Chapter 4 - Reduction of nitrogen oxide emissions

Abstract

Nitrogen oxides produced in combustion processes of boiler furnaces are a dangerous environmental pollutant. The amount of nitrogen oxides emitted by the boiler depends mainly on the fuel characteristics, the furnace type determining the temperature level in the combustion region, and the oxygen concentration in the initial section of the flame. A reduction in NOx concentration in flue gas is realized by means of primary methods, based on the modification of the furnace processes, and secondary methods, based on flue gas treatment using appropriate reacting substances. Practically all primary methods make use of the following two basic processes: staging of air and staging of air and fuel, sometimes supported by recirculation of flue gases to the combustion area. Secondary methods are realized on the basis of two fundamental processes: SCR - Selective Catalytic Reduction and SNCR - Selective Non-Catalytic Reduction. There are other technologies also, but their dissemination is limited.