Hydrocarbon species, formation pyrolysis reactions

Even comprehensive mechanisms, however, must be utilized with caution. The GRI-Mech fails, for instance, under pyrolysis or very fuel-rich conditions, because it does not include formation of higher hydrocarbons or aromatic species. Its predictive capabilities are also limited under conditions where the presence of nitrogen oxides enhances the fuel oxidation rate (NO f sensitized oxidation), a reaction that may affect unbumed hydrocarbon emissions from some gas-fired systems, for example, internal combustion engines. 

PAHs are recognized as cosmochemically important molecules, because they are abundantly detected in interstellar media, carbonaceous chondrites, and interplanetary dust particles. Although FTT reactions [42], the pyrolysis of hydrocarbons such as tlie polymerization of acetylene [138], and ion-molecule reactions [148] liave been accepted as responsible for tlie PAH genesis, the shock process discussed in the present study can be suggested as a strong candidate for PAH formation. Most species of PAHs detected in meteorites and interplanetaiy dust... 

The first step in the soot formation mechanism is the formation of the first aromatic ring when aromatic species are not directly present in the feed. Thus, the formation mechanisms of benzene and the first aromatic species in hydrocarbon pyrolysis, particularly at high temperatures, have been investigated in great depth. Nevertheless, several uncertainties about these formation paths remain. Due to the high temperatures, acetylene addition reactions on unsaturated radicals play a key role (Frenklach and Wang, 1994) ... 

Carbon black is formed by pyrolysis of hydrocarbons. The process of carbon formation is complex, involving cracking, dehydrogenation and subsequent accretion into large molecular species. It is probable that there is a stage in the reaction sequence at which liquid droplets are formed, which then undergo partial graphitization with additional... 

To make the issue even more complex, two radicals can occasionally react with each other (dimerize) in a very exothermic formation of new hydrocarbons (dimers) that can reenter the reaction sequence and provide new species. For example, two propyl radicals might combine to form hexane (Fig. 11.8). Further pyrolysis of hexane would lead to many other products. 

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