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 ID fan sits
- Tail gas burners
- Furnace
- Superheater and boiler bank
- Economiser
- ID fan
- Stack
The fan sits after the economiser at the base of the stack. Tail gas boilers rarely have a dust collector because the fuel is already filtered, so the fan sees the flue gas as it leaves the economiser. The suction it works against is moderate, typically 100 to 200 mmWC, but the volume is large because of the inert nitrogen and water in the fuel. Furnace draft must be held steady as the tail gas flow from the carbon black units changes with grade changes and reactor trips.
Gas at the ID fan
| Gas temperature, normal | 180 to 220°C after the economiser |
| Maximum or upset | Up to 280°C with fouled economiser or support fuel only |
| Dust at the fan | Low; traces of carbon black and soot, more during reactor upsets or filter bag failures |
| Moisture and dew point | Water vapour often 25 to 35% by volume, water dew point near 65 to 75°C |
| Corrosive species | SO₂ and SO₃ from feedstock sulphur; acid dew point often 130 to 160°C |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 carbon black tail gas fired boilers
| Blade form | Backward curved, sized on the true wet gas volume and density |
| Material of construction | Mild steel casing with insulation; corrosion-resistant steel or SS 316 for the impeller and inlet where metal can fall below the acid dew point |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing drain, insulated casing and inspection door; stable fan curve over the tail gas range |
What this ID fan has to cope with
Undersized fan from dry gas assumptions
The large water and nitrogen content of tail gas makes the flue gas volume far higher than a standard fuel calculation suggests. Fans sized from oil firing data run out of capacity. The selection is made from the actual tail gas analysis and measured flue gas flow.
Acid corrosion at low load
When the carbon black units run at reduced rate, the economiser outlet temperature falls towards the acid dew point and the casing and blades corrode. Casing insulation, a minimum gas temperature control and corrosion-resistant wetted parts protect the fan.
Carbon deposits and unbalance
Carbon fines released during reactor upsets or filter bag failures stick to damp blade surfaces and build up unevenly, and the deposit can fall off a few blades at a time. An inspection door, a cleaning routine after every upset and vibration monitoring on the bearings catch this before the bearings suffer.
Energy and control
The boiler load follows the carbon black reactors, and tail gas flow changes with each grade and reactor outage. A variable speed drive on furnace draft saves considerable power compared with damper control on such a large-volume fan. Reducing excess air on lean tail gas, without losing flame stability, cuts the flue gas volume and the fan power together.
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 Forced Draft (FD) Fan for carbon black tail gas fired boilers.
Questions buyers ask
Why is the ID fan so large on a carbon black tail gas boiler?
Tail gas is mostly nitrogen and water vapour, with only a modest share of hydrogen and CO, so for the same heat it produces much more flue gas than oil or natural gas. The ID fan has to move that extra volume, at the low density of a hot, wet gas, which makes it large for the steam output of the boiler.
What corrodes the ID fan on a tail gas boiler?
Sulphur compounds in the tail gas, mainly H₂S, COS and CS₂ from the feedstock oil, burn to SO₂ and some SO₃, and the very wet flue gas raises the acid dew point. Wherever the casing or impeller cools below it, at low load, start-up or near uninsulated flanges, sulphuric acid condenses and corrodes the steel.
Does a carbon black tail gas boiler need a dust collector before the ID fan?
Usually not, because the tail gas has already passed the carbon black bag filters before it is burned. Carbon fines reach the fan only during reactor upsets or bag failures. The fan is built to be inspected and cleaned when that happens, and its vibration monitoring warns when deposits start to build.
Information needed to quote an ID fan for carbon black tail 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.
- Tail gas analysis. Composition including H₂, CO, H₂O, N₂ and sulphur species, and the range of tail gas flow.
- Support fuel. Type and share of support fuel and the boiler load when firing it alone.
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.