How vent gas incinerators for PVC and chlor-Alkali plants work
PVC and chlor-alkali complexes collect vent streams from EDC cracking, VCM recovery, oxychlorination and storage, and burn them in a vent gas incinerator rather than releasing chlorinated organics. The vents carry ethylene dichloride, vinyl chloride, other chlorinated hydrocarbons, hydrogen and nitrogen, often with wide swings in heating value. They are oxidised at typically 1,100 to 1,200°C with about two seconds of residence time, supported by natural gas or fuel oil. The flue gas carries hydrogen chloride and some free chlorine. It is quenched rapidly to below 100°C to limit dioxin re-formation, then passes an HCl absorber that recovers dilute hydrochloric acid and a caustic scrubber before the ID fan and stack. A combustion air fan supplies the burner and the oxidation air.
Where the FD fan sits
- Inlet filter
- FD fan
- Air flow measurement
- Burner air register
- Secondary air ports
- Combustion chamber
The fan sits at grade beside the incinerator, drawing air from a point away from chlorine and HCl handling areas. It feeds a split air system: combustion air to the support burner and secondary or oxidation air to ports around the chamber, each with its own control damper. The pressure needed covers the burner register, air ducts and the chamber, which runs under slight suction from the ID fan, so FD fan pressures are moderate, typically 150 to 350 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Inlet atmosphere | Plant air that may carry traces of chlorine or HCl near leaks |
| Dust | Clean; inlet filter used where the site is dusty |
| Pressure at the fan outlet | Typically 150 to 350 mmWC across burner and ports |
| Flow range | From minimum burner air to full purge and peak vent load |
| 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 vent gas incinerators for PVC and chlor-Alkali plants
| Blade form | Backward curved, for efficiency and a stable curve across a wide flow range |
| Material of construction | Mild steel with an epoxy or chlorinated rubber coating inside and out, or stainless steel where the plant atmosphere is aggressive |
| Drive and arrangement | Direct coupled with a flameproof or suitably rated motor if the area classification requires it |
| Bearings and cooling | Standard pedestal or motor bearings |
| Flow control | Variable speed drive with separate dampers for burner and oxidation air |
| Also specified | Inlet filter, flow measurement for the burner management system, and a pressure switch for the air-proving interlock |
What this FD fan has to cope with
Purge and air-proving
Before ignition the burner management system needs a timed purge of several chamber volumes, and the burner will not light without proven air. The fan is sized for purge flow as well as running flow, and the air pressure switch and flow element are placed where they read reliably.
Variable vent heating value
Vents rich in hydrogen or hydrocarbons need much more oxidation air than nitrogen-rich vents. If the fan cannot supply the peak, unburnt chlorinated compounds pass to the scrubber. Air flow is sized on the richest credible vent combination, with oxygen trim on the flue gas.
Corrosion from the plant atmosphere
Chlor-alkali sites have chlorine and acid traces in the air, which rust uncoated fans and motor frames quickly. A protected coating, stainless fasteners and an inlet drawn from upwind of the cell house extend life. Motor terminal boxes and cable glands need the same protection, since they are often the first parts to fail on a coastal chlor-alkali site.
Energy and control
The incinerator runs continuously, but the air demand moves with vent flow and composition, and is often far below the purge rate for most of the year. A variable speed drive tracking chamber temperature and oxygen keeps the combustion air fan near its required duty instead of throttling a damper, and reduces support fuel too, since every unnecessary unit of air must be heated to 1,100°C.
ID fan and FD fan in vent gas incinerators for PVC and chlor-Alkali plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the quench and scrubbers, before the stack | At grade, feeding burner and oxidation air |
| What it handles | Saturated gas with HCl and chlorine traces | Clean ambient air |
| Temperature at the fan | 50 to 75°C, saturated | Ambient |
| Main design concern | Chloride corrosion and liquid carry-over | Purge capacity and air for rich vents |
| Control | Combustion chamber pressure | Chamber temperature and oxygen |
See also the Induced Draft (ID) Fan for vent gas incinerators for PVC and chlor-Alkali plants.
Questions buyers ask
What does the combustion air fan do on a vent gas incinerator?
It supplies air to the support burner and the oxidation air that destroys the vent gases. On a PVC or chlor-alkali plant incinerator it is usually called the FD fan or combustion air blower, and its flow must cover both normal operation and the pre-ignition purge.
Why does excess air matter on a chlorinated vent incinerator?
Too little air leaves unburnt chlorinated organics, and too much wastes support fuel and enlarges the scrubber and ID fan duty. Oxygen trim on the forced draft fan keeps excess air in a narrow band as the vent composition changes. The setpoint is raised briefly during purge surges, when vent composition is least predictable.
Does the FD fan need special materials in a chlor-alkali plant?
It handles ambient air, so mild steel is usually adequate, but the air around chlor-alkali units can carry chlorine and acid traces. A durable protective coating and stainless fasteners are normal, and stainless steel construction is used on sites with an aggressive atmosphere. Locate the intake upwind of the cell house.
Information needed to quote an FD fan for vent gas incinerators for PVC and chlor-Alkali plants
- 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.
- Vent schedule. Each vent stream with its flow range, composition and heating value.
- Area classification. Hazardous area zone and gas group at the fan location.
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.