Sulfur Trioxide FGC Systems

Sulfur Trioxide FGC is the most common with several hundred systems installed worldwide. It is principally used to reduce fly ash resistivity when medium or low sulfur coal is burned. Sulfur trioxide gas is produced by oxidizing sulfur dioxide gas in a catalytic converter. The sulfur dioxide can be from on site storage or produced by burning elemental sulfur. Due to improvements with sulfur burning systems and increasingly stringent regulations governing storage of hazardous gasses, all modern systems use elemental sulfur. The sulfur feed stock may be molten sulfur or granular sulfur. In the United States, molten sulfur is readily available and is generally preferred since it is much less expensive, requires less handling, and offers higher reliability.

Chemithon Sulfur Trioxide FGC Equipment

Molten sulfur is pumped from a storage tank to a refractory lined sulfur burner. There the sulfur burns in air to form a hot gaseous SO₂/air mixture. This mixture flows to a multi-stage catalytic converter where conversion of SO₂ to SO₃ exceeds 95%. It is then transported through insulated piping to where it is injected into the ductwork ahead of the electrostatic precipitator. Typical injection rates are 5 to 10 ppm or 40 to 80 lb./hr. of sulfur for a 600 MW boiler. Chemithon’s patented process is unique in that the energy consumption of the system is as little as 10% of other systems. This reduction in energy consumption is done by maintaining a high concentration of sulfur to air over most of the operating range so the heat of combustion is sufficient to maintain the system at required temperatures. Also because the Chemithon system uses a higher gas strength, our process air requirement is about half of competitor’s systems. The result is that our SO₃ piping to the injectors is usually half the diameter of other systems. You save on both installation and operating costs.

Ammonia and Dual FGC Systems

Unlike sulfur trioxide flue gas conditioning which only reduces ash resistivity, ammonia conditioning is used for a variety of reasons:

• Improve fly ash absorption of sulfur trioxide for low alkali ash
  
• Agglomerate fly ash and reduce rapper re-entrainment
  
• Increase space charge effect to optimize collection efficiency
  
• Improve high carbon fly ash collection
  
• Lower acid dew point when high sulfur coal is burned, minimizing precipitator corrosion
  
• Reduce SO₃ and slip and resulting SO₃ plume
  
• Dual conditioning is simply independent injection of both sulfur dioxide.and ammonia to achieve the benefits of both.
  

Ammonia is vaporized and mixed with process air from a PD blower. A plant boiler load signal controls ammonia vapor flow rate. The ammonia air mixture is injected into the flue gas ahead of the precipitator. Typical injection rates are usually about 4 to 8 ppm in the flue gas which is about 20 to 40lb./hr. of ammonia for a 600 MW unit. Feed stocks for these systems can be anhydrous ammonia, aqueous ammonia, or urea. Capital and operating costs are usually the lowest for anhydrous ammonia although permitting costs and risk management programs may offset the financial benefit. Urea based systems are the safest and have lower operating costs than the aqueous systems.

Flue Gas Conditioning Equipment Retrofits

Modern FGC systems are far more reliable, more efficient and offer lower operating costs than older systems. Retrofitting the equipment to modern designs can often dramatically improve precipitator performance, reduce the potential for plant derates, and often recover the cost in weeks or months of operation. The most common retrofits are:

• Replacement of outdated control systems with new PLC designs
• Retrofitting in-tank sulfur pumping systems to existing sulfur tank to eliminate inaccurate and messy external sulfur pumping systems
• Conversion of ‘Granular Sulfur Feed’ to Molten Sulfur
• Injection probes that properly distribute sulfur trioxide
• Replacement Sulfur Burners or Converters
  

Ammonia Systems for DeNOx

Chemithon supplies anhydrous, aqueous, or urea based systems for Selective Catalytic Reduction (SCR) and for Selective Non Catalytic Reduction (SNCR).

Chemithon Ammonia Systems for NOx Reduction

In the selective catalytic reduction process (SCR) dilute ammonia gas is injected into the hot side of the flue gas duct upstream of the catalyst bed where it oxidizes the NOx in the flue gas stream to form Nitrogen (N₂) and water vapor. The Chemithon Corporation provides ammonia systems (using anhydrous, aqueous, or urea) for both Selective Catalytic (SCR) and Selective Non Catalytic (SNCR) systems for NOx reduction.

For More Information:

Michael W. Rollock, Chem. E.
Regional Manager - Americas
Chemithon Corporation
5430 West Marginal Way SW
Seattle, WA 98106-1598
Phone: (206) 937-9954 ext. 1108
Fax: (206) 932-3786
mrollock@chemithon.com

© 2007 Chemithon Enterprises, Inc.