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 ID fan sits
- BFG and support fuel burners
- Furnace
- Superheater and boiler bank
- Economiser
- Air heater
- ID fan
- Stack
The fan sits after the air heater, often without a dust collector, because the fuel is cleaned before it reaches the burners. Where coal is co-fired, an ESP comes first and the fan follows it. The suction at the fan includes furnace draft, the boiler, economiser and air heater, typically 150 to 250 mmWC. Because BFG produces roughly one and a half to two times the flue gas of a coal or oil boiler for the same steam, the fan is large for the boiler rating.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 230°C with air heater fouling |
| Dust at the fan | Low on BFG alone; fine zinc and iron oxide carried through gas cleaning; higher where coal is co-fired |
| Moisture and dew point | Moderate moisture from scrubbed gas; water dew point well below fan temperature |
| Corrosive species | Low sulphur on BFG; SO₂ only from co-fired coke oven gas or oil |
| Gas density | Higher than air because of high CO₂ content, about 1.35 kg/Nm³ |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 blast furnace gas fired boilers
| Blade form | Backward curved or aerofoil for efficiency on clean, high-volume gas |
| Material of construction | Mild steel; wear liners only where coal is co-fired |
| Drive and arrangement | Direct coupled, arrangement 8 or 3 for large double inlet fans |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft, inlet damper for isolation |
| Also specified | Gas-tight shaft seal, interlocks with the purge sequence, and often two fans at 60% each on larger units |
What this ID fan has to cope with
Underestimating gas volume and density
BFG flue gas has more mass and volume per unit of heat than any common fuel. Fans selected on oil or coal ratios fall short at full BFG firing. Sizing on the actual gas analysis and the worst firing mix, at the true density, avoids a fan that limits boiler output.
Purge and unburnt CO
If a BFG flame is lost, CO accumulates quickly in the furnace. The ID fan must give a reliable purge flow of at least the specified air changes before relight, and trip logic must keep it running on a burner trip. Reliable draft is a safety function here.
Zinc and alkali deposits
Zinc and alkali fume that passes gas cleaning deposits on air heater tubes and, at times, on fan blades. The deposits are light but form unevenly and can shed from a few blades at once. Regular inspection at shutdowns, on-site balancing and vibration monitoring on the bearings keep the fan running between overhauls.
Energy and control
BFG availability follows the blast furnace, and the boiler absorbs surplus gas, so load changes several times a shift. A variable speed drive on furnace draft gives fast response and large savings over damper control on what is often one of the biggest fans in the power plant. Keeping air heater leakage low matters because leakage air adds directly to an already large 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 Forced Draft (FD) Fan for blast furnace gas fired boilers.
Questions buyers ask
Why is the ID fan bigger on a blast furnace gas boiler than on a coal boiler?
BFG is a lean gas, mostly nitrogen and CO₂, so for each unit of heat it produces far more flue gas than coal or oil, and that gas is denser than air. A boiler of the same steam rating therefore needs a larger ID fan with a larger motor, and the fan must be sized on the BFG-rich firing case.
Does a BFG fired boiler ID fan need erosion protection?
On BFG alone, no. The fuel is cleaned to a few mg/Nm³ of dust before firing, so the ID fan handles nearly clean flue gas and a backward curved or aerofoil wheel in mild steel is normal. Wear liners are needed only where coal or other solid fuel is co-fired and the fan follows the ESP.
What safety interlocks apply to the ID fan on a blast furnace gas boiler?
The ID fan must be proven running before purge and gas admission, must purge the furnace with the specified air volume before every light-up, and should stay in service on a burner trip so that any unburnt CO is removed. Its trip is normally a boiler trip, and its status forms part of the burner management logic.
Information needed to quote an ID fan for blast furnace 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.
- Firing mix. BFG share and the support fuels, coke oven gas, oil or coal, at minimum and maximum load.
- Gas analysis. BFG composition and calorific value range, and dust content after gas cleaning.
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.