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Show EXPERIMENTAL RESULTS Experimental studies were conducted at different operating conditions and for five combustor configurations. The purpose of the parametric study was to evaluate the technical approach adopted for achieving ultra-Iow-combustion emissions at a high turndown ratio and to explore the effects of the various parameters on burner performance for optimal design. Flame-Stabilization Enhancement by the Stabilizer One of the major technical approaches for achieving high excess air operation is the use of the flame stabilizer. By inducing hotter combustion products from the stabilizer into the ignition zone of the main combustor, main flame stabilization can be enhanced. As a result, the main combustor can be operated at higher excess air to further reduce NOx formation. This concept has been proven experimentally. Figure 4 shows an increase in the maximum excess air in the main combustor with stabilizer operation and a corresponding reduction of NOx emissions when the test burner was operated at high load. For example, the maximum excess air was increased from 65% to 73% at a 470-kW firing rate, and NOx emissions were reduced by 40% from 1.6 to 1.0 vppm. However, when the burner was operated at low load, the use of the flame stabilizer did not reduce overall NOx emissions but even resulted in a slight increase in NOx, as shown in Figure 4, because the flame stabilizer made a larger contribution to overall NOx formation. For the best results, the flame stabilizer should be applied for main flame stabilization at a relatively high load as well as for high turndown operation at low load. Figure 5 shows NOx produced in the stabilizer operating at different ratios of firing rates to that of the main combustor, as well as NOx that contributed to the overall emissions, at a total firing rate of 300 kW. NOx emissions from the stabilizer were reduced to as low as 0.9 vppm at a 44-kW firing rate. However, NOx that contributed to the overall emissions generally increased with the firing rate in the stabilizer. On the other hand, the higher firing rate in the stabilizer is beneficial for enhanced flame stabilization in the main combustor, particularly at higher load operation. Therefore, low-firing-rate operation of the stabilizer will not make a positive contribution for overall NOx reduction at both low and high load operation of the main combustor. An appropriate operating range for the stabilizer is between 8% and 15% of the overall firing rate, based on lower NOx formation in the stabilizer as well as higher capability for flame stabilization, as shown in Figure 5. Within this firing-rate range, the NOx formation for overall emissions is almost constant (0.12 vppm). As a result, a portion of NOx formed in the stabilizer was reduced to a level of 20% in the overall NOx emissions. A further increase in the firing rate of the stabilizer will dramatically increase the overall NOx emissions. Effect of Main Nozzle Velocity As discussed earlier, nozzle velocity is an essential parameter for combustion control and performance. The effect of the main nozzle velocity on combustion emissions was studied for different burner configurations. It was found that, within a practical range of the main nozzle velocity, from 24 to 76 mis, the effects of the nozzle velocity on both NOx and CO emissions are less than 30%. Figure 6 presents the results obtained for a typical burner configuration. In general, NOx and CO emissions abated with an increase in nozzle velocity because flue-gas recirculation and mixing in the combustor are intensified at a higher swirl intensity. However, at a higher nozzle velocity, CO emissions, especially in the region near the wall, are increased because this portion of the reactant flow will have less residence time in the chamber due to the deterioration of uniformity of the cyclonic flow field when the swirl intenSity is too high. This prinCiple was explored using the results of residence-time distribution 7 |
