OCR Text |
Show k> k TH Ol H + O H + M<^>H20 + M,with K=-J-= . * r, n . (2.5) k_5 [OH][H] k- k, [O,] O + O + M <=> 0 2 + M,with K = -- = -T7. (2.6) k_6 [O]2 where (2.5) and (2.6) are three body reactions. Using these assumptions, the N O reaction rate can be expressed as(12] flNO] nik,lk+3C[N2][Q2]3/4 - k+1k,3C2[NQ]2[Q2]-'" at " L 2 J k+1Kl/4K;/2[NO] + k+3K'/2Kl/2[H20],/2[02],/4 This mechanism has been internally validated for calculations involving laminar counter-flow oxy-diffusion flames, by comparing it with a detailed kinetics scheme (GRI 2.11). The calculations show the need to take into account reaction (2.3), and the possibility to neglect reaction (2.2). It is also shown that the thermal mechanism is the main mechanism for N2 consumption leading to N O formation in natural gas flames, while the Fenimore-prompt mechanism has a negligible contribution. In general the models ignore the reaction (2.3) (except for fuel rich combustion), and include the prompt-NO mechanism using a global model such as developed by De-Soetel14! or Williams.I15] The model presented here, using a partial equilibrium assumption, leads to correct predictions of the N O emissions of a perfectly stirred reactor for high temperatures, as shown in Fig. 2. This fact suggests that the non-equilibrium effects may be of limited importance for oxygen flames due to intense recombination reactions. 6 |