OCR Text |
Show 7 heat loss, are entrained into each blue flame zone providing further quenching of the flame temperature. Both of these effects work to reduce the flame temperature and the corresponding NOx generation rate. In the conventional Zeldovich mechanism, NO is generated by the following reaction set: o + N2 ---> NO + N N + O2 ---> NO + 0 N + OH ---> NO + H In the flame and the post-flame zone, the majority of the N created by the first reaction is rapidly converted to NO by the second and third reaction. Thus the rate of NO formation is approximately twice the rate of the first reaction. Although the 0 atom concentration is well above equilibrium values in the narrow flame zone, it rapidly approaches equilibrium in the post-flame zone. Using this well established IIpartial equilibrium" assumption, the rate of NO formation can be calculated from simple equilibrium calculations and the well established rate of the first reaction. Figure 3 represents such a calculation for methane and air combustion with 20% excess air. At the adiabatic flame temperature of this mixture (3210°F) the peak NO production rate is 1640 ppm/second. By the time this mixture has cooled two hundred degrees, this value has plummeted to 224 ppm/second. Because of the radiation from the radiant zones of the burners, peak temperatures of the flames attached to those zones never exceed about 2800°F. Little NO is thus formed in these zones. These products serve to initially stabilize the attachment of the blue flame above the adjacent perforated portion of the burner as well as introduce their somewhat lower energy gases into that blue flame. Downstream, the high velocity distributed blue flames serve to effectively induce combustion products from the larger chamber volume to cause rapid quench downstream of the flame front. The peak velocity of the combustion products above the blue flame zones is in excess of 20 feet per second. This assures the rapid quench desired. |