OCR Text |
Show mixing regime determined from the acid/alkali model. The heat flux pattern was found to be rather flat and its absolute peak value less than optimum, figure 12. These results were very encouraging since they indicated that it should be feasible to improve the fuel/air mixing with the new gas burner and hence improve or maintain the heat flux values. The proposed burner was modelled and its design adjusted for both natural gas and standby oil firing. Figures 13 and 14 shows the simulated natural gas flames at different excess air levels. The operation at 30% excess air, shows that the flame mixing length is shorter than at 8% excess air. This is caused by the higher oxygen concentration available with the excess air ensuring more rapid burnout of the fuel. The flame length excess air relationships extrapolated from the acid/alkali model results for the gas and oil flames are shown in Figure 15. It can be seen that the flame length profiles are similar to each other indicating the good jet mixing ability of the burner with either fuel. It will be noted that the gas flame is longer than the oil flame, for a given excess air level. This is due to the higher air requirement of the natural gas giving rise to a longer jet mixing length for this fuel. Mathematical modelling was undertaken to enable comparative assessments to be made of the flame heat flux distribution, temperature and burnout for different operating conditions and fuels. Figure 16 shows the difference in heat flux distribution to the kiln wall for the two fuels fired by the high momentum FCT burner. The differences between the wall heat flux distribution for the gas and oil flames is immediately obvious. The oil flame has its maximum heat release close to the burner at about 10 to 60 feet from the nozzle. The natural gas firing conditions however move the peak heat flux region some 60 feet further down the kiln, due to the reduced emissivity of the natural gas flame. By comparison the heat flux profile produced by the original burner, Figure 12, can be seen to be somewhat flat with a low maximum peak value. This flattish profile is typical of that produced by a low momentum burner with a non-recirculatory flame where the fuel/air mixing is marginal. This leads to a slower combustion rate and hence a long flame. Flames of this nature tend to produce an unstable kiln burning zone and can also give elevated exhaust gas temperatures. Hence, the flame length and heat flux profile produced by the proposed gas burner will enable kiln operation at lower excess air than the existing oil burner, whilst maintaining an adequate heat flux to the charge. There will still be differences in operation and an increased exhaust temperature owing to the changes in heat flux pattern between oil and gas but these effects are minimized. The standby oil burner has the potential for even better performance. However, this is not an issue since its operation is likely to be extremely infrequent, if it is ever used. |