| OCR Text |
Show temperature combustion processes where dissociation will be significant and partly due to numerical diffusivity. Oxyll'ueicolnbustion The numerical predictions are presented in Figs. 7 to 9 and show a good correlation with the experimental results. The investigation confirms that the multi-step global mechanism for CH4/02 oxidation can adequately predict temperature and major species concentration at high temperature conditions. The reversibility for reactions 5 and 6 is required to predict the approximate equilibrium product distribution. Figs. 7(a) and (b) show a comparison of the temperature predictions and demonstrate a far higher flame temperature in the case of normal firing than for reverse firing. This is due to the higher combustion intensity from the jets, shown in Fig. 8 where the higher jet velocity issuing from the normally fired burner is demonstrated. This clearly displays the influence of both the mixing angle and general flow direction on the mixing process. For the reverse case, oxygen and fuel are generating a slow mixing flame, which results in slaw Rnd incomplete burnout, leading to a high CO concentration in the flue gas. In the case of normal firing, the furnace behaviour is dominated by a high velocity jet through the centre coupled with high intensity combustion. Numerical predictions of thermal. prompt and total-NO (Figs. 9(a-f)) are lower than for the reverse fired burner, despite the higher predicted temperatures. Prompt-NO is more significant in the reverse fired case, representing nearly 23% of the total-NO formed as opposed to 17% from normal firing; values being taken from approximately halfway along the furnace. These values change to 17% and 14% of the total-NO formed in the exhaust gases for reverse and normal firing respectively. This is due to less efficient mixing giving rise to a larger fuel-rich zone with high fuel concentrations offuel fragments such as CH radicals, which are precursors of prompt-NO. Finally, Table 1 summarises the measured and predicted NOx in the flue gas for the cases considered during air-fuel and oxy-fuel combustion. Conclusions Preheating of combustion air without staging is disadvantageous to NOx emissions. If preheating of the air is necessary from a thermal efficiency point of view, staging the air into a flame may reduce NOx emissions to more acceptable levels. Lower EINOx numbers are achieved with oxy-fuel firing. Here, high thermal efficiencies are generated at low NOx emission levels. This investigation indicated that CFD modelling can be used as a design tool to provide information on the trend of the pollutant emissions during the combustion process. However 1(") |
