OCR Text |
Show burner showed an improvement in the NOx emissions at turndown (Figure 9), partly because of the lower thermal loading on the burner and the furnace, lowering the flame temperatures, but also because of the fact that the relative FGR rate increased as the burner was turned down (to as much as 36% at 4: 1 turndown). The efficiency of a jetpump is a strong function of the mass-flow-rates and velocities in the jet-pump. In the design of the jet-pump, advantage can be taken of this feature to match the recirculation rates with the needs of the burner at any firing rate. During the detailed flame characterization experiments, spatially detailed measurements were made of temperatures, velocities, and concentrations of major species (Figures 10 -12). In addition, several chemiluminescence images were take of the flame under a number of operating conditions (Figures 13). The detailed measurements showed a relatively homogeneous temperature and species distribution, key to low NOx emissions (Figure 10). The temperature was highest in the fuel rich flame core which promotes NOx reburn in that zone. Also, the plots of the CO and hydrocarbon distribution showed that burnout was virtually complete at a distance of 2.5 burner diameters from the tile exit (Figure 11). The detailed flame map revealed a certain degree of asymmetry in the flame. This asymmetry was attributable to a combination of inhomogeneities in the mixture exiting from the mixing throat of the burner into the diffuser, and separation of the flow in the diffuser. As can be seen from the plot of the NOx iso-mole-fraction lines (Figure 10), this asymmetry lead to a region of NOx formation near one of the secondary air jets. After the walls had been changed to their water-cooled configuration, the tile of the burner w~ -equipped with a flame holder, to improve-the symmetry of the flame, and to minimize NOx formation at the secondary air jets. At the same time the mixing between the fuel and the air in the jet-pump was improved. Although these modification reduced the maximum achievable FGR rate (due to the higher pressure drop across the burner tile), the symmetry was indeed improved (Figure 12), and the region of high NOx formation at the one secondary air jet was reduced. This finding indicates that if a recirculation zone is introduced in the burner tile and the. fuellairlFGR mixture is more premixed, even lower NOx emissions can be achieved. At the same time, the modifications will lead to an improvement in the stability of the flame at low air temperatures. 4 Benefits and Conclusions In conclusion, the optimization and detailed characterization of the StAR II burner at BERL has achieved several significant improvements to the StAR burner and provided valuable insights in its operation. First, the NOx emissions from the StAR prototype were reduced by 5-10 ppm (depending on the operating conditions) compared to the original configuration. The turndown performance of the burner was improved so that even lower NOx emissions were achievable, even at firing rates as low as 20% of full fire. Furthermore, the stability of the flame when operated with cold air was improved, and the NOx emissions were reduced. |