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negative). The greater the burner heat release the greater the reduction in NOx emissions with an increase in LHV off gas (the rate of change of relative NOx with fraction LHV off gas). Finally, the effect of increasing the excess air levels up to 5 % O2 in the flue gases varied depending on the burner ftring rate: at 25% design heat release, an increase in excess air levels decreased the effectiveness of LHV off gas utilization; at 50% design heat release, the excess air levels had apparently little effect on the effectiveness of the LHV off gas in reducing NOx emissions; and at 75 % and 100% design frring rates, the reduction in NOx emissions caused by the introduction of LHV off gas increased as the excess air levels approached 5%02• Figure 15 shows the results of a similar analysis carried out on the same data, but considering the rate of change of NOx with O2, and the effect of the fraction LHV off gas on this relationship. The all negative gradients of these curves show that increasing the LHV off gas proportion of the fuel decreases the rate of increase (or even increases the rate of decrease) in NOx production as the combustion air levels are increased towards 5 % flue gas O2• At below 40% LHV off gas the increase in excess air levels reduces NOx formation (the 25 % design heat release curve drops below the zero (NOx)/(02) level. The suggested explanation for the phenomena outlined above is as follows: When operating at 25 % design frring rate the flame is relatively cool due to an increased surface area to volume ratio and a low mixing intensity resulting from low turbulence levels, a consequence of low fuel and air velocities/supply pressures. Increasing the excess air levels leads to increased quenching effects on this flame. The effect of increasing the proportion of LHV off gas therefore is to cool an already cool flame, that is, the effect of the NOx minimization technique, in this case of increasing the proportion of low Btu fuel, is reduced when other NOx controlling features, i.e. flame cooling by low mixing rates and high excess air levels, are also in effect (Figure 9 - NOx V O2, 25% Design Heat Release Rate - shows that with 40% and above LHV off gas increasing O2 reduces NOx emissions, when frring at 50% design heat release and above the increase in combustion air increases NOx emissions with all variations in fraction LHV off gas). In contrast, in the 75% and 100% design cases, the NOx minimizing 12 |
