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Show 5 Alzeta modified a uniformly perforated burner as described in References 1 and 2 to reduce the NOx emissions to below 30 ppm at 3%) oxygen. The surface of the burner was modified to create alternating regions of perforated and non-perforated areas, and retrofit into a commercial, gas-fired hot water boiler. The boiler was instrumented to measure flue gas emissions, thermal efficiency, firing rate and stoichiometry. Alzeta discovered that selectively perforating the metal mat lowered the NOx emissions compared to the uniformly perforated burner surface as shown in Figure 1. In this arrangement, part of the burner surface operates as a traditional radiant burner while the perforated zones operate in the blue flame mode at much higher surface firing rates as shown in Figure 2. The entire burner surface still operates at surface firing rates of 1 million Btu/hr-ft2 The selectively perforated pattern also eliminated most of the combustion induced noise generated at low excess air levels. Alzeta called this new burner the IIradiation stabilized burner" . The perforated regions of the burner element are able to operate at very high surface firing rates because the radiant zones surrounding each blue flame zone anchor the flame to the surface of the burner element. As a result the burner has a large turndown ratio, and operates over a wide range of stoichiometries. In contrast, perforated burners made from non-porous surfaces, such as stainless steel, have a much narrower operating envelope and a limited turndown. Two mechanisms contribute to the NOx reduction in the radiation stabilized burner. The first mechanism is a rapid post-flame quench of each blue flame zone. Each blue flame zone acts as a free jet transferring its momentum to the surrounding flue gases and entraining them into the blue flame zone. This entrainment rapidly cools the flame lowering the NOx emissions. A second effect is the direct IIflue gas recirculation" effect produced by the entrainment of the products of combustion from the adjacent radiant zones into the blue flame. In the radiant zone, the combustion reaction is completed a few millimeters downstream of the burner surface. The flue gases, cooled by radiant |