OCR Text |
Show 12 flame. The area of internal circulation is known to play an important role in flame stabilisation, as hot products of combustion are carried through it near the burner tip. The bigger the area of the internal circulation is, the higher are the temperatures near the burner zone resulting in a better flame stability. As can be observed in Aappendices 8 and 9, the internal circulation zone of the new burner configuration is evidently bigger than the corresponding zone in the existing burner. Furthermore, the simulation results indicate that the separation of the tertiary air from the main combustion zone is more effective in the new burner configuration, because of the optimised tertiary air guide sleeve. In addition, according to the computational flow fields, the flame shape and the flame width are almost the same as in the original burner. This means that the corrosion rate will not increase after the burner modification. Temperature fields The computed temperature field showed (Appendices 10 and 11) that, with the new burner structure, several degrees higher temperatures occur near the burner tip as compared to the existing structure, resulting from the higher internal circulation area of the new burner structure. Higher temperatures near the burner tip accomplish a better flame stability. Also, the higher temperature near the burner tip means that the combustion in that area is vigorous and the O2 content reduces rapidly to near zero, which means that the already formed N O x emissions will be reduced to N 2 by reducing the species. The higher the near burner temperature is, the lower are the N O x values. The relationship between the temperature of the near burner areas and the N O x values has been proven in the simulations and in the combustion tests with the test rig and the actual boiler. 02 fields A key factor in designing a low-NOx burner is forming a large, fuel-rich zone in the reaction field, i.e. in regions with a high concetration of reducing species and low levels of 0 2. To achieve this goal, it is necessary to insure that the ignition of the fuel occurs rapidly and to promote the O2 consumption in the primary flame zone. Because of the more rapid ignition of the fuel in the new burner construction and the higher temperatures in the near burner area, the area of the low O 2 content will be bigger than in the existing burner (appendices 12 and 13). O n the basis of the O 2 fields, lower N O x values can be expected with the new burner construction than in the existing burner. 5.4 Conclusion In conclusion, the new burner structure, with optimised air guiding techniques and flame stabilisation, will produce a flame with lower N O x emissions and a better flame stability. However, the flame shape is almost the same as in the original burner, only the size of the internal circulation area is different. Two of this type of new burners have been installed at Meri-Pori during the summer overhaul of 1998, and the performance of these burners will be studied during the forthcoming operating period. |