How sulphur recovery units work
A sulphur recovery unit converts the H₂S in refinery or gas plant acid gas into liquid sulphur by the Claus process. About a third of the H₂S is burned with air in a refractory-lined reaction furnace at 1,000 to 1,300°C; the gas is cooled in a waste heat boiler, sulphur is condensed, and the remaining H₂S and SO₂ react over two or three catalytic converters with a condenser after each. The whole Claus train runs under positive pressure set by the main air blower. The tail gas, still carrying H₂S, COS and sulphur vapour, goes to a tail gas treating unit or directly to a thermal incinerator, where it is burned to SO₂ at 650 to 800°C. An ID fan appears only where waste heat recovery or a scrubber after the incinerator adds more resistance than the stack can overcome.
Where the ID fan sits
- Claus tail gas
- Thermal incinerator
- Waste heat coil or boiler
- Caustic scrubber (if fitted)
- ID fan
- Stack
Most incinerators are natural draft, with the stack alone pulling the flue gas. Once a steam superheater, a waste heat boiler or an SO₂ scrubber is added after the incinerator, the stack cannot overcome the added resistance and an ID fan is placed just before the stack. After heat recovery the gas reaches the fan at about 250 to 350°C; after a caustic scrubber it is saturated at 55 to 70°C. The fan holds the incinerator firebox at a slight suction, typically −5 to −15 mmWC.
Gas at the ID fan
| Gas temperature, normal | 250 to 350°C after heat recovery; 55 to 70°C after a scrubber |
| Maximum or upset | Up to 600 to 700°C if the waste heat coil is bypassed or trips |
| Dust at the fan | Negligible; occasional sulphate and catalyst fines |
| Moisture and dew point | 25 to 35% water vapour; SO₃ raises the acid dew point to about 150 to 180°C |
| Corrosive species | SO₂ at several thousand ppm, some SO₃; residual H₂S if the incinerator runs short of air |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 sulphur recovery units
| Blade form | Backward curved, on clean flue gas with high efficiency |
| Material of construction | Mild steel or boiler plate with insulation for dry hot gas; SS 316L or FRP for the wet scrubber outlet duty |
| Drive and arrangement | Direct coupled, arrangement 8, with casing on sliding feet for thermal growth |
| Bearings and cooling | Cooling disc and heat shield for hot duty; standard bearings on the wet duty |
| Flow control | Variable speed drive on incinerator draft, with a bypass to natural draft through the stack |
| Also specified | Gas-tight shaft seal with purge connection, casing drains and insulation to hold metal above the dew point |
What this ID fan has to cope with
Acid condensation in a hot fan
If the casing or inlet box runs below the acid dew point, sulphuric acid condenses and eats through mild steel within months, usually at the inlet cone and casing bottom. Full insulation, avoiding cold air leaks at the shaft and keeping the gas above about 180°C prevents it.
Leakage of toxic gas
The incinerator outlet can carry SO₂ and, in an upset, unburnt H₂S. Any leak at the fan shaft or casing joints is a hazard to people. A purged gland seal and fully welded or gasketed casing joints keep the gas inside.
Overheating when heat recovery trips
A tube leak or steam drum trip can send incinerator gas at 650°C to the fan. Either the fan and shaft are designed for this, or a dilution air damper and a temperature trip open a natural draft path to the stack.
Energy and control
Sulphur recovery units run continuously, but acid gas flow follows the refinery or gas plant throughput and the incinerator fuel gas is often set by temperature, not load. A variable speed drive matched to incinerator draft saves fan power at low throughput and avoids pulling excess air, which would raise the incinerator fuel gas needed to hold its combustion temperature.
ID fan and FD fan in sulphur recovery units
| ID fan | FD fan | |
|---|---|---|
| Position | After incinerator heat recovery or scrubber, before the stack | At the head of the Claus train, feeding the reaction furnace |
| What it handles | SO₂-bearing incinerator flue gas | Clean ambient air |
| Temperature at the fan | 250 to 350°C hot, or 55 to 70°C wet | Ambient |
| Main design concern | Acid dew point and gas-tight sealing | High pressure and precise flow |
| Control | Incinerator draft | Air to acid gas ratio |
See also the Forced Draft (FD) Fan for sulphur recovery units.
Questions buyers ask
Does a sulphur recovery unit need an ID fan?
Usually not on the Claus train itself, which runs under pressure from the air blower. An ID fan is used on the tail gas incinerator when a waste heat boiler, superheater or SO₂ scrubber adds resistance that natural draft from the stack cannot overcome.
What temperature does the SRU incinerator ID fan see?
After a waste heat coil the flue gas is typically 250 to 350°C, which allows a mild steel fan with a cooling disc. After a caustic scrubber it is saturated at 55 to 70°C and the fan is built in SS 316L or FRP for wet acidic service.
Why is acid dew point important for an incinerator flue gas fan?
The SO₃ in incinerator gas raises the dew point well above that of water. Metal surfaces colder than this collect sulphuric acid, so a hot fan must be insulated and kept above the dew point at every load, including start-up and low firing.
Information needed to quote an ID fan for sulphur recovery units
- 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.
- Incinerator outlet train. Whether the fan follows a waste heat coil, boiler or scrubber, with gas temperature at each point.
- Sulphur species. SO₂, SO₃ and possible H₂S content at the fan, and the required shaft seal standard.
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.