How refinery waste gas incinerators work
Refineries use thermal incinerators for sulphur recovery unit tail gas, sour water stripper off-gas, and vent and waste gas streams that cannot go to the flare. In the commonest case, tail gas carrying residual H₂S, COS, sulphur vapour and hydrogen is burned at typically 650 to 850°C with fuel gas support, converting sulphur compounds to SO₂. Many incinerators discharge directly up a tall stack on natural draft. Where heat is recovered, a waste heat boiler or steam superheater cools the gas to about 250 to 320°C, kept above the sulphuric acid dew point, and an ID fan then draws gas through the boiler and delivers it to the stack. A combustion air fan supplies the burner and secondary air.
Where the ID fan sits
- Waste gas and tail gas
- Burner and thermal oxidiser
- Waste heat boiler
- Superheater or economiser
- ID fan
- Stack
The fan sits after the waste heat boiler and economiser, at grade near the stack base. It works against the boiler tubes, economiser, ducting and dampers, typically 150 to 300 mmWC. Upstream, the gas is controlled to stay above the acid dew point, so the fan casing must also be kept warm. A bypass to the stack lets the incinerator run on natural draft if the fan or boiler is out, which is common on units where sulphur release limits must be met at all times.
Gas at the ID fan
| Gas temperature, normal | 250 to 320°C after the waste heat boiler |
| Maximum or upset | Up to 450 to 500°C if the boiler is bypassed or tubes are dry |
| Dust at the fan | Very little; occasional sulphur and soot during upsets |
| Moisture and dew point | Sulphuric acid dew point typically 140 to 170°C, depending on SO₃ |
| Corrosive species | SO₂, SO₃ and water vapour; attack is severe if any surface falls below dew point |
| Density at the design temperature | About 0.63 kg/m³ at 290°C, 53% 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 refinery waste gas incinerators
| Blade form | Backward curved, medium tip speed, for efficiency on clean hot gas |
| Material of construction | Boiler quality steel for 300°C service; heat-resisting steel where the boiler bypass case exceeds about 400°C |
| Drive and arrangement | Direct coupled, arrangement 8, with a motor rated for the hazardous area classification |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc and heat shield |
| Flow control | Variable speed drive on chamber pressure, with an inlet damper for isolation |
| Also specified | Insulated casing, warm-up connection, tight shaft seal, and a bypass damper to the stack |
What this ID fan has to cope with
Acid dew point corrosion
If the casing or inlet duct cools below the acid dew point during low load or after shutdown, sulphuric acid condenses and corrodes steel rapidly. Insulation, a controlled boiler outlet temperature, and a warm-up procedure that runs hot gas or air through the fan before tight shut-off keep surfaces dry.
Thermal growth on boiler bypass
When the boiler is bypassed, gas temperature at the fan can jump by 150°C or more. The casing, shaft seal and bearing pedestals must accept the extra growth without rubbing, and the fan is checked for the hot case even if it is rare. Bearing temperatures are monitored.
Sulphur and soot during upsets
Poor combustion in the oxidiser, often at start-up or with a rich tail gas, sends sulphur vapour and soot to the fan, where they deposit on cooler parts. Access doors and a heated casing let deposits be cleared and stop them becoming acid sites. Burner tuning helps most.
Hazardous area requirements
The fan stands inside a refinery process unit, so the motor, instruments and any sparking parts must suit the area classification, and refinery specifications often call for API 673 features. These are agreed at enquiry stage, not after design. Changing a motor or seal after the fan is built is slow and costly.
Energy and control
Tail gas flow follows the sulphur plant load, which is set by crude slate and hydrotreater operation, so the incinerator runs continuously but often at 50 to 80% of design. A variable speed drive on chamber pressure tracks this and saves energy compared with a throttled damper. Excess air control matters more for fuel than for the fan; every unnecessary unit of air adds both support fuel and ID fan volume.
ID fan and FD fan in refinery waste gas incinerators
| ID fan | FD fan | |
|---|---|---|
| Position | After the waste heat boiler, before the stack | At grade, feeding the burner and secondary air |
| What it handles | Hot flue gas with SO₂ and SO₃ | Clean ambient air in a classified area |
| Temperature at the fan | 250 to 320°C | Ambient |
| Main design concern | Acid dew point and thermal growth | Reliability and ignition safety |
| Control | Chamber pressure | Air flow and oxygen |
See also the Forced Draft (FD) Fan for refinery waste gas incinerators.
Questions buyers ask
Does a refinery tail gas incinerator need an ID fan?
Not always. Many tail gas incinerators run on natural draft from a tall stack. An ID fan is needed when a waste heat boiler or other heat recovery equipment adds resistance that the stack cannot overcome, or where the stack is short. A stack draft calculation decides it.
What temperature should the ID fan run at on a sulphur incinerator?
The flue gas at the fan is usually kept at 250 to 320°C, safely above the sulphuric acid dew point. Running colder risks acid condensation on the casing, inlet box and impeller. The casing and inlet box are insulated, and the fan is warmed with hot gas or air before the incinerator is brought to full load after a shutdown.
What standards apply to fans in refinery service?
Refinery specifications often reference API 673 for centrifugal fans, along with the site hazardous area classification for motors and instruments. These requirements should be stated in the enquiry so the fan is designed to them from the start. Testing to IS 4894 or BS 848 and balancing to ISO 21940-11 can be specified.
Information needed to quote an ID fan for refinery waste gas incinerators
- 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.
- Waste gas streams. Each stream with flow, H₂S and sulphur content, and heating value.
- Specification. Applicable refinery specification, API requirements and hazardous area classification.
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.