OCR Text |
Show Figure 6 shows that even under the optimum rich zone stoichiometry, an excessively lean second stage can impair NO reduction. The model suggests that under high excess air conditions, the [O]/[OH] ratio in the burnout region increases. Thus, obtaining sufficient OH to ensure ammonia oxidation necessarily entails the generation of excess oxygen atoms. These promote Global Reaction A which competes with NO reduction. Note that it is the same high [O]/[OH] ratio that limits thermal deNOx efficiency at SR = 1.25. In applications where high excess air is desirable, the air addition could be accomplished in two stages, e.g., to SR = 1.02 and 1.25. The NO reduction would occur to high efficiency during the first addition. SUMMARY AND CONCLUSIONS Two commercial NOx control technologies involve the addition of a selective reducing agent (NH3 or urea) to fuel lean combustion products. The effectiveness of these technologies is reduced at low initial NO concentrations, which prevents the achievement of very low NO emissions. The results of this study indicate that these deficiencies can be avoided by the implementation of a hybrid scheme. This hybrid scheme would combine combustion modification with the injection of a selective reducing agent. The data presented in this paper indicate that the optimum conditions for NO reduction with the hybrid approach are: Reaction temperature: BOOoC Stoichiometry at the injection location: SR = 0.99 Minimal air addition to give slightly fuel lean conditions: SR = 1.02 Under these conditions the NO removal window is extended to lower temperatures and higher efficiencies relative to the overall fuel lean process. The kinetic modeling suggests that the critical factor is an external source of OH radicals to initiate the decomposition of the agent. In the present approach the oxidation of the CO from the fuel rich stage provides the OH. A second critical point is that an excess of radicals can reduce the NO reduction efficiency by promoting the oxidation of the key intermediate, NH2. Thus, the level of 02 in the final stage and particularly the level of CO entering that stage must be set to provide an optimum OH profile. Finally, a comparison of the different agents suggests that ammonium sulfate is superior for removing NO. This is both because of the higher NO removal efficiency at the optimum temperature, and because of the broader temperature range over which it is active. The modeling implies that a -NINO ratio closer to 1.0 will still yield high NO reductions under optimum conditions, but will result in much lower ammonia emissions. 18 |