Fenimore mechanism

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. 

Prompt Nitrogen Oxides emerge because of the lack of oxidizer in the reaction medium. Their formation (Fenimore mechanism) is based on the following reactions ... 

This mechanism of NO formation is believed to be basic for burning lean mixtures, when the Fenimore mechanism is already inefficient because of absence of CH radicals. Reaction (88), being termolecular, notably accelerates at high pressures and is considered to be limiting in this case. Relatively low activation energies of reactions (88) and (89) make this mechanism responsible for nitrogen oxides formation at low temperatures and pressure of several MPa, when the thermal nitrogen oxides are not virtually formed. Since coal is burnt, as a rule, at the pressure close to atmospheric, this mechanism may not be considered below. 

In the flue gases of power installations with gas flow rates of 25,000—32,000 m /h, nitrogen oxides concentration is 400—800 mg/m, whereas the figures for installations with gas flow rates of 35,000—80,000 m /h are higher, 700—1500 mg/m [296]. Of the three major kinetic mechanisms of the formation of nitrogen oxides during the combustion of hydrocarbon fuels, thermal (Zeldovich mechanism), prompt (Fenimore mechanism) and fuel NOx, the major role in the combustion of natural gas belongs to the thermal mechanism [297], This mechanism, described by the two main reactions... 

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. 

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... 

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... 

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,... 

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. 

It appears that the quantitative prediction of Fenimore s simple [no]sat parameter will require a large kinetic mechanism. The combined NH3 and NO doping experiments test Fenimore s two step mechanism in a new way, but it is found that the single [NO]sat parameter can still correlate the results(14). 

In spray combustion, the prompt NO formation is initiated by HC radicals, which are present in fuel-rich regions. Therefore, prompt NO is generated directly in the main reaction zone. This mechanism has first been described by Fenimore [16] and, subsequently, has been refined by various other researchers. For a more detailed account of prompt NO formation, together with additional references (see [20, 52]). 

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