Aliphatic flames, formation

Additives, inhibition of detonation, 186-90 Air jet surface, I69f Aliphatic flames, formation of aromatic species, 3-16 Allene, rate coefficients for cyclopentadienyl cation reactions, 59t,60t,6lt Ammonia combustion, kinetic mechanism, 93,94f... 

For aliphatic hydrocarbons a close relationship exists between knock and low-temperature, two-stage ignition (110). In both cases two induction periods are observed. One, Ti, extends up to cool flame formation. The other, r2, follows ti and lasts up to autoignition. 

Role of C4 Hydrocarbons in Aromatic Species Formation in Aliphatic Flames... 

In aliphatic flames aromatic rings must be formed from nonaromatic precursors, but an accepted mechanism for this critical step has not appeared in the literature. Attempts to derive mechanisms for benzene formation in flames have suffered primarily from the lack of pertinent kinetic or thermodynamic data. Furthermore, we have found no literature wherein formation rates of benzene, or other aromatics, predicted from a mechanism are compared with rates measured in a flame. 

We will not attempt to review here the many mechanisms which have been proposed to account for aromatic formation in aliphatic flames. Suffice it to say that these fall basically into three categories ionic mechanisms concerted, pericyclic mechanisms and free radical mechanisms. 

The behavior of C4H4 relative to benzene and PAH has been observed in other aliphatic flames, including those of methane (25,26), acetylene (7,27), and ethylene (27), as well as benzene flames (1, 10). As an example. Figure 13 shows data for ethylene and acetylene flames extracted from the works of Crittenden (28) and Crittenden and Long (27). This correlation may be explained if 1,3-butadienyl can be shown to be the primary precursor for formation of C4H4, as well as PAH. 

Aromatic species formation in aliphatic flames, 3-16 Aromatic species formation in... 

J.A. Miller and C.F Melius. Kinetic and Thermodynamic Issues in the Formation of Aromatic Compounds in Flames of Aliphatic Fuels. Combust. Flame, 91 21-39, 1992. 

CJ. Pope and J.A. Miller. Exploring Old and New Benzene Formation Pathways in Low-Pressure Premixed Flames of Aliphatic Fuels. Proc. Combust. Inst., 28 1519-1527,2000. 

With aliphatic aldehydes, condensation takes place in the presence of concentrate hydrochloric add at room temperatures, yielding derivatives of the type shown in equation (1). In the ease of aromatic aldehydes the reaction takes place with or without a solvent when dry hydrogen chloride is passed in. Heating these condensation products in the free flame decomposes them, with formation of aldehyde and phenylarsine, the latter being immediately oxidised to arsenobenzene in air, this in turn being transformed completely to triphenylarsine and arsenic ... 

Preformed aromatic hydrocarbons appear to serve as templates for the formation of PAHs thus, burning benzene leads to a far greater yield of PAHs than burning aliphatic hydrocarbons. Unsubstituted PAHs are typical of high-temperature combustion processes, whereas lower-temperature flames afford a large number of PAHs substituted with alkyl (primarily methyl) groups (Hites,... 

Aromatic hydrocarbons are known to be important in soot formation in flames. The aromatic structure may abet molecular growth leading to PAH and soot formation through its ability to stabilize radicals formed from addition of aromatic radicals to unsaturated aliphatics such as acetylenic species (jL>2.). Accordingly, both aromatics and unsaturated aliphatics would be important for growth processes. Both types of species are prevalent in the flame zone where growth occurs. Aromatic structures with unsaturated side chains also are observed there (1 >3). 

There is another condensed-phase mode of action that is specific for aliphatic bromine, and it is the opposite of char formation. Bromine radicals generated thermally at low temperature in the polymer melt can cause chain scission at tertiary C atoms.Examples of polymers where this mechanism is operational are polystyrene (foams) and polypropylene (preferably thin parts, films, or fibers). The decreased molecular weight causes fast dripping of the hot polymer, which cools the flame and eventually extinguishes it ... 

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