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Show INTRODUCTION Many manufacturers of burners for boilers and process heating equipment have recognized that lean premixed combustion will be an important technology for natural gas systems to meet future air-quality rules. In the laboratory, it has been proven that premixed flames emit only 50 ppm N O x at stoichiometry (i.e. 0 % excess air). Increasing the air/fuel ratio to 4 0 % excess air (i.e equivalence ratio (J) < 0.7) can reduce N O x by ten times to less than 5 ppm. These low emission levels are accessible without relying on fuel-air staging, flue gas recirculation (FGR) or selective catalytic reduction (SCR). In practical situations, these low N O x levels are rarely attainable. This is due to factors such as burner design, flow turbulence, flame stability, non-homogeneous mixing, heat loss and burner/furnace coupling. As lean premixed combustion is a direct means to prevent the formation of N O x , it is an ideal solution for both small and large systems. However, this clean combustion technology has yet to be deployed extensively because lean premixed flames are more difficult to stabilize than the non-premixed reactions found in most conventional burners. They tend to rumble and can become unstable when operating at near limit conditions or during load following. D u e to the fundamental differences in their flame behavior and fluid mechanical processes, the large body of knowledge gained from developing non-premixed or partially-premixed burners is not directly transferable to lean premixed burners. Currently, the 25 p p m N O x limit is considered a critical operational threshold. This is because burners offering < 25 p p m N O x can come with significant penalties in terms of cost increases and limitations in load flexibility. Lowering the emissions closer to 10 p p m N O x will invoke complicated safety issues and will require very expensive controls to mitigate. To reach the so-called single digit N O x level, selective catalytic reduction is considered the only alternative. Despite these potential technological barriers, the market for < 25 p p m gas-fired lean premixed burners is considerable. The technology challenge is to develop new burner design concepts that will be much lower in capital, manufacturing and operating costs but match or exceed the N O x performance of F G R and S C R systems. The low swirl burner developed at the Lawrence Berkeley National Laboratory is a promising new lean premixed burner concept that seems ripe for commercial development [1]. The L S B is optimized for lean premixed combustion and has a simple design that provides a direct means to achieve consistent ultra-low N O x performance without using F G R , S C R or specialized materials. Preliminary laboratory research shows that the L S B delivers stable ultra-low N O x performances, requires only simple controls, and has high combustion efficiency, a high turndown ratio and a low pressure drop. Therefore, this concept has great potential for use in an affordable, efficient, and robust commercial product for a variety of applications. This burner is called weak-swirl burner in previous publications 2 |
