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 FD fan sits
- Inlet filter and silencer
- FD fan
- Air flow measurement
- Burner air register
- Secondary air
- Thermal oxidiser
The blower stands at grade near the incinerator, taking air from outside any vapour cloud boundary. It feeds air to the burner register and to secondary air ports where the waste gas is injected. Outlet pressure is set by the burner, typically 150 to 400 mmWC. On natural draft incinerators the blower is the only fan, and the chamber runs under stack suction; on units with an ID fan both fans operate together under the burner management system.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Inlet atmosphere | Plant air in a classified area; possible hydrocarbon vapour near leaks |
| Dust | Clean; inlet filter against sand and debris |
| Pressure at the fan outlet | Typically 150 to 400 mmWC |
| Duty | Continuous, with standby blower usual |
| 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 refinery waste gas incinerators
| Blade form | Backward curved, for a stable curve over the full turndown |
| Material of construction | Mild steel with non-sparking inlet ring or aluminium-bronze rub ring where specified |
| Drive and arrangement | Direct coupled, flameproof or increased safety motor to suit the area classification |
| Bearings and cooling | Pedestal bearings sized for long continuous runs, with vibration and temperature monitoring |
| Flow control | Variable speed drive or inlet vanes with separate primary and secondary air dampers |
| Also specified | Inlet silencer, standby blower with auto changeover, air flow measurement and air-proving switch |
What this FD fan has to cope with
Loss of air in a running incinerator
If combustion air fails while tail gas is still entering, H₂S passes unburnt to the stack. The burner management system trips the waste gas feed on low air, and a standby blower with automatic changeover avoids sulphur plant trips for a single fan fault. Both blowers are tested regularly.
Ignition sources in a process area
A fan rub, a static spark or a hot motor can ignite a vapour release. Non-sparking materials at the inlet and tip clearance, an earthed and bonded assembly and a motor certified for the area remove these sources. Vibration monitoring also gives early warning before a bearing failure causes a rub.
Air for variable waste gas
Tail gas heating value changes with sulphur plant performance, and rich gas needs much more air. The blower is sized on the richest case with oxygen trim, so excess air stays in a narrow band. Sour water stripper gas, rich in ammonia, adds a further air demand that is included in the sizing case.
Energy and control
The incinerator runs year round, and the blower follows tail gas flow and composition rather than a daily cycle. A variable speed drive on air flow, trimmed by oxygen, avoids running the blower at the purge rate with a closed damper. With a standby unit installed, rotating duty between the two blowers spreads bearing wear and confirms the standby actually starts.
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 Induced Draft (ID) Fan for refinery waste gas incinerators.
Questions buyers ask
What is the FD fan on a refinery incinerator?
It is the combustion air blower that supplies air to the support burner and secondary air to burn the waste gas. On many tail gas incinerators it is the only fan, because the stack provides draft and no ID fan is needed. Its air flow sets the destruction efficiency.
Why do refinery incinerators have standby combustion air blowers?
Without air the incinerator must trip, which forces the sulphur recovery unit and often upstream units to cut back. A standby forced draft fan with automatic changeover keeps the incinerator running through a single blower fault. The changeover is tested regularly so the standby is known to start on demand.
What motor is needed for a combustion air blower in a refinery?
The motor must suit the hazardous area classification at the fan location, usually flameproof or increased safety construction for the gas group present. The fan itself may also need non-sparking rub rings and earthing. State the zone, gas group and temperature class in the enquiry so the motor and fan are built to them.
Information needed to quote an FD fan for refinery waste gas incinerators
- 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. Burner type, air pressure required and primary to secondary air split.
- Redundancy. Whether a standby blower is required and how changeover is to work.
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.