Zeldovich mechanism

Zeldovich mechanism Zeldovich reactions Zelec DP Zenite Zenz plot... 

Theimal NO The formation of thermal NO is described by the Zeldovich mechanism ... 

A reaction mechanism is a series of simple molecular processes, such as the Zeldovich mechanism, that lead to the formation of the product. As with the empirical rate law, the reaction mechanism must be determined experimentally. The process of assembling individual molecular steps to describe complex reactions has probably enjoyed its greatest success for gas phase reactions in the atmosphere. In the condensed phase, molecules spend a substantial fraction of the time in association with other molecules and it has proved difficult to characterize these associations. Once the mecharrism is known, however, the rate law can be determined directly from the chemical equations for the individual molecular steps. Several examples are given below. 

For the prediction of NOx formation, the extended Zeldovich mechanism described by Heywood[603] was implemented. The soot emission modelis a modified version of previously published models for soot formation and oxidation. Details of the soot emission model have been discussed by Han et al.[604]... 

Prompt NO mechanisms In dealing with the presentation of prompt NO mechanisms, much can be learned by considering the historical development of the concept of prompt NO. With the development of the Zeldovich mechanism, many investigators followed the concept that in premixed flame systems, NO would form only in the post-flame or burned gas zone. Thus, it was thought possible to experimentally determine thermal NO formation rates and, from these rates, to find the rate constant of Eq. (8.49) by measurement of the NO concentration profiles in the post-flame zone. Such measurements can be performed readily on flat flame burners. Of course, in order to make these determinations, it is necessary to know the O atom concentrations. Since hydrocarbon-air flames were always considered, the nitrogen concentration was always in large excess. As discussed in the preceding subsection, the O atom concentration was taken as the equilibrium concentration at the flame temperature and all other reactions were assumed very fast compared to the Zeldovich mechanism. 

Equally important is the fact that Fig. 8.2 reveals large overshoots within the reaction zone. If these occur within the reaction zone, the O atom concentration could be orders of magnitude greater than its equilibrium value, in which case this condition could lead to the prompt NO found in flames. The mechanism analyzed to obtain the results depicted in Fig. 8.2 was essentially that given in Chapter 3 Section G2 with the Zeldovich reactions. Thus it was thought possible that the Zeldovich mechanism could account for the prompt NO. 

The early experiments of Bowman and Seery appeared to confirm this conclusion. Some of their results are shown in Fig. 8.3. In this figure the experimental points compared very well with the analytical calculations based on the Zeldovich mechanisms alone. The same computational program as that of Martenay [11] was used. Figure 8.3 also depicts another result frequently observed fuel-rich systems approach NO equilibrium much faster than do fuel-lean systems [12]. 

Although Bowman and Seery s results would, at first, seem to refute the suggestion by Fenimore that prompt NO forms by reactions other than the Zeldovich mechanism, one must remember that flames and shock tube-initiated reacting systems are distinctively different processes. In a flame there is a temperature profile that begins at the ambient temperature and proceeds to the flame temperature. Thus, although flame temperatures may be simulated in shock tubes, the reactions in flames are initiated at much lower temperatures than those in shock tubes. As stressed many times before, the temperature history frequently determines the kinetic route and the products. Therefore shock tube results do not prove that the Zeldovich mechanism alone determines prompt NO formation. The prompt NO could arise from other reactions in flames, as suggested by Fenimore. 

Fortunately, OH radicals do not attack N2 efficiently. However, it is more likely that the effect of water on NO emissions is through the attendant reduction in combustion temperature. NO formation from atmospheric nitrogen arises primarily from the very temperature-sensitive Zeldovich mechanism. 

Although the prediction of N0X emissions under lean and stoichiometric combustion with the extended Zeldovich mechanism is adequate for certain applications, predictive methods for fuels containing bound nitrogen and for rich combustion conditions require substantial improvement. However, the early studies of Fenimore (13, 14) demonstrated the potential importance of HCN and NH type species in fuel-nitrogen interactions. To illustrate the critical importance of the coupling of nitrogenous species reactions in rich combustion, predictions of NO emissions from rich iso-octane combustion in a jet-stirred combustor are shown in Table III. C2 hydrocarbon fragmentation and oxidation creates... 

Prompt NO mechanisms In dealing with the presentation of prompt NO mechanisms, much can be learned by considering the historical development of the concept of prompt NO. With the development of the Zeldovich mechanism, many investigators followed the concept that in premixed flame systems, NO would form only in the post-flame or burned-gas zone. Thus, it was thought possible to experimentally determine thermal NO formation rates and, from these rates, to... 

These experimental measurements on flat flame burners revealed that when the NO concentration profiles are extrapolated to the flame-front position, the NO concentration goes not to zero, but to some finite value. Such results were most frequently observed with fuel-rich flames. Fenimore [9] argued that reactions other than the Zeldovich mechanism were playing a role in the flame and that some NO was being formed in the flame region. He called this NO, prompt NO. He noted that prompt NO was not found in nonhydrocarbon CO-air and H2-air flames, which were analyzed experimentally in the same manner as the hydrocarbon flames. The reaction scheme he suggested to explain the NO found in the flame zone involved a hydrocarbon species and atmospheric nitrogen. The... 

From the view-point of determination of recombination rate coefficients using measurements of H atom concentrations for example, the overshoot phenomena mentioned do not invalidate the p.e. approach, since the concentrations of the overshooting species are too low to contribute to the overall radical concentrations in the recombination region. It is more likely that the conditions in many actual flames are such that the p.e. assumption will predict slightly too rapid a recombination rate from a given set of rate coefficients. In some circumstances, however, O atom overshoot may influence the accuracy of prediction of rates of O atom reactions in flames using the p.e. assumptions. This may need careful consideration, for example, before attempting to calculate nitric oxide formation by the Zeldovich mechanism. 

Sarofim and Pohl (16) used this same technique and found fair agreement with their data on premixed, atmospheric pressure flat flames. Iverach et al, 17) used a similar partial equilibrium assumption to correlate their data on hydrocarbon flames and found good agreement under fuel-lean (excess air) conditions. Poor agreement was observed under fuel-rich conditions unreasonably large radical concentrations were required to make the Zeldovich mechanism account for the measured NO. Iverach, therefore, suggested that reactions such as those proposed by Fenimore may be important under fuel-rich conditions. 

There are three generally accepted mechanisms for NOx production thermal, prompt, and fuel. Thermal NOx is formed by the high-temperature reaction of nitrogen with oxygen, by the well-known Zeldovich mechanism.4 It is given by the simplified reaction ... 

The kinetics of these reactions are such that their role becomes significant only above 1500°C. Consequently, diffusion flames are particularly prone to higher levels of thermal NO production because of their higher peak flame temperatures. Besides the Zeldovich mechanism, NO formation can also occur via the prompt-NO mechanism and from fuel nitrogen sources. In the prompt-NO mechanism, the reactions of CH radicals, produced by the sequential degradation of hydrocarbon fuels, with N2 are responsible for NO production ... 

NOx formation can occur by three mechanisms a) the Zeldovich mechanism for thermal NOx formation, b) the prompt mechanism and c) the fuel nitrogen mechanism. For clean nitrogen-free fuels, only the first two mechanisms occur. 

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