How tar and heavy residue fired boilers work
Tar and heavy residue fired boilers burn by-products such as coal tar, pitch, vacuum residue, asphalt and visbreaker tar, usually in chemical plants, coke works and refineries that produce them. The fuel is heated to about 110 to 160°C to bring its viscosity down for steam atomisation. Calorific value is high, typically 8,500 to 9,800 kcal/kg, but so are carbon to hydrogen ratio, sulphur, often 1 to 5%, and metals such as vanadium and nickel. Combustion produces soot and unburnt carbon cenospheres. The gas passes the boiler bank, economiser and air heater, then usually a multiclone, ESP or bag filter to meet particulate limits, before the ID fan and stack. The FD fan supplies combustion air through a steam coil air heater and the main air heater.
Where the FD fan sits
- Inlet screen
- FD fan
- Steam coil air heater
- Air heater
- Wind box
- Burner registers
The fan sits at ground level and pushes ambient air first through a steam coil air heater, which warms it enough to keep the cold end of the main air heater above the acid dew point, then through the main air heater to the wind box. Its pressure covers both heaters, the ducts, the wind box and the burner registers, which need a good pressure drop for the mixing that heavy fuels demand.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the steam coil heater | Typically 60 to 90°C, downstream of the fan |
| Air after the main air heater | 180 to 250°C, downstream of the fan |
| Dust | Clean air; screen against debris |
| Pressure at the fan outlet | Typically 200 to 400 mmWC, including both heaters and burner registers |
| 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 tar and heavy residue fired boilers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings; no cooling needed |
| Flow control | Variable speed drive or inlet vanes, with oxygen trim on the combustion control |
| Also specified | Inlet screen and silencer, and an air flow measurement in the duct for ratio control |
What this FD fan has to cope with
Narrow window of excess air
Heavy residues need enough air to burn out soot and carbon, yet extra oxygen converts more SO₂ to SO₃ and raises the acid dew point. The fan and its control must hold excess air within a tight band at every load, using an air flow measurement and oxygen trim.
Register pressure for atomised mixing
Viscous fuels atomise into larger droplets than light oil, and good burnout depends on strong air swirl at the register. The fan must deliver enough wind box pressure at full load so the register can generate it. If the fan is short of pressure, the burners smoke at high fire even with the damper fully open, and the boiler is derated to keep the stack clear.
Air heater leakage
A leaking regenerative or tubular air heater passes FD air directly into the flue gas, starving the burner and loading the ID fan. The FD fan is selected with the measured leakage included. Leakage grows as the air heater ages and corrodes, so the margin should reflect the condition expected before the next overhaul, not only the new heater.
Energy and control
Residue boilers usually run at steady base load, so the FD fan runs near one point for most of the year. A high efficiency backward curved wheel selected close to that point, with a variable speed drive for the periods of lower firing, keeps power low. Accurate air flow measurement and oxygen trim save far more in fuel and in avoided cold-end corrosion than the fan power itself.
ID fan and FD fan in tar and heavy residue fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the dust collector, before the stack | Before the steam coil and main air heaters |
| What it handles | Acidic flue gas with soot and cenospheres | Clean ambient air |
| Temperature at the fan | 170 to 220°C, up to 280°C fouled | Ambient, 25 to 45°C |
| Main design concern | Acid dew point and sticky deposits | Tight excess air at the burners |
| Control | Furnace draft | Air flow and oxygen trim |
See also the Induced Draft (ID) Fan for tar and heavy residue fired boilers.
Questions buyers ask
Why does a heavy residue boiler need a steam coil air heater before the main air heater?
Cold ambient air would cool the cold end of the main air heater below the acid dew point, and sulphuric acid would corrode the tubes. The steam coil heater raises the air temperature first. The FD fan must overcome the resistance of both heaters. It is designed as one system.
How much excess air does a tar fired boiler need?
Typically more than a light oil boiler, often 15 to 25%, depending on burner and fuel. Too little gives soot and smoke, too much increases SO₃. The FD fan should be able to hold the chosen value accurately across the load range. Oxygen is measured at the economiser outlet.
Is the FD fan on a residue boiler different from one on a furnace oil boiler?
The fan type is the same, but residue firing usually needs higher register pressure and an extra steam coil air heater, so the pressure duty is higher. Air flow control is also more important because the fuel is harder to burn cleanly. Conversions should be checked.
Information needed to quote an FD fan for tar and heavy residue 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 of burners, atomising method and the register pressure at full load.
- Air heating. Steam coil and main air heater pressure drops and the air temperatures after each.
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.