How coke oven gas fired boilers work
Coke oven gas is the by-product gas of a by-product coke oven battery, cleaned of tar, ammonia, benzol and, where a desulphurisation plant is fitted, most of its H₂S. It is a rich fuel, typically 55 to 60% hydrogen and 23 to 27% methane, with a calorific value of about 4,000 to 4,500 kcal/Nm³. Residual H₂S can range from a few hundred mg/Nm³ after desulphurisation to several g/Nm³ without it, and traces of naphthalene and tar remain. Steel plants fire it in captive boilers, often mixed with blast furnace gas. Hydrogen-rich firing gives a hot, fast flame and wet flue gas. An FD fan pushes air through the air heater to the burners, and an ID fan draws the gas to the stack.
Where the FD fan sits
- Inlet screen
- FD fan
- Steam coil air heater
- Air heater
- Windbox
- COG burners
The fan draws ambient air at grade and pushes it through a steam coil air heater and the main air heater into the burner windbox. The burners are designed for the high flame speed of hydrogen, and each needs a steady air pressure at the register. The FD fan must cover the windbox pressure, air heater and duct losses, typically 200 to 350 mmWC, and must provide purge air 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 280°C, downstream of the fan |
| Dust | Steel plant atmosphere; coal and coke dust near the battery; screen or filter needed |
| Pressure at the fan outlet | Typically 200 to 350 mmWC |
| Air demand | About 4 to 5 Nm³ of air per Nm³ of coke oven 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 coke oven gas fired boilers
| Blade form | Backward curved or aerofoil |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive with fast response on combustion control |
| Also specified | Inlet filter or screen and inlet silencer |
What this FD fan has to cope with
Fast air-fuel response
Hydrogen-rich gas reacts quickly, and a short shortage of air produces CO and soot, while a sudden air surge can unsettle the flame. The FD fan and its drive are selected so the air flow follows fuel changes without lag, with the fan working well away from its stall region.
Coal and coke dust in the intake
Plant air near a coke battery carries coal and coke fines, and in humid weather they cake on the impeller and on the fins of the steam coil air heater. The fan drifts out of balance and the heater loses output. Taking air from a clean area, upwind of the battery, through a screen or filter keeps balance and air heater performance stable.
Mixed gas air demand
Switching between coke oven gas and blast furnace gas changes air per unit of fuel by a factor of five or more. The fan is sized for the COG-rich case, where air demand per unit heat is the largest, and must still give stable control at low flow.
Energy and control
With firing that follows surplus gas, the FD fan rarely runs at full output. A variable speed drive tied to combustion control saves substantial power and gives the fast response hydrogen-rich firing needs. Oxygen trim to keep excess air low reduces both FD and ID fan power and lowers the flue gas loss.
ID fan and FD fan in coke oven 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 | Wet flue gas with SO₂, almost no dust | Ambient air |
| Temperature at the fan | 150 to 180°C | Ambient |
| Main design concern | Acid dew point and corrosion | Fast air-fuel response |
| Control | Furnace draft | Combustion control and oxygen trim |
See also the Induced Draft (ID) Fan for coke oven gas fired boilers.
Questions buyers ask
How much combustion air does coke oven gas need?
Roughly four to five cubic metres of air per cubic metre of coke oven gas, plus excess air. That is several times what blast furnace gas needs, so the FD fan on a COG fired boiler is large in relation to the fuel flow, and its duty changes a great deal when the plant switches between gases.
Is the coke oven gas boiler FD fan exposed to hot air?
No. The fan pushes ambient air into the air heater, and the air is heated downstream. The fan is therefore selected at ambient density in mild steel, which keeps it simple and efficient. Only the ducts after the air heater are insulated for hot air.
Why does the FD fan need a purge interlock on a coke oven gas boiler?
Hydrogen-rich gas has a wide flammable range and a fast flame. Any gas left in the furnace after a trip can ignite violently on relight. The FD and ID fans must purge the furnace with a set volume of air before every light-up, and the burner logic checks fan status first.
Information needed to quote an FD fan for coke oven 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.
- Burner data. Number and type of burners, windbox pressure, and air demand for COG and mixed gas firing.
- 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.