Hydrocarbon combustion mechanism

Detailed finite rate calculations were used from CO and H2 on as before poor agreement was still observed. Poor agreement may be a result of the lack of a detailed hydrocarbon combustion mechanism or of other reactions with nitrogenous intermediates. 

McEnally, C.S. et al.. Studies of aromatic hydrocarbon formation mechanisms in flames Progress towards closing the fuel gap. Prog. Energy Combust. Sci., 32, 247, 2006. 

The reaction 0(3P) + C2H2 plays a key role not only in the combustion of acetylene itself,53 but also in the overall mechanism for hydrocarbon combustion, since acetylene is an important intermediate in the combustion of methane, larger aliphatic hydrocarbons, and aromatics.54-57 There are two energetically-allowed channels ... 

The solution procedure to this equation is the same as described for the temporal isothermal species equations described above. In addition, the associated temperature sensitivity equation can be simply obtained by taking the derivative of Eq. (2.87) with respect to each of the input parameters to the model. The governing equations for similar types of homogeneous reaction systems can be developed for constant volume systems, and stirred and plug flow reactors as described in Chapters 3 and 4 and elsewhere [31-37], The solution to homogeneous systems described by Eq. (2.81) and Eq. (2.87) are often used to study reaction mechanisms in the absence of mass diffusion. These equations (or very similar ones) can approximate the chemical kinetics in flow reactor and shock tube experiments, which are frequently used for developing hydrocarbon combustion reaction mechanisms. 

The approach is to start with analysis of the smallest of the hydrocarbon molecules, methane. It is interesting that the combustion mechanism of methane was for a long period of time the least understood. In recent years, however, there have been many studies of methane, so that to a large degree its specific oxidation mechanisms are known over various ranges of temperatures. Now among the best understood, these mechanisms will be detailed later in this chapter. 

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. 

The relative importance of these three mechanisms in NO formation and the total amount of prompt NO formed depend on conditions in the combustor. Acceleration of NO formation by nonequilibrium radical concentrations appears to be more important in non-premixed flames, in stirred reactors for lean conditions, and in low-pressure premixed flames, accounting for up to 80% of the total NO formation. Prompt NO formation by the hydrocarbon radical-molecular nitrogen mechanism is dominant in fuel-rich premixed hydrocarbon combustion and in hydrocarbon diffusion flames, accounting for greater than 50% of the total NO formation. Nitric oxide formation by the N20 mechanism increases in importance as the fuel-air ratio decreases, as the burned gas temperature decreases, or as pressure increases. The N20 mechanism is most important under conditions where the total NO formation rate is relatively low [1],... 

In terms of temperature regions, low-temperature combustion occurs over the range 298-550 K, whereas high-temperature combustion mechanisms dominate at temperatures over 1000 K. Intermediate temperatures, from 550 to 700 K, demonstrate an unusual phenomenon called the negative temperature coefficient (NTQ, which is observed for methane and larger hydrocarbon fuels. As shown in Fig. 3, when the correct alkylperoxy radical chemistry is included in a fuel s combustion mechanism, a NTC range exists (Fig. 3, plot C) where an increase in temperature causes a decrease... 

The simplest hydrocarbon, methane, has posed a wealth of challenges to experimentalists and theoreticians seeking to discern its combustion mechanism. Methane s reactions have been explored in a wide variety of contexts over the past several decades. We have discussed these briefly the interested reader is referred to the reviews cited in our previous discussion for further details. Due to the scope of this review, we are primarily interested in these reactions insofar as they provide useful benchmarks for the reactions of larger alkylperoxy (RO2 ) and alkoxy (RO ) systems. With respect to the reactive intermediates present in methane combustion and their implications for larger systems, Lightfoot has published a review on the atmospheric role of these species, while Wallington et al. have provided multiple overviews of gas-phase peroxy radical chemistry. Lesclaux has provided multiple reviews of developments in peroxy radical chemistry. Batt published a review of the gas-phase decomposition reactions available to the alkoxy radicals. ... 

There has been a great deal of research on the combustion of small hydrocarbons, including nitrogen-cycle chemistry leading to nitric-oxide formation and abatement [138]. There are a number of methane-air reaction mechanisms that have been developed and validated [274,276,278], the most popular one being GRI-Mech [366]. There is also active research on the kinetics of large hydrocarbon combustion [81,88,171,246,328-330,426]. 

The role of formaldehyde in hydrocarbon combustion varies with the hydrocarbon and the experimental conditions, principally the temperature. There are two distinguishable regions characterized by fairly distinct oxidation mechanisms. The lower region is considered first. 

Similar reaction sequences have been identified in other chemically reacting systems, specifically catalytic combustion (52, 53), solid-fuel combustion (54), transport and reaction in high-temperature incandescent lamps (55), and heterogeneous catalysis (56 and references within). The elementary reactions in hydrocarbon combustion are better understood than most CVD gas-phase reactions are. Similarly, the surface reaction mechanisms underlying hydrocarbon catalysis are better known than CVD surface reactions. 

Hydrocarbon combustions are highly complex with very many reactions participating. Nonetheless, a simplified mechanism can be written containing all the essential features to explain cool flame behaviour. 

One of the important hydrocarbon combustion reaction intermediates is the CH radical. Although CH chemiluminescence (.42 A — X2ir) has been observed in many hydrocarbon flames, the mechanism of CH formation and its reaction kinetics have been difficult to unravel in situ due to the low steady-state concentrations and the complex nature of combustion reactions. This project was undertaken to investigate a means of CH radical production and to study its reactions with various important species so that an overall picture of the oxidation processes, particularly with regard to the mechanism of NO formation, may be better understood. 

Production of soot in hydrocarbon combustion is a complicated kinetic process involving transformations of fuel molecules in fuel-rich zones under the influence of heat (called pyrolysis) to produce new fuel molecules that contain more C and less H than the original molecules. The elementary steps involved are so numerous that simplifying approximations are essential for both efficient computation of soot production and development of understanding of the kinetic mechanisms. Progress is being made toward discovery of suitable simplications [49]. It is known that small... 

The other segment of the mechanism would include the formation of chem-ions, HaO and CsHs, which are the principal positive ions in hydrocarbon combustion. The formation of HsO" is relatively well understood, and the reader is referred to the review article by Fontijn (IS) for particulars. The same certainty cannot be applied to the reaction mechanism leading to CsHs, It appears, see Kistiakowsky and Michael (19)), that the most probable reaction leading to this ion in the absence of oxygen is ... 

Van der Honing, G. Volatile and Char Combustion in Large Scale Fluidized Bed Coal Combustors", Ph.D dissertation, Twente University, Netherlands, 1991. Konnov A. A., Detailed Reacrion Mechanism for Small Hydrocarbons Combustion, Release 0.4, http //homepages.vub.ac.be/ akonnov/, 1998. 

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