Prompt NO mechanisms

There are three major sources of the NO formed in combustion (1) oxidation of atmospheric (molecular) nitrogen via the thermal NO mechanisms (2) prompt NO mechanisms and (3) oxidation of nitrogen-containing organic compounds in fossil fuels via the fuel-bound NO mechanisms [1], The extent to which each contributes is an important consideration. 

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. 

While Table C8 includes reactions for the formation of thermal NO, it does not include those for prompt NO. Mechanisms and reaction rate data for prompt NO formation and various methods for the reduction of NO have been described by Miller and Bowman [Prog. Energy Combust. Sci. 15, 287(1989)]. 

The prompt-NO mechanism forms NO earlier in the flame than the thermal mechanism and is initiated by reaction (4) [12]. Both N and HCN react rapidly with oxidant to form NO in the flame ... 

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... 

Following the conclusions of Bowman [1], then, from the definition of prompt NO, these sources of prompt NO in hydrocarbon fuel combustion can be identified (1) nonequilibrium O and OH concentrations in the reaction zone and burned gas, which accelerate the rate of the thermal NO mechanism (2) a reaction sequence, shown in Fig. 7, that is initiated by reactions of hydrocarbon radicals, present in and near the reaction zone, with molecular nitrogen (the Fenimore prompt-NO mechanism) and (3) reaction of O atoms with N2 to form N2O via the three-body recombination 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 ... 

Figure 25.27 illustrates the contributions of the three mechanisms to NO formation. It can be seen from Figure 25.27 that both the fuel and prompt NO are weakly dependent on temperature. Below around 1300°C thermal NO formation is negligible. However, at the highest temperatures thermal NO is the most important. Once NO has been formed, it can then oxidize to NO2 according to ... 

One early controversy with regard to NO chemistry revolved around what was termed prompt NO. Prompt NO was postulated to form in the flame zone by mechanisms other than those thought to hold exclusively for NO formation from atmospheric nitrogen in the high-temperature zone of the flame or post-flame zone. Although the amount of prompt NO formed is quite... 

The term prompt NO derives from the fact that the nitrogen in air can form small quantities of CN compounds in the flame zone. In contrast, thermal NO forms in the high-temperature post-flame zone. These CN compounds subsequently react to form NO. The stable compound HCN has been found in the flame zone and is a product in very fuel-rich flames. Chemical models of hydrocarbon reaction processes reveal that, early in the reaction, O atom concentrations can reach superequilibrium proportions and, indeed, if temperatures are high enough, these high concentrations could lead to early formation of NO by the same mechanisms that describe thermal NO formation. 

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. 

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. 

From other more recent studies of NO formation in the combustion of lean and slightly rich methane-oxygen-nitrogen mixtures as well as lean and very rich hydrocarbon-oxygen-nitrogen mixtures, it must be concluded that some of the prompt NO is due to the overshoot of O and OH radicals above their equilibrium values, as the Bowman and Seery results suggested. But even though O radical overshoot is found on the fuel-rich side of stoichiometric, this overshoot cannot explain the prompt NO formation in fuel-rich systems. It would appear that both the Zeldovich and Fenimore mechanisms are feasible. 

The relative importance of these three mechanisms in NO formation and the total amount of prompt NO formed depend on conditions in the combustor. Acceleration of NO formation by nonequilibrium radical concentrations appears to be more important in non-premixed flames, in stirred reactors for lean conditions, and in low-pressure premixed flames, accounting for up to 80% of the total NO formation. Prompt NO formation by the hydrocarbon radical-molecular nitrogen mechanism is dominant in fuel-rich premixed hydrocarbon combustion and in hydrocarbon diffusion flames, accounting for greater than 50% of the total NO formation. Nitric oxide formation by the N20 mechanism increases in importance as the fuel-air ratio decreases, as the burned gas temperature decreases, or as pressure increases. The N20 mechanism is most important under conditions where the total NO formation rate is relatively low [1],... 

Prompt NO Hydrocarbon fragments (such as C, CH, CH2) may react with atmospheric nitrogen under fuel-rich conditions to yield fixed nitrogen species such as NH, HCN, HjCN, and CN. These, in turn, can be oxidized to NO in the lean zone of the flame. In most flames, especially those from nitrogen-containing fuels, the prompt mechanism is responsible for only a small fraction of the total NO,. Its control is important only when attempting to reach the lowest possible emissions. 

The formation of prompt NO increases the complexity of the nitrogen chemistry in gas flames considerably. This is illustrated in Fig. 14.9, which shows the most important reaction paths in prompt NO formation, as well as fuel nitrogen conversion—two mechanisms that share some common features. Prompt NO is, as the name indicates, a very rapid mechanism. The initiating step (R117) takes place in the flame zone, where methylidyne radicals (CH) may be formed in significant quantities. 

Because thermal NO is the dominating source under conditions with high temperatures and excess air, it was once assumed that prompt NO formation is negligible in most practical applications. This assumption is hardly valid, however. Turbulent diffusion flames are the most common practical flame configuration. In these flames the reaction zone is typically somewhat fuel rich, providing favorable conditions for prompt NO formation. While the relative contributions of the two formation mechanisms is still in dispute, there is little doubt that prompt NO is an important source of NO in most practical gas-diffusion flames. 

Use GRI-Mech (GRIM30. mec) and a laminar premixed flame code to simulate a stoichiometric, burner-stabilized methane-air flame at a pressure of 20 Torr and an unbumed gas velocity of 1 m/s. Evaluate the contribution to NO formation by the N2O mechanism and prompt NO, respectively, by removing the initiation steps in these mechanisms and repeat the flame calculations. 

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... 

An alternative explanation of prompt NO proposed by several investigators is as follows it is known that the combustion reactions proceed by a chain-branching mechanism involving the rapid buildup of such species as H, OH, and O to high levels, in some cases considerably above the values predicted by assuming equilibration of the reactions ... 

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