NOX (and SOX) emissions are components of acid rain - when mixed with water vapor in the clouds they form nitric (and sulfuric) acid.

NOX participates in smog (photochemical ozone) formation by reacting with VOCs in the presence of sunlight - smog is a respiratory and eye irritant. NOX is primarily composed of NO and NO2, and most thermal NOX is NO.

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 wind box. 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 wind box 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 wind box inlet, where mixing of the air and flue gas must be uniformly achieved by installation of appropriate mixing devices.

IFGR is a very effective technique in reducing NOx emissions due to:

• low total cost relative to competing technologies such as low-NOx burners or selective catalytic reduction
• speed of application installation;
• Providing the same levels of NOx reduction as competing technologies.