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Show The burner design, shown in Figure 3, incorporates a jet pump, driven by the combustion air, which enables the recirculation of a large volume of flue gas into the burner for mixing with the primary combustion air and the fuel prior to ignition. The recirculated flue gas not only reduces NOx emissions by suppressing peak flame temperatures, but also improves heat transfer in the furnace by increasing the mass flow through the burner. Use of the jet pump eliminates the need for a separate fan, thereby reducing cost and improving reliability. Due to the inherent self-proportioning entrainment of the jet pump, the recirculated flue gas fraction is maintained substantially constant over the entire fIring range, without the help of external controls. The balance of the combustion air is injected into the flame envelope to ensure complete burn-out of the fuel. This staged air process provides additional NOx control by tailoring the flame's temperature and stoichiometry distribution in a way that avoids the high temperature, fuel lean conditions that favor NOx fonnation. Concept Development The prototype StAR burner was designed with the aid of two combustion models. A combustion kinetics model was used to optimize the operating conditions for NOx control and to define residence times required for ignition and flame stabilization. A computational fluid dynamic model was then used to optimize the mixing process and thereby establish critical burner dimensions. This design process provided the team with considerable knowledge regarding the impact of key variables on burner performance. It also significantly reduced the required experimental effort, since burner modifications were not required during the prototype testing. As discussed below, this initial design surpassed all program goals. Prototype Testing The 3 MMBtulhr prototype StAR burner was manufactured by Hauck and tested at MIT's Combustion Research Facility [Ref 2]. This burner achieved very low NOx emissions, even when operating with combustion air preheated to 1090 K (15OO°F) and with furnace temperatures of about 1530 K (23(xtF). The results, presented in Figure 4, show that by optimizing the primary zone equivalence ratio and level of flue gas recirculation, NOx emissions could be reduced from an uncontrolled concentration of about 500 ppm to levels in the range of 20 - 40 ppm. Achieving 20 ppm required a high level of flue gas recirculation, while 30 - 40 ppm was reached with only moderate recirculation. These NOx levels are significantly below the project's goal of 100 ppm for highly preheated combustion air. At moderate preheat temperatures (530 K or 500°F), NOx emission levels below 20 ppm were achieved. In all cases, carbon monoxide (CO) emissions were well below the target of 100 ppm. The NOx control effectiveness demonstrated by the prototype StAR burner (92-96% NOx reduction) is comparable to the best achievable with expensive post-combustion 6 |
