| OCR Text |
Show the breaking of the strong N 2 triple bond (dissociation energy of 941 kT mol-I). This effect is represented by high activation energy of reaction R 1, which makes this the rate - limiting step of the extended Zeldovich mechanism. However the activation energy for oxidation of N atom is small and providing there is sufficient oxygen (fuel- lean flame), the rate of consumption of free nitrogen atoms becomes equal to the rat of its formation and therefore a quasi-steady state can be established. This assumption is valid for most combustion cases except for extremely fuel - rich combustion conditions. In order to solve equation (1), the concentration of quasi-steady species N, stable species, i.e. O 2 and N 2 , the concentration of 0 atoms as well as free radical 0 H will be required. Following a suggestion by Zeldovich, the thermal-NO formation mechanism can be decoupled from the main combustion process, by assuming equilibrium values of temperature, stable species and 0 atoms and OH radicals. However an error may be involved by this approximation. The effect of super equilibrium 0 atom on NO formation rates has been investigated (6) during CH 4/ Air combustion. The results from these investigations indicate that the levels of NO emissions can be underpredicted by as much as 2 % In the flame zone, by assuming equilibrium 0 atom concentrati n . In order to overcome this inaccuracy one of th p ibi approaches would be to couple the xt nd d Z ld vi h m hani n1 with actual hydrocarbon combustion mechanism whi h will inv Iv n1any reactions, species, steps, and will b compl x t s Iv - and (4) |
