How coke oven waste heat boilers work
Heat recovery (non-recovery) coke ovens carbonise coal in horizontal ovens under negative pressure. Volatile matter released from the coal burns inside the oven and in sole flues beneath it, so no by-products are recovered; instead the hot waste gas, at about 950 to 1,100°C, flows through a common tunnel to waste heat boilers. Each oven depends on draft to pull combustion air in and gas out. After the boilers the gas, at about 180 to 230°C, passes through flue gas desulphurisation, commonly a spray dryer absorber with lime, and a bag filter before the ID fan and main stack. Emergency stacks on the common tunnel open if the boilers or the ID fan are lost. The fan therefore sets the draft for the entire battery.
Where the ID fan sits
- Coke ovens and sole flues
- Common tunnel
- Waste heat boilers
- Spray dryer FGD
- Bag filter
- ID fan
- Main stack
The main ID fan, often a duty and standby pair, sits after the bag filter and before the main stack. It works against the oven suction, tunnel, boilers, the spray dryer absorber and the bag filter, so the total suction is high, frequently several hundred mmWC. It sees cleaned, desulphurised gas, cooled in the spray dryer to about 140 to 170°C and close enough to its dew point that the margin needs watching.
Gas at the ID fan
| Gas temperature, normal | 140 to 170°C after the spray dryer and bag filter |
| Maximum or upset | Up to about 230°C if the spray dryer is bypassed or water injection fails |
| Dust at the fan | Low after the bag filter; fine lime and calcium sulphite and sulphate if bags fail |
| Moisture and dew point | Moisture raised by spray dryer water; the approach to saturation is controlled, typically 20 to 30°C above dew point |
| Corrosive species | Residual SO₂ and chlorides; corrosion if the gas reaches dew point |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 coke oven waste heat boilers
| Blade form | Backward curved for efficiency on cleaned gas, with a robust blade section |
| Material of construction | Mild steel with corrosion allowance; corrosion-resistant steel for drains and lower casing where condensate can collect |
| Drive and arrangement | Direct coupled, arrangement 8, usually duty and standby |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc and continuous vibration monitoring |
| Flow control | Variable speed drive on common tunnel draft |
| Also specified | Interlocks with emergency stack caps, insulated casing, casing drain and access doors |
What this ID fan has to cope with
Battery draft must never be lost
If the fan stops, the ovens lose suction, gas leaks out at doors and the emergency stacks open to vent untreated gas. Duty and standby fans, fast automatic changeover, and vibration and bearing monitoring are normal practice. Fan reliability is part of the plant's emission performance, not only its steam output.
Running close to dew point after the spray dryer
Spray dryer absorbers work best when the gas is cooled close to saturation. If water injection overshoots, wet, acidic deposits form in the bag filter and the fan casing. Insulation, a low-point drain and an interlock that cuts water when the outlet temperature falls protect the fan.
Draft changes through the coking cycle
Each oven releases volatiles at a rate that falls through its cycle, and ovens are charged in sequence. The gas flow to the tunnel changes continually. The fan holds a steady tunnel pressure while the volume drifts, so it needs a wide stable range and a responsive speed control loop.
Energy and control
A heat recovery coke plant runs continuously, and the main ID fan is one of its largest power consumers because of the high suction across the boilers, FGD and bag filter. Speed control on tunnel draft, an efficient wheel and keeping bag filter differential pressure in check reduce that load. Lower fan power adds directly to the electricity available from the boilers.
Questions buyers ask
Why is the ID fan critical on a heat recovery coke oven?
The ovens run under suction and burn their own volatile matter with air drawn in by that draft. The ID fan creates the draft for the whole battery. If it stops, the ovens leak gas and the emergency stacks vent untreated gas, so the fan is usually duplicated.
What gas does the coke oven waste heat boiler ID fan handle?
On most modern plants, cleaned and desulphurised gas from a spray dryer absorber and bag filter at about 140 to 170°C. It is low in dust but humid and close enough to the dew point that insulation and drains matter. Older plants without FGD put hotter, SO₂-bearing gas through the fan, which changes the materials needed.
How much suction does a coke oven ID fan provide?
It must cover the oven and tunnel draft, the boilers, the FGD unit and the bag filter, which commonly adds up to several hundred mmWC. The exact figure depends on the battery and gas cleaning design and is calculated from the plant data. Margin is added for bag fouling.
Information needed to quote an ID fan for coke oven waste heat 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.
- Battery and gas cleaning. Number of ovens, gas flow, and the FGD and bag filter type and pressure drop.
- Draft control. Tunnel draft set point and the duty and standby arrangement required.
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.