Combustor axial temperature profile

Experimental results (Li et a/., 1991) indicate that the axial temperature distribution is highly dependent on the secondary-to-total air ratio. When this ratio is less than about 0.3, the axial temperature profile is essentially uniform except for the region approaching the exit of the combustor, as shown in Fig. 17. When this ratio exceeds about 0.4, however, the temperature in the middle of the combustor becomes lower than those at the two ends. On the other hand, experimental results also indicate that increasing the secondary air ratio is favorable for supressing NO emission. If this ratio is greater than 0.5, NO emission can be controlled to less than 65 ppm. Therefore, it is necessary to locate the secondary air inlet properly and choose the secondary air ratio in order to optimize between efficient combustion and low NO emission. 

Temperature profiles were measured at several axial locations to locate the peak temperatures in the combustor. The axial distance between the nozzle and the temperature-measurement cross-section is denoted by Lf With one insert in place, the peak gas temperature immediately downstream of the insert was lowered but the high-temperature region was extended radially, i.e., the pattern factor was improved, as shown in Fig. 28.2. The peak temperatures at each axial location are shown as a function of the distance from the nozzle, or Lt/D, in Fig. 28.3. For the baseline case the highest temperature of 1418 K was found at 1.8 pipe diameters downstream of the nozzle. With one porous layer present, the peak gas temperature was about 200 K lower at Lt/D = 1.8 2.2 but increased by up to 120 K and 200 K at 0.5 and 3.2 pipe diameters downstream of the nozzle, respectively. The highest flame temperature was lowered but the high-temperature region was extended to upstream and downstream. 

With fitted reflection factors, this model does an excellent job of predicting axial density profiles in the upper part of a range of different columns with secondary air addition (Senior and Brereton, 1992), not only for a small-scale pilot plant CFB combustor but also for a prototype boiler of cross-sectional area 0.43 m operated at high temperature (845°C). 

---