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Show It is fairly well established that NO formation is strongly temperature-dependent, based on the Zeldovich mechanism. The second mechanism has been invoked to explain the anomalously high concentration of NO that has been observed in flame fronts.4 Because the reactions are endothermic and the activation energy and reaction heat are relatively low,s prompt NO formation can occur in relatively low-temperature flames and can still be accelerated by increasing flame-front temperatures. The third mechanism is used to describe how fuel-bound nitrogen is converted to NO. Conversion of fuel-based nitrogen to NO also increases with increasing flame temperatures. NOx generated by the combustion of natural gas is primarily controlled by the Zeldovich mechanism. The rate of formation of thermal NOx is highly temperature dependent. 6 Therefore, the technical direction for minimizing NOx formation is to reduce temperature in the flame zone. NOx Reduction - Approach and Challenges With an understanding of NOx formation, the basic approaches toward lower NOx emissions focus on reducing the concentration of free oxygen, residence time, and combustion temperature, including eliminating the "hot spots" in the combustion zone. The various proven practical combustion technologies for reducing NOx formation include high excess air combustion, homogeneous combustion, staged firing, recirculation of combustion products, cooling of the combustion zone, and heat removal from the flame. However, the only approach that has reduced NOx emissions to the levels of a few vppm is premixed gas combustion. For most kinds of space and process heating applications for which ultra-Iowcombustion emissions are targeted but high combustion temperatures are not required, the favorable approach is high excess air premixed combustion because of its simple structure and operation and, thereby, low cost. Consistent reduction of NOx emissions with an increase in excess air has been proven experimentally in many gas-fired test combustors.7-12 The data show a clear trend that NOx could be reduced to a level less than 1 vppm by increasing excess air greater than 80%. However, problems with combustion stability, especially at low load operation, can result. Moreover, any attempt to reduce NOx by lowering combustion temperatures and minimizing the residence time may lead to an increase in CO and THC. Therefore, an advanced combustor to achieve ultra-low emissions for all combustion pollutants (NOx, CO, and THC) should apply the following techniques: • Premixed fuel/air combustion • High excess air operation • Excellent flame stabilization • Intensive mixing in the combustor to minimize CO and THe formation. For the applications of these techniques, the cyclonic combustion concept has been adopted at IGT as the ideal candidate. Cyclonic Combustion A conceptual cyclonic combustor is shown in Figure 1. Through the evenly spaced tangential nozzles, a premixed gas/air mixture is injected into the combustion chamber at a 3 |
