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 FD fan sits
- Inlet filter and silencer
- FD fan (air blower)
- Air flow control valves
- Reaction furnace burner
- Waste heat boiler
- Claus converters
The blower takes ambient air through a filter and delivers it to the reaction furnace burner, and on many units to the inline burners or reheaters and to the incinerator. Because the whole Claus train runs under pressure, the blower discharge is high for a fan, typically 3,000 to 7,000 mmWC, which on larger units calls for a multistage centrifugal blower rather than a single-stage fan. A separate, smaller combustion air fan normally serves the tail gas incinerator.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Typically 3,000 to 7,000 mmWC, depending on the train and tail gas unit |
| Dust | Clean, filtered air; plant areas may carry sulphur dust |
| Flow control accuracy | Air must track acid gas flow closely to hold the H₂S to SO₂ ratio near 2:1 |
| Turndown | Acid gas flow can fall to 20 to 30% of design at low refinery throughput |
| 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 sulphur recovery units
| Blade form | Backward curved, high pressure single-stage or multistage, with a stable rising curve |
| Material of construction | Mild steel casing; high strength steel or aluminium alloy impellers at high tip speed |
| Drive and arrangement | Direct coupled at two-pole speed, or gear driven on high pressure units; standby blower common |
| Bearings and cooling | Rolling element bearings on small units; sleeve bearings with lube oil on larger high speed machines |
| Flow control | Inlet guide vanes or speed control for main flow, with a fine trim valve on the tail gas analyser |
| Also specified | Blow-off valve for surge protection, inlet filter and silencer, and check valve against backflow of acid gas |
What this FD fan has to cope with
Surge at low acid gas flow
At low throughput the blower may be pushed into surge, causing pressure pulsation that upsets the furnace flame and the ratio control. A blow-off or recycle valve and a curve with sufficient rise to surge keep it stable.
Backflow of acid gas on trip
If the blower stops while the furnace is under pressure, acid gas can flow back into the air line and reach the blower. A fast-closing check valve and the shutdown interlock keep toxic gas out of the air system.
Air demand error and sulphur slip
Small errors in air flow shift the H₂S to SO₂ ratio, lower recovery and raise SO₂ at the stack. The blower and its flow measurement must give a steady, repeatable flow, which a throttled, oversized fan rarely does.
Energy and control
The air blower is the largest power user on most sulphur recovery units and runs around the clock. It is often selected with large margins for future acid gas and then throttled. Inlet guide vanes or a variable speed drive cut power at the usual operating point, while a trim valve gives the fine control the tail gas analyser needs.
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 Induced Draft (ID) Fan for sulphur recovery units.
Questions buyers ask
What is the main air blower in a Claus sulphur recovery unit?
It is the forced draft fan or blower that supplies combustion air to the reaction furnace. It sets the amount of H₂S burned to SO₂, so its flow control directly affects sulphur recovery efficiency.
What pressure does an SRU air blower need?
Typically 3,000 to 7,000 mmWC, enough to push the gas through the furnace, waste heat boiler, condensers, converters and tail gas unit. Smaller trains can use a single-stage high pressure fan; larger ones use multistage blowers.
Is the incinerator air fan the same as the Claus air blower?
No. The tail gas incinerator has its own combustion air fan at much lower pressure, since the incinerator runs near atmospheric pressure. It is a separate, smaller FD fan.
Information needed to quote an FD fan for sulphur recovery units
- 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.
- Acid gas range. Minimum, normal and maximum acid gas flows and H₂S content, including any ammonia-bearing sour water stripper gas.
- Train pressure drop. Required discharge pressure at the reaction furnace burner at maximum flow.
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.