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 ID fan sits
- Furnace
- Boiler bank
- Economiser
- Air heater
- Multiclone or ESP or bag filter
- ID fan
- Stack
The fan sits after the dust collector, which removes most of the cenospheres and soot, and before the stack. Gas reaches it at about 170 to 220°C, deliberately kept higher than on a clean oil boiler because the acid dew point with 2 to 5% sulphur fuel can be around 130 to 160°C. The fan suction covers the furnace draft, boiler and economiser, the air heater and the collector, typically 150 to 300 mmWC.
Gas at the ID fan
| Gas temperature, normal | 170 to 220°C after the air heater |
| Maximum or upset | Up to 280°C with heavy soot fouling or soot blower faults |
| Dust at the fan | Fine soot and carbon cenospheres; low after a bag filter or ESP, several hundred mg/Nm³ after a multiclone only |
| Moisture and dew point | Acid dew point typically 130 to 160°C depending on fuel sulphur and excess air |
| Corrosive species | SO₃ and sulphuric acid, vanadium and sodium compounds in the ash |
| 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 tar and heavy residue fired boilers
| Blade form | Backward curved with a self-cleaning blade profile, or radial tipped where soot load is high |
| Material of construction | Mild steel with corrosion allowance; corrosion-resistant steel for the casing bottom and drains where cold spots occur |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | External insulation and cladding on casing and inlet box, a low-point drain, and an access door for washing |
What this ID fan has to cope with
Cold-end acid corrosion
Where the casing is uninsulated, at drains, inspection doors and the inlet box corners, the metal runs below the gas temperature and acid condenses. It mixes with soot to form a corrosive paste. Full insulation, heated or insulated doors and a fan that is kept running during short stops keep the metal above dew point.
Sticky soot build-up on the blades
Residue soot is oily and acidic and sticks to the blade pressure faces, unevenly. Vibration rises as the deposit grows and then drops when pieces break away. A self-cleaning blade shape, access for washing during shutdowns and in-situ balancing manage it. Keeping excess air correct at the burners reduces the soot reaching the fan in the first place.
Fouled air heater raising duty
Heavy fuels foul the air heater quickly, raising resistance and gas temperature together. The fan is selected for the fouled condition and the higher temperature, so it still holds furnace draft before the next soot blowing or water washing. Monitoring the fan current and inlet temperature trend shows when the heater needs cleaning, well before draft is lost and the boiler must shed load.
Energy and control
Residue fired boilers usually run continuously at fairly steady load, since the plant burns the by-product as it is made. Fan energy depends heavily on how fouled the boiler is: a choked air heater can add a large share to the ID fan power. A variable speed drive holding furnace draft saves energy at part load, but regular soot blowing and controlled excess air save more by keeping resistance and gas volume down.
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 Forced Draft (FD) Fan for tar and heavy residue fired boilers.
Questions buyers ask
Why does the ID fan on a tar fired boiler corrode?
Tar and residues contain sulphur, and part of it forms SO₃, which condenses as sulphuric acid wherever the metal is below the acid dew point, often 130 to 160°C. Uninsulated casings, drains and doors run cooler than the gas and corrode first. Insulation and higher gas temperatures prevent it.
Can an existing furnace oil boiler ID fan be used for vacuum residue firing?
Sometimes, but check the duty. Residue firing usually needs more soot blowing, higher exit gas temperature and an added dust collector, all of which raise volume and resistance. The fan, its materials and its insulation should be reviewed for the new fuel. Measuring the present gas flow and suction gives the starting point.
Does a heavy residue boiler need a dust collector before the ID fan?
In most cases yes. Residue and tar combustion produces soot and carbon cenospheres, and particulate limits generally require an ESP or bag filter. Placing the ID fan after it protects the fan from deposits and wear. The collector also captures the vanadium-bearing ash that would otherwise build up on the blades and corrode the casing at cold spots.
Information needed to quote an ID fan for tar and heavy residue 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.
- Fuel analysis. Fuel type, sulphur, vanadium and sodium content, and the atomising temperature.
- Dust collection. Type of collector before the fan and the measured dust and gas temperature at the fan inlet.
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.