Nitrogen post-flame production

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. 

The NO thus formed reacts with hydrocarbon radicals to form HCN or CN radicals which then convert to NHj, similarly as described above. Due to the high activation energy of the reaction 0 + N2 of = 335 kJ/mol, the NO (the NH2 precursor molecule) appears relatively slowly in the high-temperature, post-flame thermal NO . There also is a direct HCN production channel, CH + N2 HCN + N [28]. The fate of NH2 depends on the equivalence ratio. The reaction mechanisms in the different types of flames are more complicated than can be indicated here, since it is very important whether the radicals are present in the burning zone or in the burnt gases. The importance of a specific elementary reaction depends on the concentration of both reaction partners as much as on other kinetic parameters like temperature. The amount of NO produced in these flames is generally smaller than in combustion systems in which fuel nitrogen is present. 

In gas-phase combustion science and the area of premixed flames, the premixed gas enters the reaction zone, also referred to as flame front. The flame front is defined as the volume where the ignition and the main part of the instantaneous and exothermic combustion reactions take place. After the flame front, that is, in the post combustion zone, the rest of the combustion reactions take place. The products from the gas-phase combustion are the hot combustion gases, which are dominated by carbon dioxide, water vapour and inerts, such as nitrogen. [9,50]... 

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