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Flue Gas Conditioning

Fuel Tech Air Pollution Control TechnologiesFlue Gas Conditioning

Improve Performance of Electrostatic Precipitators (ESPs) and Capture of Flyash and Particulate


  • Injection of SO3 and ammonia into flue gas to improve operation of Electrostatic Precipitators (ESPs)
  • Reduces flyash resistivity and improves ESP capture of fine particulate for reduced opacity and emissions
  • Technology proven on over 500 ESPs worldwide

FGC SO3 Module ContainerFlyash precipitators do not operate effectively when there is not sufficient sulfur trioxide (SO3) is not present in the flue gas. By injecting SOand, in some cases, ammonia into the flue gas the performance of Electrostatic Precipitators (ESPs) is improved without increasing SOX emissions. The conditioning gas necessary for the FGC process is produced by burning elemental sulfur to produce SO2, which is converted to SO3 with a catalyst and injected into the flue gas at levels normally less than 20 ppm.

System installation costs can be greatly reduced by designing modular equipment. Whenever possible, Fuel Tech system design is “containerized” by factory-assembling the process equipment inside a permanent enclosure prior to shipment. This minimizes installation time and expenses, and negates the need for separate, dedicated buildings.

Chemical feedstock costs and external energy input are significant. As such, catalytic converters should be as efficient as possible to minimize the flow of molten sulfur feedstock to the system, as well as, the cost of unused chemical exiting the stack. For this reason, the standard catalytic converter is a two pass design and 95% conversion efficiency is guaranteed. All systems are designed for high SO2 and SO3 gas concentrations to minimize external energy input and reduce the size of the field-installed piping needed to transport the conditioning gas to the injection probes mounted in the ductwork.


There are two sulfur options: molten sulfur and pelletized dry sulfur technology. Ammonia systems can be operated independently or added to SO3 injection for "dual conditioning." The system best for you is unique to your operation’s capacity and flyash conditioning needs, as well as, the type of coal burned. 

Molten Sulfur

Molten sulfur is the most economical and common feedstock in the United States and commonly available in bulk shipments. It’s non-hazardous and allows complete combustion within sulfur burner. It is reliably and accurately metered and transferred to the burner; as well as, heated to temperature with plant steam or from a small, electric steam boiler.

Equipment Sequence:

  • Molten tank
  • Metering pumps
  • SOProduction
    • Air blower
    • Air heater
    • Sulfur burner
    • SO2/SO3 Converter
  • Injection probes

FGC Molten Sulfur System - Fuel Tech


Pelletized Sulfur System

Pelletized Sulfur technology provides substantial benefits, especially for smaller capacity units. It eliminates unloading of hot molten sulfur from a tanker truck and reduces steam usage and amount of jacketed interconnecting process piping. Pelletized sulfur is shipped on pallets or in silo and can be stored indefinitely without need for steam heating. It has more flexibility in sulfur storage location and equipment layout and is widely available in one-ton super-sacks or bulk deliveries. As with molten sulfur, it is also non-hazardous.

Equipment Sequence:

  • Dry unloading
  • Dry silo w/conveyor
  • Sulfur melter
  • Metering pumps
  • SOProduction
    • Air blower
    • Air heater
    • Sulfur burner
    • SO2/SO3 Converter
  • Injection probes

FGC Pelletized Sulfur System - Fuel Tech

Ammonia Systems

Dual Flue Gas Conditioning

Dual FGC - Ammonia System Ammonia systems are available using anhydrous or aqueous ammonia. NH3 is effective alone in certain precipitator enhancement applications, or also in conjunction with SO3 creating a "dual"  gas conditioning process. NH3 can also be used as SO3 Mitigation Technology.

  • Wide range of system size availability.
  • When Dual System is installed, some SO3 equipment (such as the air blower & PLC) can be shared with NH3 dilution.
  • Urea based systems provide safer reagent
  • Dual FGC required for high-ash coals and flyash where SO3 alone is not sufficient
  • Works well on high ash coals (China and India)
  • Supplied as modular system

Fly Ash Resistivity v. Flue Gas Temperature 

Flyash Resistivity

Cold side ESPs are located downstream of the air heater.

Typical flue gas temperatures at the ESP inlet are in the range of 120-204°C (250-400°F).

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