Fuel staging (FS), air staging (AS), and selective noncatalytic reduction (SNCR) are techniques for abating nitrogen oxides (NOx ) NO + NO 2 ) 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.

Fine particle emissions from combustion sources have gained attention recently due to their adverse effects on human health. The emission depends on the combustion process, fuel, and particulate removal technology. Particle concentrations at Kraft recovery boiler exits are very high, and the boilers are typically equipped with electrostatic precipitators (ESP). However, little data are available on the ESP performance in recovery boilers. Particle concentrations and size distributions were determined at two modern, operating recovery boilers. In addition, we determined the fractional collection efficiency of the ESPs by simultaneous measurements at the ESP inlet and outlet and the particulate emissions of trace metals. The particle mass concentration atthe ESP inlet was 11-24 g/Nm3 at the two boilers. Particle emissions were 30-40 mg/ Nm3 at boiler A and 12-15 mg/Nm3 at boiler B. The particle size distributions had a major particle mode at around 1 microm. These fume particles contained most of the particle mass. The main components in the particles were sodium and sulfate with minor amounts of chloride, potassium, and presumably some carbonate. The ESP collection efficiency was 99.6-99.8% at boiler A and 99.9% at boiler B. The particle penetration through the ESP was below 0.6% in the entire fume particle size range of 0.3-3 microm. Trace element emissions from both boilers were well below the limit values set by EU directive for waste incineration.

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