Understanding the Formation of NOx in Pulp Mill Boilers

This paper presents a model for calculation of NOx chemistry in pulp mill boilers. A detailed chemical kinetic mechanism is used in combination with a simplified description of the furnace fluid dynamics. A reactor network is used to describe the mixing of furnace gas into the combustion air jets and the furnace gas flow between air levels. The model is applied to a 107 MWth bubbling fluidized boiler (BFB) and a 3000 tds/day Kraft recovery boiler.In the BFB, NH3 and NO in the furnace gas originating from the bed are entrained into the secondary and tertiary air jets, resulting in considerable NOx reduction in the jets, and low NOx emissions from the boiler. In contrast, in the Kraft boiler, NOx precursor release is not restricted to the char bed, but takes place in a large volume of the furnace. In the Kraft boiler NH3 is largely oxidized to NO between the air levels due to excess of oxygen and high temperatures. In addition, the importance of NO release from carry-over droplets for boiler NOx emissions is evident.For efficient in-furnace NOx reduction, NH3, NO and O2 should be simultaneously present in the furnace gas at a temperature favoring the reduction. Such conditions can be obtained in the combustion air jets, provided that the entrained furnace gas contains NH3. Calculations suggest that extreme air staging would be needed in order to obtain such conditions in a recovery boiler.