NOx Chemistry in Recovery Boilers under Staging Conditions, 2004 International Chemical Recovery Conference

Two approaches involving detailed description of the nitrogen chemistry kinetics are used to study staging in black liquor recovery boilers. First, CFD is used to study the effect of staged tertiary (burnout) air on NOx emission. Second, kinetic calculations are used to study fuel staging and of adding air staging to fuel staging. The CFD study showed that staging the tertiary air had only minor effects on NOx emission if the operational mode of the lower part of the boiler is not modified. In the present case the temperature in the lower part of the tertiary air register was too high. The kinetic study of fuel staging, on the other hand, showed that fuel staging alone can reduce NOx emission by the order of 50%. According to the kinetic study adding air staging to a fuel staged boiler only slightly improved the overall performance of the boiler with respect to NOx emission. Higher NOx reduction can probably be obtained if both the fuel staging and the air staging steps are optimized simultaneously rather than individually.