OCR Text |
Show O.lm form the burner exit. The measured temperature profile shows that the gas temperature is only approximately symmetrical about the burner centreline (position 0), indicating a maximum temperature of 2180K at the central point of the air-staged burner which gradually decreases either side to a temperature of 1600K. The predicted temperature shows reasonable agreement with the measured data therefore providing reasonable flow and temperature fields for NOx emission simulation. The temperature reached by the unstaged burner exceeded * at the burner centreline. A comparison of the measured and predicted NOx emitted from the unstaged and airstaged type burner, at a distance ofO.lm from the burner exit, is illustrated in Fig. 5(a) and (b) respectively. In Fig. 5(a), the experimental result indicates that the maximum NOx concentration reaches approximately 2784ppm. The predicted result shows good agreement with the experimental data and the variation may be due to the non-symmetrical behaviour of the burner and measurement errors. Predicted results indicate that the contribution of the prompt-NO is about 12 - 14% of the total-NO and reaches its maximum value at the burner centreline. This could be due to the formation of prompt-NO from fuel fragments present in the centre of the flame. However, thermal-NO (which contributes in the region of 85% of the total-NO) reaches its maximum level at ±10mm from the centre of the burner. Figure 6(b) shows the measured and predicted NOx concentration for the air-staged type burner, at O.lm from the burner exit. According to theoretical predictions during the staged combustion, the contribution of prompt-NO (28 - 32% of the total-NO) increases significantly in the fuel-rich regions of the furnace. The difference between the experimental and predicted results in the near burner region are larger for the unstaged burner. However, the trends are properly predicted indicating that the essence of the problem is still retained in the global mechanisms used for fuel-oxidation and NO-chemistry. In addition, the N02 concentration result is also illustrated and the theoretical values indicate that its contribution to the totalNOx, at this position inside the furnace, is negligible at the centreline of the burner. However, in the region of ±50mm. from the centre, the total contribution is in the order 5% of the totalNOx, where there is rapid cooling of the combustion gases. Comparisons between the model predictions and experimental measurements for exhaustNOx emissions, under the same burner operating conditions as described earlier but different air preheat temperatures, are shown in Fig. 6. Generally, an increase in the air preheat temperature results in an increase in the flame and furnace temperature. Higher flame temperatures lead to higher radiative heat transfer to the load resulting in greater thermal efficiencies. Kinetically, the thermal-NO formation rate increases exponentially with the rise in temperature. However, the model predicts lower values of NO emission in the stack for unstaged combustion. The discrepancy between the measurements and predictions for NO is partly due to an error introduced by employing two-step reaction mechanisms for high 9 |