How carbon black tail gas fired boilers work
Carbon black is made by partial combustion of a heavy aromatic feedstock oil in a refractory reactor. After quenching and filtering out the black, the remaining tail gas is a lean fuel: mainly nitrogen and water vapour, with hydrogen and carbon monoxide, a calorific value of typically 500 to 900 kcal/Nm³, and sulphur compounds such as H₂S, COS and CS₂ from the feedstock. The tail gas is burned in a boiler with dedicated burners, usually with a support fuel. Because the fuel is so lean, the flue gas volume per tonne of steam is roughly twice that of an oil-fired boiler, and its moisture content is very high. An FD fan supplies combustion air and an ID fan draws the gas through the boiler and economiser to the stack.
Where the FD fan sits
- Inlet screen
- FD fan
- Air preheater where fitted
- Windbox
- Tail gas and support fuel burners
- Furnace
The fan sits at grade beside the boiler and delivers ambient air to the burner windbox, sometimes through an air preheater that raises the air temperature to help the lean tail gas burn. Air dampers at each burner split the air between tail gas and support fuel firing. The fan must supply the windbox pressure the burners need, typically 150 to 300 mmWC, both when tail gas is plentiful and when the boiler fires support fuel alone during reactor outages.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | Up to about 200°C where an air preheater is fitted, downstream of the fan |
| Dust | Carbon black dust in the plant atmosphere; inlet screen or filter needed |
| Pressure at the fan outlet | Typically 150 to 300 mmWC |
| Turndown | Wide; from low tail gas flow to full flow plus support fuel |
| 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 carbon black tail gas fired boilers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings, no cooling needed |
| Flow control | Variable speed drive on combustion control, with burner air dampers |
| Also specified | Inlet filter against carbon black dust, inlet silencer |
What this FD fan has to cope with
Flame stability on lean fuel
Tail gas burns slowly and close to its flammability limit. If the air is excessive or poorly distributed, the flame lifts or goes out. A fan with steady pressure and fine flow control lets the combustion system keep excess air low.
Carbon black drawn into the fan
Carbon black plants have fine black dust in the air. It coats the impeller and enters the air preheater. An inlet filter and an intake located away from the packing area keep the fan clean.
Two firing modes
The fan must suit tail gas firing, which needs a large air flow at moderate pressure, and support fuel firing alone. The duty is checked at both points so the fan is not in an unstable part of its curve in either mode.
Energy and control
Combustion air demand follows tail gas availability, which changes with grade and reactor status. A variable speed drive on the combustion control loop replaces damper throttling and saves power for most of the year. Oxygen trim matters more on lean tail gas than on rich fuels because every extra percent of air adds to an already large flue gas volume.
ID fan and FD fan in carbon black tail gas fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the economiser, before the stack | At the windbox, ahead of the burners |
| What it handles | Wet flue gas with SO₂ and traces of carbon | Ambient air |
| Temperature at the fan | 180 to 220°C | Ambient |
| Main design concern | Large wet gas volume and acid dew point | Stable flame on lean tail gas |
| Control | Furnace draft | Combustion control and oxygen trim |
See also the Induced Draft (ID) Fan for carbon black tail gas fired boilers.
Questions buyers ask
Is the FD fan on a tail gas boiler the combustion air fan?
Yes. On a carbon black tail gas boiler the forced draft fan supplies all the combustion air to the tail gas and support fuel burners. It handles ambient air only. The tail gas itself is delivered by the process, so the FD fan duty is the air needed for the lean gas plus the support fuel, at the windbox pressure the burners need.
Does the FD fan supply the tail gas itself?
No. The tail gas comes from the carbon black process, pushed by the process blowers or a dedicated tail gas booster blower. The FD fan supplies only air. Keeping the two separate means the combustion air fan can be a simple clean-air machine, while the tail gas booster, where fitted, is built for a wet, flammable and sulphur-bearing gas.
Why is excess air so important on a carbon black tail gas boiler?
The tail gas is lean and wet. Too little air and the CO and hydrogen do not burn out; too much and the flame becomes unstable and the flue gas volume, and ID fan load, rise sharply.
Information needed to quote an FD fan for carbon black tail 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. Burner type and windbox pressure for tail gas and support fuel firing.
- Plant atmosphere. Location of the air intake and the level of carbon black dust nearby.
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.