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NOx Control Technologies

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Flue Gas Recirculation Background

PFD Download Theory of Cost Effective NOx Control

Recirculating flue gas back to the combustion zone has been one of the most effective methods of NOx Control, reducing NOx emissions from gas and oil fired boilers since the early 1970's. Flue gas recirculation (FGR) acts to reduce NOx formation by reducing peak flames temperatures. In conventional applications, the recirculated flue gas is typically extracted from the boiler outlet duct upstream of the air heater. The flue gas is then returned through a separate duct and hot gas fan to the combustion air duct that feeds the windbox. The recirculated flue gas is mixed with the combustion air via air foils or other mixing devices in the duct. This technology is known as windbox FGR (WFGR). WFGR systems require installation of a separate hot gas FGR fan to move flue gas from the boiler exit to the air supply ducting at the windbox inlet, where mixing of the air and flue gas must be uniformly achieved by installation of appropriate mixing devices.  ETEC is one of the leaders in IFGR NOx Control.

IFGR BackgroundSince Induced Flue Gas Recirculation (IFGR) may impact forced draft fan capacity and reheat steam temperature controls, the applicability must be carefully evaluated during the initial design phase. Subsequent to installation, the IFGR system must be optimized throughout the boiler's perating load range. The following summarizes the advantages and disadvantages of IFGR.

Induced Flue Gas Recirculation Advantages:

  • Fractional cost of alternate technologies
  • Highly Effective NOx reduction (50-80%)
  • Boiler efficiency increase in many cases
  • Combustion performance is improved
  • Minimal downtime required for installation
  • IFGR may be used with existing burners, as well as Low NOx Burners
  • IFGR does not result in excessive boiler and duct pressures as LNB/WFGR systems may

Induced Flue Gas Recirculation Disadvantages:

  • IFGR operation at full load may be limited by forced draft fan capacity
  • Reheat steam temperature controls may limit IFGR operation