How gas based DRI plants work
A gas based DRI plant reduces iron oxide pellets in a vertical shaft furnace with a hot reducing gas rich in hydrogen and CO. The reducing gas is made by reforming natural gas with recycled top gas in catalyst-filled tubes inside a large reformer, or heated in a process gas heater in schemes without a reformer. The reformer burners fire on top gas and natural gas, so the flue gas leaving the reformer box at about 1,000 to 1,100°C contains a high proportion of water vapour and little dust. It passes a flue gas heat recovery section that preheats combustion air, feed gas and natural gas, and an ID fan or ejector stack carries it to the chimney at about 150 to 250°C. Process gas is moved by compressors, not fans.
Where the FD fan sits
- Inlet filter and silencer
- Combustion air (FD) fan
- Air preheater coils
- Hot air ducts
- Burner rows
The fan draws ambient air through a filter and silencer and pushes it into the air preheater coil in the flue gas heat recovery section. The hot air then passes long insulated ducts and headers to the burner rows on the reformer roof or floor. The fan pressure covers the preheater, ducts, distribution dampers and burners, typically 300 to 600 mmWC, with margin for the distribution dampers to work.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | About 450 to 650°C, downstream of the fan |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Turndown | Moderate; the reformer usually runs near rated output |
| 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 gas based DRI plants
| Blade form | Backward curved aerofoil, for high efficiency on continuous duty |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, often double inlet for large reformers |
| Bearings and cooling | Standard oil or grease lubricated bearings |
| Flow control | Variable speed drive, with inlet vanes or dampers as backup |
| Also specified | Inlet filter, silencer and interlocks with the burner management system |
What this FD fan has to cope with
Uneven burner air
Poor air distribution across the burner rows gives hot spots on some reformer tubes and shortens their life, while other rows run with excess air. The fan must provide enough pressure margin for the distribution dampers to balance the rows, and the header design must avoid starving the burners furthest from the fan.
Air preheater protection
Loss of combustion air while flue gas is still hot can overheat the air preheater coil in the recovery section. Interlocks that cut firing on low air flow, a fan with a robust drive and good condition monitoring, and in many plants a standby arrangement, protect the coil and the reformer.
Inlet filter blockage
Plants in dusty or coastal areas see inlet filters load up over weeks, cutting air flow and raising fan power. Differential pressure monitoring on the filter, and a fan selected with a stable margin on its curve so that it does not approach stall as resistance rises, avoid sudden loss of air.
Energy and control
Like the ID fan, the combustion air fan runs continuously for the whole campaign. An efficient aerofoil impeller and a variable speed drive cut power compared with an inlet damper. Accurate air to fuel ratio control matters more: excess air raises reformer fuel use and the flue gas volume, which in turn increases the load on the ID fan.
ID fan and FD fan in gas based DRI plants
| ID fan | FD fan | |
|---|---|---|
| Position | After flue gas heat recovery, before the stack | Before the air preheater |
| What it handles | Wet, clean reformer flue gas | Ambient filtered air |
| Temperature at the fan | 150 to 250°C | Ambient |
| Main design concern | Dew point margin and box pressure control | Even distribution across burners |
| Control | Reformer box pressure | Air to fuel ratio |
See also the Induced Draft (ID) Fan for gas based DRI plants.
Questions buyers ask
What does the FD fan do in a DRI plant?
In a gas based DRI plant the FD fan supplies combustion air to the reformer or process gas heater burners. The forced draft fan takes ambient air, pushes it through the air preheater in the flue gas heat recovery section, and delivers it hot to the burner rows through insulated ducts and distribution dampers.
Does the reformer combustion air fan handle hot air?
No. It handles ambient air and pushes it into the preheater. The air downstream is at several hundred degrees, but the fan itself stays cool, which keeps it smaller and simpler than a hot air fan placed after the preheater would be. The preheater air side loss is part of the fan duty.
Why is reliability critical on the reformer combustion air fan?
A combustion air trip shuts down the reformer burners, which stops reducing gas production and with it the shaft furnace. It can also overheat the air preheater coil. Many plants therefore specify a robust drive, vibration and bearing temperature monitoring, and a planned spares or standby arrangement.
Information needed to quote an FD fan for gas based DRI plants
- 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.
- Burner arrangement. Number of burners and rows, and air pressure required at the burners.
- Air preheater. Air side pressure drop and hot air temperature.
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.