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Show FUEL STAGING BURNERS FOR NOx CONTROL R. Waibel, D. Nickeson John Zink Company Tulsa, Oklahoma, USA L.Radak W.Boyd Southern California Edison Company Southern California Gas Company Rosemead, California, USA Montebello, California, USA ABSTRACT Low excess air, flue gas recirculation, air staging and burners-out-of-service have all been utilized extensively to reduce the emissions of NOx from industrial burners and utility boilers. New restrictions in parts of the US and elsewhere, however, have defined levels that are more and more difficult to achieve. Specifications of 75 ppm and even 40 ppm as the maximum allowable concentration are becoming common. Fuel staging has proven to be able to achieve these more restrictive NOx levels in most cases. The results of an extensive research and development program are presented in which the fuel staging technique has been applied to burners for industrial and utility boilers. Burners ranging in size from 25 MMBtu/hr to 100 MMBtu/hr were used in the study. The benefits of combining fuel staging and flue gas recirculation are also presented. UNTIL RECENTLY NOx emISSIon control has been primarily limited to large utility boilers or has been enforced on selected industries on a regional basis. New rules, however, such as the proposed New Source Performance Standard for industrial boilers have provided a basis for national interest in NOx control. As the major supplier of burners to the petroleum refining, petrochemical and chemical industries, the John Zink Company has gained considerable knowledge of NOx control technology. These industries have experienced environmental restrictions on the emissions of oxides of nitrogen from process heaters and boilers for several years and these restrictions have become increasingly more stringent and widespread. Although the state of California has the most stringent restrictions, the state of Alaska also has strict rules. Specifications for low NOx burners, however, are now typically received for burners going to all parts of the U. S. and are being specified more frequently for Europe, the Middle East, and Asia. Early low NOx technology involved air staging and flue gas recirculation. Air staging, however, results in extended flame lengths, difficulties in achieving low excess air operation and problems with controlling flame impingement on process tubes. Flue gas recirculation, although an effectiv~ techniq~e f~r controlling thermal NOx, can be expensIve and SInce It 345 cannot easily be employed on natural draft burners it is seldom used on process heaters. Although forced draft, preheated air burners are being used much more frequently on process heaters, such applications generally include the requirement that the burner must also be able to operate at full capacity under natural draft, ambient air conditions. Fuel staging has been developed as an alternative to air staging and flue gas recirculation and has proven to be the most effective means for overcoming operational problems of these latter two methods while still providing the lowest a vaila ble NOx emissions. The John Zink Company developed staged fuel burner technology in order to meet both the operational needs of the user and the increasingly stringent environmental restrictions. Staged fuel burners for process heaters have been provided by John Zink since 1981 and several thousand are now in service. During the past two years development efforts have been underway to apply fuel staging technology to large capacity, forced draft industrial and utility boiler burners. The bulk of the results included in this paper are a result of this boiler burner development program which is being cost shared by the Southern California Edison, Southern California Gas and John Zink Companies. FUEL STAGING PROCESS Figure 1 depicts John Zink's patented staged fuel burner design. It shows a schema tic cross-sectional view of a staged fuel process heater burner. NOx control is accomplished by injecting part of the fuel into the bulk of the combustion air and the remainder of the fuel into the tail of this primary combustion zone. Thermal NOx production in the primary combustion zone is limited by the low flame temperatures that result when fuel is burned with very high excess air levels. NOx production is low despite the high oxygen concentrations because the peak flame temperatures are low. Quenching of the flame by the high excess air levels also occurs further limiting the peak flame temperatures and providing active reducing agents for NOx reduction. The inerts in the primary zone combustion products then reduce peak flame temperatures in the secondary combustion zone and lower the local concentration of oxygen. Both of these factors limit the NOx production in the secondary combustion zone. |