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Show 3 The staged coal flame stabilizer is a low NOx enhancement of the patented conventional coal flame stabilizer which has already been installed on over 2000 MW of coal-fired utility boilers. The staged coal flame stabilizer design was established utilizing an axisymmetric twodimensional model. The intent of the optimization study was to maintain the primary air (15 %) to secondary (85 %) airflow split, but to stage the secondary airflow to achieve the optimum values required for the desired NOx reduction. The primary air was conveyed in the centrally located coal pipe. The secondary air comprised an outer zone around the flame stabilizer, unstaged and staged zones within the flame stabilizer, and an inner ignition zone. The secondary airflow characteristics were evaluated for design of the flame stabilizer at the 40 inch diameter exit plane (end of the existing coal pipe). The flame stabilizer vanes were set to achieve the unstaged and staged zones by different vane exit angles and a different number of blades in each zone. The staged section had twelve vanes and the unstaged section had four. The more axial flow angle of the unstaged zone and reduced blockage from fewer vanes permit the increased secondary flow for the unstaged zone. The design ratio of 1.35 was achieved at a five degree difference. An evaluation was made of the swirl number for the secondary airflow. Swirl number is a measure of the tangential-to-axial momentum of the secondary air exiting the plane of the flame stabilizer. The swirl number determines the size of the internal recirculation zone. For optimum combustion and low NOx emissions, a swirl number less than 1.0 is desirable when integrated for the flow. Lower swirl ( < .6) may cause burner instability and ignition problems. Higher values (> 1.5) create over swirl, which results in an oversized recirculation zone creating a hotter, more turbulent flame and the potential of gas recirculation into the register. Over swirling will increase the NOx emissions from the burner. Utilizing a 27 inch diameter staged flame stabilizer, the swirl number was calculated to be .86 for the unstaged section, .95 for the staged section and an overall integrated swirl number of 0.91. Figure 3 shows the staged flame stabilizer and Figure 4 shows the modified burner with new staged flame stabilizer and coal spreader. |