Reburning Scale-Up Methodology for NOx Control From Cyclone Boilers

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A complete numerical simulation of the reburning sys­tem, including cyclones and screen tubes, was performed. The purpose was to evaluate the effects of lower furnace velocity gradients computed by the model on the mixing performance. Similar mixing performance was achieved for the rebuming burner penettation as the previous simu­lations utilizing measured velocities from the full-scale unit. The numerical model is capable of predicting the mixing and stoichiometry distributions. Detailed combus­tion and heat transfer predictions using B& W models (5) are planned to be completed later during this project Numerous cases were studied using the validated model. These cases included the effects of size, number and location of the burners and OFA ports, and the addi­tion ofFGR on mixing. Figure 13 shows the mixing per­formance computations for three and four burners. When FGR was not used, mixing with three reburning burners was almost as good as that with four reburning burners, with approximately 60% of the flow reaching the reducing conditions. The maximum flow achieving reducing condi­tions was observed when four burners were used along with FGR. A total of 80% of the flow reached the reduc­ing environment with four reburning burners in compari­son to 62% for a three reburning burner system. The OFA mixing was also good in that all flow reached the oxidiz­ing conditions before exiting the furnace. The predicted mixing performance of the Nelson Dewey boiler with four reburning burners/OF A ports was similar to that of the SBS. Therefore, four reburning burners were recom­mended for the reburn retrofit CONCLUSIONS AND RECOMMENDATIONS Based on the pilot-scale study and modeling efforts, the following conclusions and recommendations are derived: • Nominal 50% NOx reduction is feasible without major side effects on boilez operational conditions • Pilot-scale simulation of the Nelson Dewey unit produced a close thermal and chemical environment to the full-scale. Differences are found, but they are not defeating. • Mixing performance of the SBS could be duplicated by using four reburning burners and OF A ports • The Nelson Dewey reburning system is sufficiently flexible. Mixing and penettation can be adjusted by adjusting fuel split, FGR flow rate, and rebuming burner hardware • Physical and numerical models were validated for mixing evaluation. These tools could be used in future applications. • Use four burners and OF A ports for the Nelson Dewey reburning system • The pulverizer design should be capable of providing a nominal 30% heat input with a fmeness of 85% through 200 mesh • Further study is needed to assess the impact of reburning on ESP performance. Rgure 13. Effect of tM Number of Rebumlng BumMS .nd OFA Ports on Predicted MIxing PMfotmlta (25% Fwi to the IWxImlng Burners) 12