Reducing NOx emissions using fuel staging, air staging, and selective noncatalytic reduction in synergy

Fuel staging (FS), air staging (AS), and selective noncatalytic reduction (SNCR) are techniques for abating nitrogen oxides (NOx = NO + NO2) from boilers and engines. Each of these techniques has a limited range of applicability, within which 50% to 70% NOx reduction is attained. Higher reductions are achieved by methods that use the aforesaid techniques in sequence, thus cumulating their reduction ability, but also collecting their respective limits. In this paper, we describe a new method that we call "combined staging" (CS). This method combines FS, AS, and SNCR in synergy rather then in sequence. In CS, the fuel is first staged for converting NOx precursors to hydrogen cyanide (HCN). Then, the air is staged for reducing HCN to N-2. Further reduction is achievable by optional SNCR. In the followings the basics of FS, AS, SNCR, and their sequential applications are reviewed first. Then, the combined staging is introduced, its chemical details are elucidated via kinetic modeling, and options for its application are illustrated. Finally, assumptions and limits of the kinetic models are discussed. The present work reveals that CS can reduce over 40% NOx at lower temperatures and within shorter residence time than required by other techniques and methods. Thus, CS could reduce NOx effectively in devices where other techniques fails, e.g., in kraft recovery boilers, fluidized bed combustors, low-grade fuel combustors, small and domestic boilers, and fast engines.