How blast furnace gas fired boilers work
Blast furnace gas is the top gas of an iron blast furnace after dust removal in a dust catcher and wet scrubber or dry bag filter. It is a lean fuel, typically 20 to 25% CO, 20 to 25% CO₂, a little hydrogen and the rest nitrogen, with a calorific value of about 700 to 900 kcal/Nm³ and dust below about 10 mg/Nm³ after cleaning. Steel plants burn it in captive power boilers, usually with coke oven gas, oil or coal as support fuel to keep the flame stable. The air heater preheats combustion air, and often the gas, to lift flame temperature. Flue gas leaves the air heater at about 150 to 180°C and the ID fan sends it to the stack.
Where the FD fan sits
- Inlet screen
- FD fan
- Steam coil air heater
- Air heater
- Windbox
- BFG and support burners
The fan draws ambient air and pushes it through a steam coil air heater, used at low load, and the main air heater, where the air reaches about 200 to 300°C, then into the burner windbox. On some boilers the BFG is preheated in a separate gas heater. The FD fan must supply windbox pressure for full firing plus the air heater loss, and it also carries the purge air needed before every light-up.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 200 to 300°C, downstream of the fan |
| Dust | Steel plant atmosphere with iron oxide and sinter dust; inlet screen or filter recommended |
| Pressure at the fan outlet | Typically 200 to 350 mmWC |
| Air demand | About 0.7 to 0.9 Nm³ of air per Nm³ of BFG, far less than for rich fuels |
| 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 blast furnace gas fired boilers
| Blade form | Backward curved or aerofoil |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive or inlet vanes on combustion control |
| Also specified | Inlet filter or screen, inlet silencer, and two fans at 60% each on larger boilers |
What this FD fan has to cope with
Shifting air demand with fuel mix
BFG needs little air per unit volume, coke oven gas and oil need much more. As the fuel mix shifts, the air flow changes widely. The fan is sized on the richest firing case and controlled across the full range without hunting.
Steel plant dust in the intake
Iron oxide and sinter dust in the plant air deposit on the impeller and in the air heater. An intake away from dusty areas with a screen or filter keeps the fan balanced and the air heater clean.
Purge air reliability
No BFG burner may light without a full purge. If the FD fan is unavailable the boiler cannot restart, so a second fan or a quick-change spare rotor is common in plants where the boiler absorbs surplus gas.
Energy and control
Air flow changes constantly as the boiler takes surplus gas, so a variable speed drive is the norm. Most of the savings come from avoiding throttling at the part loads common when the blast furnace is slack. Oxygen trim on BFG is especially valuable because excess air cools an already lean flame and adds to flue gas volume.
ID fan and FD fan in blast furnace gas fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the air heater, before the stack | Before the air heater, feeding the burners |
| What it handles | Large volume of CO₂-rich flue gas, low dust | Ambient air |
| Temperature at the fan | 150 to 180°C | Ambient |
| Main design concern | Gas volume, density and purge reliability | Air demand across fuel mixes |
| Control | Furnace draft | Combustion control on fuel mix |
See also the Induced Draft (ID) Fan for blast furnace gas fired boilers.
Questions buyers ask
Why is combustion air preheated on a BFG fired boiler?
BFG has a low calorific value and a low flame temperature. Preheating the air, and sometimes the gas, raises flame temperature and keeps the flame stable, especially at low load. The FD fan still handles cold air; the heating happens downstream in the air heater, so the fan is selected at ambient density.
Does the FD fan on a BFG boiler need to be large?
Not as large as you might expect. BFG needs less air per cubic metre than rich fuels, but the fan is sized for the richest support fuel case and for purge flow, which often sets the larger duty.
How many FD fans does a blast furnace gas boiler need?
Smaller boilers use one fan. Larger captive power boilers often use two fans at about 60% each, so the boiler can keep running at reduced load and keep absorbing surplus gas if one fan is out. Each fan is then checked for parallel operation so that neither stalls when both run at low load.
Information needed to quote an FD fan for blast furnace gas fired boilers
- 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.
- Fuel cases. BFG only, mixed gas and support fuel only cases, with air demand for each.
- Air heater. Air side pressure drop of the steam coil and main air heater.
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.