OCR Text |
Show temperatures. The calculated maximum flame temperatures and N O x emissions at the furnace exit are shown in Fig 15, and the profiles of T^/T^an along the furnace height are plotted in Fig 16. As expected, the flame temperature increases as the preheated air temperature increasing, and the temperature distribution tends to be more uniform. A s a result, however, the N O x emission also increases as the preheated air temperature increasing. 3.3 Optimal combination of preheated air temperature and oxygen concentration The propose of the design of advanced industrial furnace is to provide a high level and uniform temperature in the furnace and a low N O x emission. It is clearly seen from above results and discussion that the flame temperature tends to be high and uniform when the preheated air temperature increasing, while the N O x emission decreases as the oxygen concentration decreasing. A s pointed out above, N O x is hardly formed when the flame temperature is lower than a certain level, but will rapidly increase when the flame temperature is higher than a certain temperature. Considering both temperature and N O x emission, the highly preheated and diluted air combustion should be arranged to provide a uniform temperature level in the furnace as high as possible in the range where N O x is hardly formed. Such a combustion process could be realized by an optimal combination of the preheated air temperature and the oxygen concentration in the diluted air. As can be seen from the above results, for a certain preheated air temperature, there exists a so named critical oxygen concentration where the flame temperature can be maintained in a high and uniform level but the N O x emission is lower than the permitted value. For a certain oxygen concentration, on the other hand, there exists a so named critical preheated air temperature where the high and uniform flame temperature the permitted N Ox emission can be obtained. The combination of such a critical oxygen concentration with the corresponding air temperature or such a critical air temperature with * Q .S 0.11 g 2 0~ 0.09 - NOx emission at furnace exit greater than SO ppmv NOx emission at furnace exit less than 50 ppmv 1100 1150 1200 1250 1300 1350 1400 1450 Preheated Air Temperature [K] Figure 17 Optimal combination of preheated air temperature and oxygen concentration in air |