NOx formation, control and reduction techniques

The formation of nitrogen oxides (NOx) in combustion systems is an important environmental concern. The control of NOx emissions and modern reduction techniques is based on worldwide investigations performed in this field. In this chapter, developments in the understanding of nitrogen chemistry in flames during the pas twenty years following the seminal studies of Miller and Bowman [1] are summarized. The different routes of NOx formation, including the newly proposed NNH route and prompt-NO via NCN and other precursors, are discussed. The results of recent laboratory studies on NOx control through reburning are also detailed. Other approaches for NOx control, including low-NOx burners, flue gas recirculation, and over-fire air, are also outlined. Of special and practical interest are the selective noncatalytic reduction (SNCR) techniques; these include ammonia-based, urea-based, ammonium carbonate-based SNCRs, and additive-enhanced SNCR. An example of nitrogen-containing additives for the simultaneous reduction of NOx and minimization of corrosion is presented. Finally, potential future developments and areas of use of these reduction techniques are discussed.