OCR Text |
Show vertical plane of the furnace under the three simulation cases. In the cases of un-diluted air combustion, the N O x emission at the furnace exit increases more than 100 times from 16 ppm to 1734 p p m when the air temperature increases from 323 K to 1223 K. The same increment appears for the maximum N O x concentration in the flame zones. In the case of diluted air combustion, however, almost no N O x is formed because of the low flame temperature. The low N O x emission is another important benefit provided by the highly preheated and diluted air combustion. Based on the NOx model adopted in FLUENT code, thermal-NOx production doubles for every 90 K temperature increase when the flame temperature is about 2200 K. As the maximum flame temperature reaches a high level of 2627 K in the case of highly preheated air combustion, the predicted N O x concentration also reaches a very high level. Comparing with the N Ox emission measured by Hasegawa et al. [1], the N O x formation in this case is over predicted because of the over predicted flame temperature as mentioned above. It is shown from our simulations that the prompt-NOx has very little contribution on the total N O x emission. In light of this, controlling the flame temperature should be the key point to lower the N O x emission. Soot formation. Because of the low oxygen concentration and the high air temperature in the case of highly preheated and diluted air combustion, the soot formation becomes a concerned issue. The simulated soot concentration distributions in the regenerative furnace are illustrated in Fig 9. A s can be observed, the soot is mainly formed in the flame zone, and the formation rate becomes fast when the air temperature is high and the oxygen concentration is low. Almost no soot was formed in the case of normal combustion, while a maximum soot concentration was predicted to be 0.426 mg/m3 and 3.29 mg/m3 respectively for the highly preheated fresh air combustion and the highly preheated and diluted air combustion. In these two cases, however, the formed soot was almost completely oxidized in the rear flame zones, and little soot was emitted from the furnace exit. The formed soot in the flame under highly preheated air combustion conditions will contribute to the radiation heat transfer of the flame. OH radical concentration. The OH radical concentrations were also calculated in the cases of normal air combustion and highly preheated and diluted air combustion respectively. The concentration distributions on the central vertical cross section of the furnace are shown in Fig 10. A s can be seen, the O H radical concentration has a much higher level for the normal air |