The Influence of Mixing on NOx Reduction by Coal Fuel Staging

This study was conducted to evaluate the influence of reburn fuel mixing with primary combustion products on NOx reduction by coal fuel staging. Furnace experiments were conducted in a 2.5 MW furnace to determine the NOx emissions possible under conditions simulating a fuel staged boiler. NOx emissions were measured for cases when the mixing of the reburn fuel and primary combustion products at the end of the reburn zone was complete and incomplete. Reburn zone experiments were conducted in a 80 KW laboratory reactor to determine the NOx reduction possible in the reburn zone under rapid mixing conditions. In the furnace experiments, reburn zone stoichiometry was the most important variable and NOx emissions could be reduced from 1000 ppm to 350 ppm as reburn stoichiometry decreased from 1.0 to 0.8. Complete reburn fuel mixing with primary combustion products only lowered NOx emissions in the tests with reburn stoichiometry less than 0.9. In the reburn zone experiments, NOx could be reduced from 1000 ppm to 30 ppm in 0.3 s. The extent of reduction was independent of the reburn fuel fraction and primary stoichiometry as long as the reburn stoichiometry was less than 0.85. The total volatile fixed nitrogen concentration (TVFN; sum of NOx, HCN and NH3) at the end of the reburn zone could not be reduced to less than 350 ppm. The TVFN concentration was independent of the reburn zone stoichiometry less than 0.9. Comparison of the results of the two experiments indicated that NOx reduction was partly limited by mixing of the reburn fuel and primary combustion products. However, the NOx emissions in the 2.5 MW furnace equaled the TVFN in the laboratory reactor and suggested that NOx emissions were also limited by the ability to reduce HCN and NH3 in the reburn zone.