Hydrocarbon oxidation mechanisms

It is essential to establish the specific mechanisms that explain the cool flame phenomenon, as well as the hydrocarbon combustion characteristics mentioned earlier. Semenov [14] was the first to propose the general mechanism that formed the basis of later research, which clarified the processes taking place. This mechanism is written as follows  

Since the system requires the buildup of ROOH and R CHO before chain branching occurs to a sufficient degree to dominate the system, Semenov termed these steps degenerate branching. This buildup time, indeed, appears to account for the experimental induction times noted in hydrocarbon combustion systems. It is important to emphasize that this mechanism is a low-temperature scheme and consequently does not include the high-temperature H2—02 chain branching steps. 

At first, the question of the relative importance of ROOH versus aldehydes as intermediates was much debated however, recent work indicates that the hydroperoxide step dominates. Aldehydes are quite important as fuels in the cool-flame region, but they do not lead to the important degenerate chain branching step as readily. The RO compounds form ROH species, which play no role with respect to the branching of concern. 

Owing to its high endothermicity, the chain initiating reaction is not an important route to formation of the radical R once the reaction system has created other radicals. Obviously, the important generation step is a radical attack on the fuel, and the fastest rate of attack is by the hydroxyl radicals since this reaction step is highly exothermic owing to the creation of water as a product. So the system for obtaining R comes from the reactions 

Some evidence [17, 17a] suggests that both sets of products develop from a complex via a process that can be written as 

The abstraction of a hydrogen atom from an alkane first produces an alkyl radical. In the atmosphere, however, alkyl radicals have but little choice other than to combine with oxygen to yield an alkylperoxy radical. As mentioned previously, tertiary hydrogen atoms are abstracted more easily than secondary H atoms, and their abstraction, in turn, is more facile than that of primary H atoms. In the higher hydrocarbons the number of secondary H atoms usually exceeds that of primary or tertiary ones, so that secondary alkyl and alkylperoxy radicals are most frequently formed  

From rate coefficients for the attachment of CH3 and C2H5 to molecular oxygen (see Table A-4), it can be estimated that their lifetimes in air at ground-level pressure is about 10-7 s. Other alkyl radicals undoubtedly convert to alkylperoxy radicals at similarly rapid rates. Occasionally, an abstraction reaction of the type 

Higher alkylperoxy radicals are expected to react analogously to CH302 radicals, but a few supplemental remarks are in order regarding the products. 

Darnall et al. (1976b) and Carter et al. (1979) have estimated ratios of fcb/fca of 0.04 for propylperoxy, 0.09 for butylperoxy, and 0.16 for pentylperoxy radicals. Atkinson et al. (1982a) found similar values rising up to 0.32 for n-octylperoxy radicals. In addition to these authors, Demerjian et al. (1974) and Niki et al. (1972) have shown by computer simulations of smog-chamber data that higher alkylperoxy radicals react with NO at least as rapidly as CH302. 

The reaction of alkylperoxy radicals with H02 leads to the formation of stable alkylhydroperoxides. This is a radical chain termination process. 

---

The low-temperature hydrocarbon oxidation mechanism discussed in the previous section is incomplete because the reactions leading to CO were not included. Water formation is primarily by reaction (3.56). The CO forms by the conversion of aldehydes and their acetyl (and formyl) radicals, RCO. The same type of conversion takes place at high temperatures thus, it is appropriate, prior to considering high-temperature hydrocarbon oxidation schemes, to develop an understanding of the aldehyde conversion process. 

Building on the foundation of the hydrocarbon oxidation mechanisms developed earlier, it is possible to characterize the flame as consisting of three zones [1] a preheat zone, a reaction zone, and a recombination zone. The general structure of the reaction zone is made up of early pyrolysis reactions and a zone in which the intermediates, CO and H2, are consumed. For a very stable... 

Significant progress has also been made on the development of low and intermediate temperature hydrocarbon oxidation mechanisms, and the reader is again referred to the literature. 

The continental biosphere is a large source of hydrocarbons. Quantification of these sources in toms of geophysical (e.g. temperature, humidity, light levels) and biogeochemical (soil physical and chemical properties, land use) parameters is much needed for inclusion in atmospheric models. The hydrocarbon oxidation mechanisms in the atmosphere should also be better understood, so that formation of ozone, carbon monoxide, partially oxidized gaseous hydrocarbons, and organic aerosol can be better quantified. The formation of organic aerosol from hydrocarbon precursors and then-capability to serve as cloud condensation nuclei are issues which need to be studied in depth. 

Let us now pass on to hydrocarbon oxidation mechanisms at high temperatures, when cracking and dehydrogenation become the main reactions in the system. 

It is apparent that the fate of the H atom (radical) is crucial in determining the rate of the H2-O2 reaction or, for that matter, the rate of any hydrocarbon oxidation mechanism. From the data in Appendix B one observes that at temperatures encountered in flames the rates of reaction between H atoms and many hydrocarbon species are considerably larger than the rate of the chain branching reaction (17). Note the comparisons in Table 1. Thus, these reactions compete very effectively with reaction (17) for H atoms and reduce the chain branching rate. For this reason. 

In the atmosphere, hydrocarbons are subject to attack by OH radicals and ozone which initiate an oxidation mechanism whereby the materials are first converted to oxygenated compounds and then partly to CO. Hydrocarbon oxidation mechanisms are discussed in Section 6.3. Here we note that not every carbon atom is converted to CO. Accordingly, a yield estimate is required if one wishes to utilize the above data in estimating the production of CO from the oxidation of hydrocarbons. For isoprene the oxidation mechanism has been staked out and one expects a conversion yield of 80% CO, 20% C02. A laboratory study of Hanst et al. (1980) has essentially confirmed these yields. Terpenes, by contrast, pose much large uncertainties, because a substantial portion of the material may be converted to low-volatility products, which condense onto aerosol particles (see Section 7.4.3). The experiments of Hanst et al. (1980) on a-pinene indicated a total yield of CO + C02 of 30% and a CO/C02 ratio of 0.7. Thus, about 20% of carbon in a-pinene was converted to CO. If the conversion efficiencies for other terpenes were similar, one would obtain the following CO... 

A novel laser-based technique for the time resolved study of integrated hydrocarbon oxidation mechanisms,... 

Photochemical Processes Basic Gas-Phase Free Radical Chemistry The Oxidation of Methane Hydrocarbon Oxidation Mechanisms Ozone... 

Although studies on methane partial oxidation have never stopped and continued beyond this period, and even attempts to introduce new industrial processes have been made (see, e.g., [29]), none of them found practical implementation. At the same time, under the influence of the theory of branched-chain reactions, developed by Semenov and his co-workers [30], a new understanding of mechanism of the oxidation of hydrocarbons began to take shape. Results of more than half a century of research in the oxidation of hydrocarbons and new concepts of hydrocarbon oxidation mechanism were summarized in a fundamental monograph by Shtern [13]. 

Hughes, K.J., Fairweather, M., Griffiths, J.F., Porter, R., Tomlin, A.S. The application of the QSSA via reaction lumping for the reduction of complex hydrocarbon oxidation mechanisms. Proc. Combust. Inst. 32, 543-551 (2009)... 

---