OCR Text |
Show effect of LHV off gas increases with the increase in excess air levels. In these circumstances, the quantity of fuel entering the flame is greater and the mixing of the fuel and air less rapid on a proportional basis. Increasing the excess air levels will therefore increase the combustion intensity and the propensity for NOx formation. In this case the introduction of LHV off gas is the sole NOx controlling technique and demonstrates greater effect as the excess air is increased. IX. Discussion Further manipulation of the results for which the reduction in NOx emissions achieved by the use of LHV off gas fuel are simply interpreted as a reduction in NOx emissions by a flame temperature controlling technique, while not intended to be used for prediction purposes, provides some interesting observations. If the horizontal axis of Figure 14 were replaced by a generic scale of fraction of base line NOx remaining after implementation of the primary NOx controlling technique, for example, the use of flue gas recirculation or steam injection, then the relative effects of varying excess air levels on the reduced NOx emissions may be calculated; Figure 15. Given the relationship demonstrated in Figure 15, it may also be conceivable to relate the relative effect of varying flame stoichiometry on NOx emissions to reduction in adiabatic or measured flame temperature from a baseline case, Figure 16. The validity of these correlations in this latter form; however, are questionable, particularly due to the varying effectiveness of introducing inert species to the flame by different methods as discussed in section VI (flex - inspirated RFG) where reductions in theoretical adiabatic flame temperature due to inspirated furnace gases were assumed to be a less effective than when achieved by the inclusion of inert gaseous components in the fuel gas; and for variations in burner design or heat release where flame structural differences greatly effect the NOx emissions but will not effect the theoretical adiabatic flame temperature. 13 |