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Show 11 experimental finding that the pressure sensitivity of the NOx increases as the flame temperature increases. In Figure 8, the effect of residence time is shown. There are two residence times of interest. These are 1) the residence time of the zone of high, non-equilibrium free-radical concentrations (i.e., the PSR zone, which may be loosely thought of as the flame zone), and 2) the overall residence time of the combustor. In the PSR zone and very early part of the PFR zone the NOx forms quickly (mainly by the nitrous oxide mechanism; see below). In the balance of the PFR, which represents the post-flame zone, the NOx fonns slowly (by the thennal NOx mechanism). In this case, the adiabatic equilibrium temperature is about 1800K. At temperatures lower than this, the post-flame NOx behavior becomes flat, and the overall residence time has little effect on the NOx emission. This effect is also reported by Snyder et ale (1994). Chemical reactor modeling has been used also to assess the contributions of the three NOx formation mechanisms at engine pressure levels. The sample case treated assumes a PSR zone, CH4 fuel premixed with air at <t> = 0.5, and a mixture inlet temperature of 675K. Upon a pressure of 14.3 atm, the modeling indicates that the contributions of the nitrous oxide, Zeldovich, and prompt mechanisms to the NOx are 85:14:1. That is, for high pressure, lean-premixed combustion, the modeling suggests that the nitrous oxide/NOx formation route is the predominant source of NOx, whereas at atmospheric pressure, all three mechanisms contributed to the NOx. The conclusions which can be drawn from the modeling indicate the following: 1. The modeling confirms the exponential dependence of the NOx on flame temperature. The apparent activation energy (of 58.7kcal/gmol) is at the high end of the range noted for most of the experiments. 2. The modeling indicates that the effect of pressure on NOx is experienced mainly in the 1 to 8atm range. For the pressures of interest in land based gas turbine engines, the NOx is predicted to vary relatively weakly with pressure. In fact, for low flame temperatures the modeling suggests that the NOx may decrease with increasing pressure. 3. The residence time of paramount importance is that of the flame zone (i.e., the zone of high free radical concentrations). 7. CONCLUSIONS In this paper the control of NOx in gas-fired lean-premixed combustion, and in gas turbine engines has been reviewed and discussed. NOx data for a range of lean-prelnixed burners have been presented and compared, and conclusions regarding the main features have been drawn. Both similarities and differences have been seen between the different burners. Likewise, both similarities and differences have been seen between the modeling and the burner data. The results are similar in that in all cases the NOx varies significantly with flame temperature. The average apparent activation energy of the NOx formation is about 45kcal/gmol. This value should be used cautiously, however, since there is a significant range in the values of apparent activation energy determined from the different datasets. The results are different in their response of NOx to pressure. Some of the results indicate a weak to negligible (even negative) dependency of the NOx on pressure. These are the laboratory burner and the modeling results. Results from the |