically 200 to 500 mmWC |
| Air to fuel ratio | About 1.1 to 1.4 m³ of air per m³ of producer gas |
| 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 reheating furnaces
| Blade form | Backward curved, for efficiency at moderate pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or belt drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive or inlet damper, linked to gas flow |
| Also specified | Inlet filter; low air pressure interlock with the gas valve |
What this FD fan has to cope with
Changing gas quality
Producer gas calorific value shifts with coal quality, bed condition and gasifier load. A fixed air setting then runs rich or lean. Linking the combustion air fan speed to measured gas flow, with trim from flue gas oxygen, keeps combustion right as the gas changes and reduces soot and CO.
Coal dust at the inlet
The gasifier and coal yard are usually close to the furnace, so the air around the fan carries coal dust. Without a filter the dust builds up on the impeller and enters the recuperator. An inlet filter with a pressure gauge protects both. Where the bay is very dusty, the inlet can be ducted from a cleaner location.
Confusing the two blowers
The gasifier air blower and the furnace combustion air fan are different duties with different pressures and controls. Treating them as interchangeable, or sizing one from the other, leads to poor gas quality or poor combustion. Each should be specified from its own data. The furnace fan follows the burners; the blower follows the bed.
Energy and control
The combustion air fan on a producer gas furnace is smaller than the gasifier blower and the ID fan, but it runs continuously. A variable speed drive linked to gas flow saves power during holds and improves air to fuel ratio control, which reduces both gasifier coal consumption and ID fan load.
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 Induced Draft (ID) Fan for producer gas fired reheating furnaces.
Questions buyers ask
Is the gasifier blower the same as the FD fan?
No. The gasifier air blower supplies air and steam to the coal bed to make producer gas. The FD fan, or combustion air fan, supplies air to the furnace burners to burn that gas. They have different pressures, flows and controls, and each should be selected from its own duty data.
How much combustion air does producer gas need?
Roughly 1.1 to 1.4 m³ of air per m³ of producer gas, depending on its composition. That is far less than oil or natural gas per unit of fuel volume, because producer gas is diluted with nitrogen. The forced draft fan is sized from the gas flow and composition.
Why does our producer gas furnace produce CO and soot?
Usually because air and gas are not well matched as gas quality changes, or because burner mixing is poor at low pressure. Controlling the combustion air fan from gas flow, with oxygen trim, and checking burner condition, usually improves combustion. Cleaning tar from burner passages also restores mixing.
Information needed to quote an FD fan for producer gas fired reheating furnaces
- 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, composition and calorific value at normal and maximum firing.
- Burner type. Burner design and the air pressure it needs.
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.