by: Joseph Colannino, Ph.D.

Flue gas recirculation is a method of reducing NOx. NOx is a criteria pollutant. NOx comprises two main species: nitric oxide (NO) and nitrogen dioxide (NO2). One method of reducing NOx in boilers is to recirculate flue gas from the exhaust stack into the windbox. (See Figure 1.) The Faber VPSSS Burner uses a separate mixing plenum (also called a mixing box) to mix the recirculated flue gas with the combustion air to assure a homogenous mixture to the boiler windbox. In boiler environments, NOx is nearly all NO. To understand how FGR reduces NOx, consider how NO is formed. Fuels such as natural gas contain no nitrogen bound in the fuel structure. That means that all of the nitrogen used to make NOx must come from the combustion air itself. Equation (1) is known as the shorter Zeldovich mechanism. It shows the essential features of nitric oxide formation in combustion systems.

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