How coke dry quenching plants work
A coke dry quenching plant cools red-hot coke, discharged from the oven battery at about 1,000 to 1,050°C, with a circulating inert gas instead of a water spray. Coke is charged into the top of a refractory-lined cooling chamber and descends slowly while the circulating gas, mainly nitrogen with some CO₂, CO and hydrogen, is blown upward through it. The gas leaves the chamber at about 850 to 980°C, drops its coarse coke breeze in a primary dust catcher, raises steam in a waste heat boiler and passes a secondary cyclone before the circulating gas fan returns it, at about 130 to 200°C, to the bottom of the chamber. The loop is closed, so the fan is both the draft fan for the boiler and the blower for the quench chamber.
Where the ID fan sits
- Cooling chamber
- Primary dust catcher
- Waste heat boiler
- Secondary cyclone
- Circulating gas (ID) fan
- Gas inlet at chamber bottom
The fan sits after the secondary cyclone and before the gas distributor at the base of the cooling chamber. Upstream it has the chamber outlet, the dust catcher, the boiler tube banks and the cyclone; downstream it must push the gas up through the coke column itself. It therefore develops a higher pressure than an ordinary boiler ID fan, often several hundred to over a thousand mmWC, and the loop runs under positive pressure after the fan and slight suction before it.
Gas at the ID fan
| Gas temperature, normal | 130 to 200°C after the boiler and cyclone |
| Maximum or upset | 250 to 300°C if the boiler economiser section is lost or bypassed |
| Dust at the fan | Fine coke breeze that escapes the cyclone, typically up to about 1 g/Nm³, very abrasive |
| Gas composition | Mostly nitrogen, with CO₂ and several percent CO and hydrogen |
| Moisture | Low; rises sharply if a boiler tube leaks into the loop |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 dry quenching plants
| Blade form | Backward curved or radial tipped, chosen for the pressure and the dust load, with wear-protected leading edges |
| Material of construction | Wear-resistant plate or hard-faced blades on a high strength impeller; casing liners at the scroll tongue and inlet |
| Drive and arrangement | Direct coupled, double bearing, with a stiff pedestal; the casing is welded gas-tight |
| Bearings and cooling | Oil-lubricated bearings with temperature monitoring; shaft cooling usually not needed at this temperature |
| Flow control | Variable speed drive on the circulating gas flow, set from coke discharge rate and gas temperature |
| Also specified | Nitrogen-purged shaft seals, gas-tight access doors, vibration monitoring and an inspection port at the impeller |
What this ID fan has to cope with
Coke breeze erosion
Breeze is harder than most boiler ash and the fan runs continuously. Blade leading edges and the scroll tongue wear first. Keeping blade tip speed moderate, using wear plate or hard-facing on the wear zones, and maintaining the secondary cyclone so that it does not let breeze through all reduce wear and extend the interval between impeller rebuilds.
Leakage of combustible gas
The loop gas contains CO and hydrogen. Leaks at the shaft or casing release toxic gas into the plant, and air drawn in on the suction side burns combustibles and raises the loop temperature. A gas-tight casing, nitrogen-purged labyrinth or carbon ring seals, and suction-side pressure monitoring are standard on this duty.
Unbalance from deposits and wear
Uneven erosion and patchy breeze deposits shift the balance of the rotor within months. Vibration monitoring with trend alarms, and planned in-situ balancing at each shutdown, prevent bearing failures on a fan that cannot be stopped without also stopping the quench and diverting coke to standby wet quenching.
Temperature excursion on boiler upset
If feedwater is lost or part of the boiler is bypassed, the gas reaching the fan can rise well above normal. The impeller stress check and the bearing arrangement should cover that upset temperature for the time it takes the plant to cut coke discharge and bring the loop back under control.
Energy and control
The circulating gas flow follows the coke discharge rate, which in turn follows the oven pushing schedule. Running the fan on a variable speed drive, rather than holding full speed and throttling with a damper, matches gas flow to coke flow and saves power at the reduced rates between pushes and during battery delays. The gas to coke ratio also sets how much steam the boiler makes, so stable fan control improves heat recovery as well as fan power.
Questions buyers ask
What fan is used in a coke dry quenching plant?
The main fan is the circulating gas fan, which acts as the ID fan for the waste heat boiler and the blower for the cooling chamber. It handles inert gas at about 130 to 200°C carrying abrasive coke breeze, so it is built with wear protection, a gas-tight casing and purged shaft seals, and usually runs on a variable speed drive.
Why is the circulating gas in CDQ not air?
Air would burn the hot coke. The loop gas settles to mostly nitrogen and CO₂, with some CO and hydrogen given off by the coke. The fan must be gas-tight so this mixture does not leak out into the plant and air does not leak in, which would burn the combustibles and overheat the loop and boiler.
How do you reduce wear on a CDQ circulating fan?
Keep the secondary cyclone in good order so it removes most of the breeze, choose a blade form and tip speed that limit wear, and protect the leading edges and scroll tongue with wear plate or hard-facing. Trending vibration shows when wear has started to affect balance, so rebuilds can be planned for a shutdown.
Information needed to quote an ID fan for coke dry quenching plants
- 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.
- Loop data. Circulating gas flow, gas composition including CO and hydrogen, and the pressure drop across the coke column and boiler.
- Coke handling. Coke throughput, discharge pattern and the breeze loading measured or expected after the secondary cyclone.
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.