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Show 3 Results and Discussion The StAR II burner was installed in the Burner Engineering Research Laboratory where extensive parametric studies were carried out to optimize performance of the current burner design. In addition, spatially detailed flame characterizations were carried out for several operating conditions. For the parametric optimization experiments, measurements of species concentrations were made at the exit of the combustion chamber. Measurements of species concentrations were also made in the FGR return line to detect leaks in the system. In the parametric experiments, the effect of burner configuration (through the primary fuel equivalence ratio and the flue gas recirculation rate) upon the burner performance was investigated first. By moving the fuel gun further into the burner, the fuel equivalence ratio in the primary combustion zone was increased (from 1.7 to 2.7), which also resulted in an increase of the maximum flue gas entrainment rate (from 25% to 30%). Judging on a combination of the NOx emissions and flame stability and symmetry, a primary fuel equivalence ratio of 2.5 was chosen to be the optimum configuration (Figure 5). It was found that with the current burner design, the NOx emissions did not improve when the fuel equivalence ratio in the primary zone was increased beyond 2.6. Possibly, when the primary mixture becomes too fuel rich, such a large fraction of the fuel remains unbumed as it starts mixing with the secondary air jets that a reduction in NOx emission from the primary zone will be more than offset by the increase in NOx production at its interface with the secondary air jets. After optimization of the burner geometry, parametric studies were carried out to characterize the · effects of FGR rate, air preheat temperature, excess air level and turndown on the NOx emissions. As was shown in the MIT tests, the operating parameter with the strongest influence on the NOx emissions was the FGR rate (Figure 6). Besides its effect on NOx emissions, the FGR caused the flame to become transparent to the human eye, although the flame scanner was still able to detect a strong signal. Although this mode of combustion differs from a normal, luminous diffusion flame, it is also distinctly different from the so-called flameless oxidation [5] in which the flame is lifted off from the burner. For many metals processing applications, a safe, stable, flame is desirable to prevent radiation hot-spots in the furnace. The amount of FGR that can be accepted is limited by the jet-pump capacity and by the flame stability at low. air temperatures. The effect that high levels of air preheat would have on NOx formation .in conventional burner systems is offset in the StAR burner by the high FGR rates that can be achieved at those high air temperatures (Figure 7). This feature of the burner makes the StAR burner attractive for both operations that use ambient air and operations that use high levels of air preheat. As expected, high levels of excess air had a negative impact on the StAR burners low NOx emissions (Figure 8). The higher overall fuel equivalence ratio in combination with the higher oxygen levels in the FGR caused the primary fuel equivalence ratio to drop rapidly (from 2.5 at 10% excess air to 1.5 at 60 % excess air). The data shown here were obtained without re-optimizing the burner for these high excess air levels. Finally, the performance of the StAR II burner at turndown was investigated. The optimized |